Back with rough spot around 2500 rpm

Well I’m back with the same issue that has had me pulling my hair out for months now.
Everything I do seems to make it a little bit better but nothing I have done has completely eliminated the issue.
I didn’t want to create another post but I’m just lost and have no idea what’s causing it.
It’s a 300hp crate 360 Magnum, roughly 25-30k miles on engine.
Dualnplane M1 4bbl intake manifold.
Quick fuel super street series 680cfm vacuum secondary carb.
Firecore electronic vacuum advance distributor.
Crane cams hi-6 cd ignition with crane cams Ps92 e-core style coil.
Taylor thundervolt 8.2mm plug wires.
Ngk bkr5e spark plugs brand new indexed and gapped at .40.
Timing is 18 initial 32 total all in by 3400-3500 rpm.
Vacuum advance on manifold vacuum to add 12 degrees timing at idle for a total of 30 degrees at idle.
Issue is I have a rough spot around 2500 rpm and I have not been able to determine the cause.
First I though it was bad plugs and wires, so I replaced them, it helped a little bit didn’t eliminate it, next I tried a new coil, I did have a crane lx91 coil I bought a brand new crane ps92 coil didn’t notice a difference.
Then I thought it was rotor phasing, I’ve verified rotor phasing is correct on my current firecore distributor.
I have verified the timing mark on the balancer with a piston stop.
I’ve verified timing with two dial back lights bothnare within a 1/2 of a degree of each other.
Then I pulled the distributor and put stiffer advance springs inn to slow the advance rate down from all in at 2400 to all in at 3400.
That seemed to help but again did not eliminate the issue.
Then I pulled the plugs again to look at them and seen that cylinder number 8’s plug was semi fouled, not totally but it was darker then the other 7 and a slight smell of fuel to it.
So tonight I am doing a compression test on cylinder 8 with engine warm to see if there is an issue there.
If compression checks out then i don’t know where to go next.
Maybe the crane ignition box? That’s the only piece I haven’t tried swapping out.
But I’d like to confirm it is the issue before spending money on another ignition box and going through the trouble of wiring a new box inn.
I have a steady 20hg of vacuum at idle with vacuum gauge.
I did a compression test about 7-8k miles ago and all 8 cylinder cranked out 150 psi even across the board with carb wide open and engine warm.

The motor is talking to you about it not liking something at that RPM, understanding what is saying Is the hard part AKA aside, I would try reversing what you did with the mechanical advance and have it all in by 2000 instead of 3400 RPM.
If that doesn't fix it try replacing # 7 spark plug with the next hotter range plug and see if that helps it or not Maybe you have a intake gasket sealing problem on # 7 Is that plug shiny or dull looking?

at 2500 does it stay rough from then on as high as you rev it? is that part or WOT or cruise. I would measure the vac & fuel psi & timing at that exact point when it starts to act up. Yes do a compression test & maybe a leakdown & try the new plug first & see if the new plug straightens it out for awhile. Describe the roughness (shakeing/missing). EDIT if it dont continue to be rough from 2500 on up, how much more RPM does it continue to run rough?

Transmission issue?

It’s hard to describe the rough spot, I just know it happens in every gear at the same rpm.
Starts around 2500 and continues on up as far as I can tell.
It almost feels like a driveshaft vibration and sounds like one in the cab specially when I get the rpms up higher around 3500-4000 rpm.
Seems like it takes a little longer to cycle once im at a higher rpm around 3500-4000 rpm if that makes sense.
At 2500-3000 it seems like its cycling pretty quick and the higher the rpm goes up the slower it gets but it’s still there.
You can feel it in the floor boards in the steering wheel, in the seat.
I know this isn’t a drivetrain issue becuause it’s rpm dependent not speed dependent.
I have a 1350 series cv shaft for a rear driveshaft, pinion angle is set perfectly at -2 to -3 so that the pinion is inline with the shaft while driving.
For the longest time I thought this was drivetrain related but now I’m convinced it’s not.
My converter is correct I’ve verified that, flex plate is correct.
Balancer is correct.
And like I said since it’s rpm dependent the drivetrain theory kind of goes out the window.
I’m convinced it’s in the engine somewhere, ether ignition or there is an issue with one of the cylinders.
Its just strange cause the last time I did a compression test all cylinders were dead even at 150 psi.
Below is a pic of the spark plug that came from cylinder 8.
The plug on the left cane from cylinder 6 plug on the right is cylinder 8.
All the other plugs looked just like cylinder 6’s plug.
Cylinder 8 was the only one that didn’t look right and smelled like fuel a little bit.
I’m heading out right now to pull cylinder 8’s plug and do a compression test.
The engine is still warm from driving it home from work.
I’ll report back with what it puts out, last time it was 150 and that was only like 6-8 thousand miles ago and I never abuse this engine.
I baby it big time.
So I don’t see how compression could have just dropped in one cylinder all a sudden.

Can you bump the original post back up so we can reread/refresh our info?. A balanceing issue? just crossed my mind.

I’ve verified the torque converter and the flex plate, they are correct.
I have the magnum weighted converter and a standard la style flex plate.
I just pulled number 8’s plug with the engine warm and the brand new plug I just put in it a couple of days ago is already looking like it isn’t firing right just as before.
Did a compression test on that cylinder with engine warm and carb wide open cranked on it that way for about 5-6 seconds and it’s reading 155 psi.
I’m at a loss with what’s going on with cylinder 8.
I suspect cylinder 8 is causing the rough spot.
Should I pull the valve cover and inspect rocker arms and valve springs?

I’m leaning towards a bad valve spring on cylinder 8.
But what I don’t understand is why I’m not seeing something obvious with the vacuum gauge hooked up.
But it makes sense, only cylinder 8’s plug isn’t firing correctly all the others look pretty good.
It idles fine and drives perfectly fine below 2500 rpm.
Once you get up to 2500 or higher it just gets rough.
I’m seriously thinking valve spring on cylinder 8 but the only thing that has me wondering if that’s really the case is the vacuum reading at idle appears fine.
Maybe I should hook up the vacuum gauge and sling it up to 2500 rpm and watch it and see what it does then.
Cause from what I’ve read online about bad valve springs and the symptoms I’m having it sure makes sense at this point.
Specially with cylinder 8 pumping out good compression.

Just thinking out loud, I'm wondering if a badly worn guide/valve stem/seal is letting the plug, oil foul almost immediately, the compression would still be good as would the valve spring. when you put the new plug in there is it still running rough IMMEDIATELY? (before the plug has time to foul) If so I might be wrong but I keep thinking a "balancing issue".

I’m just not seeing where the balancing could be off and why that would be affecting the number 8 plug?
Like I said i have crawled under the truck and removed the converter inspection plate and verified that it is a weighted Magnum converter.
And a La style flex plate.
The engine still has the balancer on the front that came with the engine from Mopar.
And the number 8 plug isn’t oil foiled, it’s fuel.
This just doesn’t make sense to me, but I have a strong feeling the rough spot I’m feeling is connected to what’s going on with cylinder 8.
Would it be a good idea to have the engine hooked up to some kind of monitor to watch the spark events to see exactly what’s going on?
There has to be way to determine whatever is going on here.

Where would you have me start to rule out a balancing issue?
I’ve wondered the same thing in the past but like I said I’ve looked at the converter and the flex plate and have compared them to other Weighted magnum flex plates and my converter looks identical to the magnum weighted converter right down to what the weight looks like and where it is on the converter.
It’s easy to see it’s a standard La small block flex plate.
Then the front like I said still has the balancer that came with the crate engine from mopar.
And if it was a balance issue why am I not feeling anything below 2500 rpm?
Wouldn’t I be feeling it all the time.
This just has me baffled.

Would it be a good idea to have the engine hooked up to some kind of monitor to watch the spark events to see exactly what’s going on?
There has to be way to determine whatever is going on here.

I've used a timing light on plugs that are suspect. Just shine the light on a clean paper/cardboard piece while increasing rpm's. Up to a certain rpm, your eyes will notice a misfire or erratic firing. Start or with #1 for reference then proceed to #8. If it's erratic with a new plug, then problem is upstream of the plug. Just some off-the-cuff thoughts here.

So I talked to a mechanic friend of mine and he said that if there was a weak valve spring the compression test doesn’t always reveal that.
He let me barrow a leak down tester and said to make sure the valves are closed on cylinder 8 and test it.
He said something is allowing more fuel into cylinder 8 and that he agrees with me that it does sound like a weak valve spring.
And he said that could most definitely be causing the rough spot im feeling.
He said it sounds like the spring is ok at idle and lower rpms but it starts revealing its self around 2500 rpm where the rough spot comes on and it could be allowing the valve to stay open to long drowning the number 8 plug in fuel.

in that other post you said you got rid of the vibration by swapping in a new distributor. is that correct?

now the vibration is back? i see you posted a thread about fouled plugs and the #8 plug being darker.
did you make adjustments to your carb to clean your plugs up?

Well what’s throwing me off is why are the other 7 plugs looking perfectly fine but cylinder 8’s plug is looking like it’s struggling to fire.
This rough spot has been there for a long time and I’ve just kept throwing parts at it trying to figure out what’s going on.
Everything I have done has made it a tiny bit better each time but it’s never been eliminated all together.
New plugs, new wires, new distributor, new coil.
Timing is set at 18 initial and 32 total.
I’ve verified it with two different lights, both dial back lights I have show 1/2 of a degree of each other.
When I un-plug the vacuum advance it gets better.
That’s what led me to believe it was possibly a rotor phasing issue at first.
But I’ve verified rotor phasing on my new distributor and it’s fine with and without vacuum advance.
The only area I haven’t really checked out is the heads.
Compression test revealed 155psi on cylinder 8 with engine warm and carb wide open cranking on it for about 5 seconds.
I’m going to remove the valve cover and do a leak down test on cylinder 8 and hopefully that reveals something cause this is driving me insane.

I reread back thru the old thread. with these changes/adjustments makeing it better it would not be the balancing. unplugging the can changes (4) things: reluctor gap/Rotor Phasing/timing/rotor tip to cap terminal air gap (related to RP) but the RP is corrected which gets the best rotor radial air gap & iirc you bumped the timing up to make up for what the can took out correct? there might be a ring issue causing the fouling but iirc you said it was fuel not oil? How the can capped is helping things & getting #8 to fire is on my mind.

That’s whats on my mind as well.
I’m just going to do a compression test on all 8 cylinders and a leak down test and go from there for right now.
Everyone else I have chatted with about this seems to agree that something is causing number 8 to get more fuel.
And they all seem to think it’s a valve or valve spring problem.
I’m also going to remove the valve cover and see if I can see anything out of the ordinary.
I have a feeling whatever is causing this is something that’s not so obvious but will make sense once it’s found.
It’s just finding it that sucks.

We're gonna have a good laugh when we find it, as in THAT was it!.

Check out this article, this guy was having the same issue I was having except he was having problems with cylinder 6 and he had his computer throwing a miss fire code.
It ended up being a broken valve spring on cylinder 6 and the tech said historically it’s cylinder 8 that has issues with the valve springs.

https://www.justanswer.com/dodge/192b9-2001-dodge-ram-1500-truck-5-9l-v8-my-problem.html

I'm thinking I would toss in a regular MP dist/ECU for a quick test to see if that makes it dissappear. Wouldn't be hard to do with some jumpers. I keep coming back to capping/plugging the vac can hose makes it much better. We've checked that the can/hose ain't leaking & we've added back in the timing the can was providing & we've corrected the rotor phaseing.

Well isn’t possible that a broken valve spring would work better with less timing?
Take away the 10-12 degrees fromvacuum advance and your making it easier on it,
Seems logical to me.
Idk though I’ll pull the valve cover tonight and have a look at it.

Just a couple thoughts. The pic of the two plugs. They look pretty normal to me. #8 could be a bit rich but definitely not fouled. It may be just the pic but the gap looks pretty wide on those. I had a problem like you describe and found out that my plug gap was too wide. .025 should be plenty.

Did you re route the ignition wires exactly like the TSB shows?

Have you replaced #8 ignition wire and spark plug with "known good" substitutes?

Like i wrote in your previous post
looking at all 8 spark traces on an old Sun machine display
could eliminate doubts about ignition.
Those old Sun machines could also do a "cylinder power balance" test at 2500 rpm.

Looking at the valve springs, rocker arms and hyd lifter bleed downs is quite reasonable, as Magnums have been known to have problems in all 3.
A video boroscope makes this inspection quicker and easier but removing valve covers is also worth the time and gasket costs. Looking inside all 8 cylinders with the video boroscope through the spark plug hole is worthwhile. Are there deposits on intake valve stems?

Having down the static compression test on #8 is good. Your 155 is typical.
A "running compression test" also called a dynamic compression test
would also be instructive in spoting a bad valve spring or lifter,
and could even give a hint about intake manifold runner air distribution variation cylinder to cylinder.

Have all the previous worries about the Carb set ups been eliminated?

I removed the valve cover this afternoon and took a close look at both valve springs on cylinder 8 and I’m not seeing anything out of the ordinary compared to the rest of the springs.
I have a leak down tester that I have barrowed from a friend but I haven’t had time to perform the test.
I was told cylinder 8s valves need to be closed while performing the leak down test.
How do I determine when the valves are closed on that cylinder?
The plug gaps are set at .40 cause I am running a crane cams hi-6 cd ignition and a crane cans ps92 e-core coil.
I did another compression test on cylinder 8 while the engine is cold and it revealed 147-148 psi this time.
Is it normal for it to drop a few psi when it’s cold?
I haven’t ran the truck in a few days cause I’ve just been so frustrated with whatever the hell is going on with it.
So you reccomend I call a few shops and find out who has the equiptment to monitor ignition while it’s running and have them bring the throttle to 2500 where the rough spot starts and go from there?
Cause I’m throwing in the towel I have no idea what’s causing this issue.
Number 8s plug looking darker then the rest has me thinking it’s all connected some how.
One thing I can do is turn my chokes fast idle down a little bit more; throw a new plug in cylinder 8 and see what happens.
All plugs are brand new and my plug wires are also brand new.
Coil is brand new, distributor is brand new, timing has been verified with two different lights.
Timing mark on balancer has been verified with piston stop.
This is just strange and I have no idea why this rough spot is a lot more prominent when vacuum advance is connected.
With no vacuum advance it’s much better.

Just try a tighter gap. .040 to .025 isn't going to hurt anything. With high output ignitions the gap will only get wider over time. Start a little tighter.

I’m running all 8 plugs at .040.
The cd ignition should have no problem firing that.
Most company’s reccoemnd .045 to .060 with cd ignition boxes and coils.
I’m turning the choke down a little bit and turning the fast idle down a little bit as well and I’m going to try and route number 6 and number 8 plug wires as far away from each other as possible.
I just don’t know how I’m going to accomplish that cause I only have the plug wire looms that bolt to the valve covers.
I know plug wire 6 and plug wire 8 cross each other but where the do cross is up near the distributor cap.

.040" ain't an issue nor is 6/8 crossing each other (& crossing at a good angle as opposed to running parallel for a good length is the best). the problem is elsewhere.

Well tonight I turned down the electric choke a little bit and turned down the fast idle a little bit and installed a brand new plug in cylinder 8.
I’ll drive it to work tomorrow and check the plug when I get home from work tomorrow night and see if that made a difference.
Cause every morning when I fire it up it is running off the choke and fast idle, its possible the choke being a little on the rich side and the fast idle being a little too high was causing the rich condition on cylinder 8’s plug.
And maybe the rough spot around 2500 is still rotor phasing or maybe it’s just my crane ignition box.
I’m going to call around to a few shops tomorrow and hopefully find someone with a sun machine or a spark analyzer that can hook up to my ignition and bring the Rpm up to 2500 and watch the ignition to get a better idea of what’s going on.

what is the timing at 2500 ? i know you have initial at 18, and the total at 32, but what is it at 2500 ? [with and without the vacuum advance hooked up]

you never said if you did anything to the carb. did you jet it down to clean the plugs up?

is it possible the carb is rich at idle and transition and lean once it gets on the mains (your jetting)?

the color difference on the two plugs you showed doesn't seem that drastic to me given you have a dual plane manifold.

if it were me i would jet the carb up 4 steps, let it get wamr, then set idle speed and adjust idle mix to the highest vacuum reading, then go see if it's still missing.
it's a simple thing to check.

Well like I said I turned down the choke a little bit on the carb and turned down the fast idle a little bit and installed a brand new plug on cylinder 8.
Started it this morning and let it run on the choke and fast idle for about 10 minutes this morning when I first started it.
Left for work and to my surprise it felt a lot better at 2500 rpm in 2nd gear cruising down the road at 35-37 mph.
It wasn’t shaking nearly as bad and felt smoother.
I’m starting to think I just simply had the fast idle on the choke turned up a little too high and the choke was maybe opening a little too slow.
I do know that with this quickfuel carb when I first got it I was fouling plugs like crazy, like 3 or 4 plugs would be completely black and I realized the fast idle being too high was the cause of it cause with the engine idling on fast idle at night I could see the herders turning red and I called quickfuel about the issue and the guy said the engine was loading up with too much fuel on fast idle and to turn the fast idle and the choke down a little bit.
So I did that and I thought it was good but I’m thinking I didn’t have it turned down enough and it was still affecting cylinder 8’s plug.
Time will tell I guess I will run it a couple days like this and check number 8s plug after a few days of running it like I normally do.

Well turning down the fast idle on the choke circuit as well as turning down the choke it’s self has seemed to help a lot here.
One thing I didn’t consider until tonight is should I try running a 1 step hitter plug?
Right now I’m running the factory rated heat range, but I am carbureted and I am also 3500 feet above sea level.
I’m wondering if maybe the combination of higher elevation and running a carburetor might call for a hotter plug?
It’s possoble right? I mean these engines were speced with the plug heat range with efi and sea level in mind.
Not 3500 feet above sea level and a carburetor.

Issue is back and seems to be getting worse with the cold weather....
Now I’m back to thinking it’s an issue on cylinder 8 specifically.
Quick re-fresh, I have an awful miss around 2500-3000rpm it starts around 2000 rpm and feels like a slight surge then around 2500 it starts getting shakey and feeling rough.
Compression test on cylinder 8 revealed 155psi.
I removed the valve cover and couldn’t see anything odd with the valve springs on cylinder 8 but I’m convinced it’s an issue on cylinder 8.
Is there a way to diagnose a valve spring issue on a specific cylinder?

can you jet it up (richer) 4 or 6 sizes and see if it gets any better?

I’ve already increased the idle feed restrictors up 3 sizes, I’ve increased the main jet 1 size, and I’ve decreased idle air bleeds 5 sizes.
There is no way this thing is lean.
I even took the carburetornoff thenither day tire it all down and cleaned it very well used 3 cans of carb cleaner and blew out every orfice in the carb main body and the metering blocks and put it back together with new gaskets.
I have a strong feeling it’s strictly cylinder 8 that’s the issue.
Another reason I believe this is cause of all the things I’ve read online with people having issues with cylinder 8 in particular.
But the compression test on cylinder 8 checks out.
So I’m kind of stuck not knowing what to do next to pin point what the issue is.
Tonight I’m going to hook my timing light up to number 8’s plug wire and bring it up to 2500 where it gets rough and see if the flash from the timing light is erratic.

So tonight I hooked up the timing light to cylinder 8s plug wire and brought the rpm up to 2500 rpm and sure enough you can see the break in the light from the missfire on cylinder 8.
So there is definitely something going on with cylinder 8.
I’m going to go around all the other 7 plug wires and do the same thing just to make sure there isn’t a miss fire on any of the other cylinders but I’ll bet money it’s just cylinder 8 cause like I said every time I pull the plug from cylinder 8 it also looks semi fouled with fuel and looks like it’s struggling to fire compared to the other 7 plugs.
So what are your guesses? Compression test reveals 155 on all cylinders including cylinder 8.
I’m back to suspecting a broken or weak valve spring again.

I wondering if the oil rings are fubared on #8. A quick visual would confirm the spring ain't broken & did we check the seal? I was thinking we did. but if #8 blinks differently than the rest of em, plug wire or check the underside of the cap, the # 8 brass terminal. didn't we replace the plug wires?

Yep brand new plug wires, brand new plugs, brand new cap and rotor.
And number 8’s plug is never oil fouled it’s always fuel and it even smells like fuel.
I’m seriously thinking valve spring.
Something is letting more fuel into cylinder 8 which is causing the plug to semi fuel foul.

It’s ether the valve springs or a valve it’s self but if it was a valve wouldn’t that be showing something on the compression test?

I would think, but the compression rings could be adequate & the oil rings can still be bad. First we gotta resolve the blinking light potential.

So where do I go from here? Should I buy a couple valve springs and put them inn on cylinder 8!and see if that fixes it?

I would put a clean plug in #8 & see if the light blinks differently on #8 than on the rest of em & if so we might swap some plug wires & see if the intermittent blinking follows the wire (that is on #8 now). I wouldn't buy a valve spring right now.

Ok the only issue is I don’t have a shop and my truck is on the side of the road and it would enrage the entire neighborhood if I sat out there Reving the engine.
My neighbor already hates my truck says it’s too loud.
My main thought on that is I have tried multiple plugs on cylinder 8 and the result is always the same the number 8 plug always ends up semi fuel fouled and smelling like fuel.
These are brand new plug wires with less then 100 miles on them, same with the coil and same with the distributor.
The only part of the ignition that hasn’t been replaced and isn’t brand new is the crane cd ignition box.
This issue has been present for a long time and at first I thought it was a vibration from drivetrain then realized that wasn’t the case because of how it’s rpm dependent.
Then I buy new plugs and wires and a new distributor and a new coil all at once and that doesn’t make a difference.
I’m just so annoyed with running in circles with this thing and never pin pointing exactly what the issue is.
It’s been driving me insane.

I will go back thru everything & see if anything jumps out at me.

Change the springs, they aren't that expensive.

What is the timing at 2500 rpm?

so I was thinking today and I remembered awhile back a few months ago I was sitting in neutral and held the throttle at 2500 rpm where the missing starts and I remember my crane ignition box was getting pretty warm to the touch.
And the red light on the box was flickering rapidly at 2500 rpm.
I also know that I have gone through an un-usual amount of pick up coils with this crane ignition box.
Before I purchased this firecore distributor I was running a Mopar unit that was customized and set up by Don at fbo systems.
I went through 2 or 3 pick up coils on the distributor from don.
They would last like 6-9 months and then all a sudden they would just die.
Then another time a couple years ago I had a Premium borg Warner distributor cap on my distributor from don and the coil button on the inside of the distributor cap melted away completely.
That crane box is the only thing I haven’t tried replacing.
It just never occurred to me that it could be causing this issue but now I’m kind of wondering if I got a bad box.
I bought it from a whole saler on eBay and they said it was brand new and it was still packaged Ike it was brand new when I got it, and I got a really good price on it.
Is there any tests I can do to determine if the box is the cause of this missfire?
I don’t have the factory ignition module and wiring anymore otherwise I’d just wire that back up and see what it did.
I’ve triple checked the polarity of the pick up coil wiring I know that’s correct.
I’ve ohmed the new plug wires they all check out at 50 ohms per foot.
I’ve ohmed the coil out it checks out.
Pick up coil checks out on the ohm test.
That basically leaves that crane ignition box.
I’m going to try the timing light on the other cylinders while bringing up the rpm to see if the miss fire shows up on other cylinders or if it’s just strictly cylinder 8.
I got off work an hour and a half late and it’s already too late to be outside reving the engine and testing the other cylinders with the light.
That might have to wait till this weekend.
But if I can get off work at a decent time before the weekend I’m going to try it out and see what happens.
The only thing that’s throwing me off is how number 8s plug always looks like it’s struggling to fire compared to the others, of it was the ignition box you’d think it wouldn’t be affecting just one cylinder.
But we’ll find out when I get the light on the other 7 cylinders and bring the rpm up to where it starts missing

So I did some more testing tonight and I found out that this missfire that occurs around 2000-2500 rpm is not just limited to one cylinder.
It’s happening on all 8 cylinders.
I hooked the timing light up to every cylinder and brought the rpms up to 2500 rpm and I could see the break in the light on every cylinder at 2500 rpm.
I checked the timing at 2500 rpm with vacuum advance and it’s at 43 degrees at 2500 with vacuum advance.
Without vacuum advance it’s at 33 degrees at 2500 rpm.
So based on what I found tonight my current thought is the crane ignition box has something wrong with it internally.
I talked to a tech tonight about this issue and he agreed that since the miss is showing up on all 8 cylinders it sounds like the ignition box or the pick up coil in the distributor.
I explained to him that this issue was present with my previous distributor and that o bought a brand new firecore distributor and the issue is still there.
He said based on the fact that you have replaced everything else in the ignition system it sounds like the box.
Cause I told him the coil is brand new and that made no difference, the plug wires are brand new that didn’t make a difference, plugs are brand new.
And cap and rotor are also brand new a lot with distributor.
Vacuum stays steady at 20hg while idling, when I hold throttle at 2500 the vacuum gauge kind of oscillates a little bit but not by much maybe 1/2 an hg.
I’m not seeing anything on the vacuum gauge to indicate major problems and compression test revealed 155psi on all 8 cylinders while warm.
He asked if I have noticed anything strange with the crane box now or ever since I got it and I told him how it’s always made a pretty loud buzzing sound.
He said Heaney heard some boxes that are quiet and some that buzz so he doesn’t know if that is something to consider or not.
Since there is no way of testing a cd ignition box that I am aware of I was thinking about pulling the box out and taking it apart and seeing if I can see anything un-usual with the wiring and transistors.
Any burned wires or burned area’s.

I wish you were nearby as we could put a spare OE MP ign on it & see if it straightens out. The box IS beginning to look guilty.

Well I used to have a the factory Mopar wiring harness and a chrome Mopar ignition box and a canister oil filled coil.
But now the harness is all hacked up, I still have the chrome box, and I don’t have a coil or a ballast anymore, I don’t know what happened to them.
I moved a couple years back and I suspect it’s probly all buried in my storage unit somewhere.
I could just go buy an msd digital 6 box for a couple hundred bucks at my local Oreilly auto parts store, it would be super easy to wire inn since I already have a cd ignition box wired inn and all the wiring is there for a cd ignition.
I just hate spending money on things without knowing without a doubht that what I’m replacing is the cause of the problem.
But everything at this point seems to be point at the crane box.
Specially when you consider I went through an un-usual amount of pick up coils in my previous Mopar distributor.
I always thought that was strange, I went through 3 pick up coils in about 10k miles and I think whatever issue this crane box has is what was wiping those pick up coils out.
And when you also consider the box gets kind of warm to the touch when you hold the throttle at 2500 and it starts missing and the red led light on the box starts flickering while the throttle is held at 2500 and it’s missing.

I’m going to turn thenrev limiter all the way up tonight and see if the missfiring moves uonhigher in the rpm, if it does then I’ll be convinced it’s the rev limiter malfunctioning inside the box.
I’ve said it before that it almost feels like it’s bouncing off a rev limiter just not quite as harsh as I would expect it to be but then again that could be because it’s not totall kicking inn but it kind of is.
I just don’t know, I do know if I end up replacing the ignition box and it fixed the issue I’m going to send this box inn to fast who boughtnout cranes ignition line and ask them to diagnose the problem and if it isn’t too severe of an issue I’ll have them repair it and send it back that way I’d have a spare box in case I ever run into this kind of thing again.

i've got an msd streetfire on my truck and haven't had any problems. pretty sure they were only $100 when i bought it. seems to be 150-ish now...

https://www.msdperformance.com/products/ignitions/street_fire_cd_ignition/parts/5520

you can disable the rev limiter all together by setting your rotary switches to 00. also if you want to disable the multi spark & rev limiter, you set them to 01

I thought if you set the rev limiter to 0 the engine won’t start?

is this for your ignition box?
http://www.cranecams.com/uploads/instructions/9000-6000c.pdf

00 disables rev limiter, 01 disables multispark AND rev limiter...

Yes I still have the instructions and I have went over the wiring multiple times.
Pick up coil wires are wired correctly, the purple crane wire goes to orange positive on distributor and green crane wire goes to black negative wire on pick up coil.
The yellow rev limiter wire is wired directly to battery positive.
The main power wire on crane box is 10 gauge wire going straight to battery positive and the negative main wire on crane box goes directly to negative battery terminal.
Coil wires are correct.
Thin red wire is going to it’s own switched 12v toggle switch to power the box on and off.
I have pick up coil wires routed as far away from the other wires as I can.
Rev limiter is set at 6k rpm on both dials.

have you tried bypassing the toggle switch and going direct for a test?

Going direct to what? Since this truck was efi before I got it and I put a carbed engine in it I’ve always used a toggle switch to power my ignitions, first with the factory Mopar electronic ignition and now with this crane ignition box.
Could powering the cd box off of a toggle switch be an issue or cause an issue?
It’s a 50 amp toggle switch that turns the box on and off.
I didn’t know what else to use to power the box on and off.

if it's like an MSD it should have a low current turn on lead that should be a switched 12v source.
the thicker red wire should be wired through a fuse to basically the battery.

still doubt any of that is going to give you a miss only after 2500rpm. and especially be worse when it's colder...

Ya it’s exactly like an msd, the thin red wire is a switched 12v wire.
I have that going to a 50 amp toggle switch.
The big main power wire is going directly to positive terminal on battery and the big ground is going directly too negative battery terminal.
Coil wires are correct, pick up coil wires are correct.
It’s just strange whatever it is, I would be confident it was the ignition box at this point except for the fact that the issue seems to get better when I take away the vacuum advance.
That’s what led me to rotor phasing a few months back.
But I have checked rotor phasing with my current firecore distributor with and without vacuum advance and it looks good.
So that’s the only thing that has me a little baffled.
I’m still leaning towards the ignition box, I just wish there was a quick and easy way to test the box before spending the money on another one.

my thinking on the toggle switch is that it may be causing resistance thereby lowering the voltage/amperage draw run a jumper from the battery directly to the box that the toggle switch controls

the turn on lead doesn't need to be high current, it should work all way down to 10v or so. all it does is tell the ignition box to turn on. the bigger voltage wire that he has running to the battery is what provides the ign the real power.

seems you have good ignition and timing.
you have good compression.
the only other thing needed is fuel right?

what is the baseline tune on the carb you have and how is the carb setup now???

Carburetor is a quickfuel super street series 680cfm vacuum secondary carb.
I’m running 65 main jets and 75 secondary jets.
.031 idle feed restrictors, .065 idle air bleeds, .033 high speed air bleeds.
A 9.5hg power valve with .049 pvcr’s.
Floats are set half way up the sight glasses.
Idle mixture screws are about 3/4 to 1 turn out.
This isn’t a fuel issue I know it’s ignition.
I keep thinking it’s the crane box but what’s throwing me off is how this missfiring clears up once I take away the vacuum advance.
So that leads me back to rotor phasing.
But I have checked rotor phasing and it’s good.
So this is very strange I just don’t know what’s causing this.

the vac can does three things, adds timing/shifts RF/increases reluctor gap. Are you saying the problem completely dissappears when the can is capped? something might be loose inside the dist that is being moved when the can shifts. I would cap the can & bump the timing up to where it is when the can is operational & see if it straightens out (grab your dialback). You're sure the RF is close enough when the can is deployed? iirc you said the reluctor gap does not change when the can is pumped up (it changes-gets wider on a regular dist) not sure how this dist is getting around that but it is a good thing). if you duplicate the timing & that ain't it then it has to be something in the dist functioning. (& this is a steady 2500+ miss correct?) EDIT & the can has no vac leak?

Can does not have a vac leak, I have checked the air-gap with and without vacuum advance while holding the distributor in my hand and applying vacuum to the vacuum advance with a mighty vac.
But when it’s in the engine and spinning at 2500 rpm it’s possible it looks a little different then how it looks in my hand.
Someone had mentioned that maybe the oil pump drive bushing could be wiped out causing the inconsistency in timing at 2500.
And it is a steady miss at 2500 that’s affecting every cylinder.
I will cap the vacuum and bring the distributor timing up to 43-44 degrees where it is when vacuum advance is hooked up at 2500 rpm and I’ll bring the rpm up and check every cylinder with the light and report back.
Since it’s chiristmas weekend I’m not sure when I’ll get to it but I’m gonna try ether today or tomorrow or Monday.

Turn the photo upside down and that’s a picture of the rotors position directly under the number cap terminal.
That’s with 18 degrees of initial timing plus another 12 degrees added by the vacuum advance.
So that’s a total of 30 degrees of timing at idle with initial plus vacuum.
That’s idling with 20hg of vacuum.
When I have the throttle held at 2500 rpm in nuetral the vacuum is around the same as it is at idle, maybe slightly less around 18-19hg.
So at 2500 rpm in nuetral the rotor should be roughly in the same exact spot that it’s inn while idling with 20hg of vacuum.
To me the rotor looks like it’s within range as to not cause any misfires or spark scatter inside the cap.

I still have the modified reluctor That will move the rotor forward 5, 10, or 15 degrees.
Looking at the picture of the rotors position with vacuum advance while zoomed in on it close I
Guess the rotor could come forward a little more, I’d guess maybe 5-10 degrees.
I just don’t know if that’s going to solve the issue or not.
But I can give it a try easily enough.
Last time I tired moving the position of the rotor I made my own slot on an old reluctor which moved the rotor a little too far forward, probly around 20-25 degrees and that made the missing worse at 2500.
That was before I got the modified reluctor from ehernberg.
I’m not an expert on rotor phasing and I’m not sure if rotor phasing can cause a missfire on all 8 cylinders or not.
Maybe at 2500 rpm the rotor is being pulled back a little bit further then what the picture shows when it’s idling and that’s whats causing the missfiring.
The most interesting part of this issue to me is how taking the vacuum advance away seems to clean it up.
So again that leads me back to rotor phasing.

You've got the hole cut. Shine the timing light in there when it's running and having a problem.

RP is the same on all cyls (the spatial distance gap) but if the "gap" is borderline it can be random missing as different cyls have different charateristics but to KISS, the ans is yes. some of these Q's are redundant but you say capping the hose completely elims the issue? What Dave said, shine your light in there while IDLING & see where RP is at in relation to the cap terminal "bulge with (A) the can capped & (B) with it plumbed into manifold & the can will shift it CCW (SB). I'm assuming you have a dialback, get it running at 2500 & note the (total) timing then plug the hose & twist the dist to get the timing degrees back up to where it was with the can operational. NAPA has a rotor (# MO-3000 iirc) that has a .060" longer blade that helps with phasing ($8.xx out the door iirc) cuz it is the total gap that is in question, the circumferential gap (that you see with the light) & the radial gap (longer rotor helps with that) & if the total gap makes the "required" voltage greater than the "available" voltage from your particular ign setup then it will misfire & the required voltage is greatest at WOT at which point the can is non op (but this is a steady cruise from 2500 on up correct?. Short version: circumferential & radial distance is too great/reluctor gap issue (but we covered that potential)/too much timing (way less likely) but do get a number/cracked cap/bad plug wire(s).

a tip on drilling the inspection hole. i use a step drill. cuts a nice hole and doesn,t leave a raggedy outside edge on the cap.

Ive checked timing with and without vacuum advance at 2500 .
With vacuum where it starts missfiring is 42-43 degrees.
Without vacuum it’s 32-33 degrees.
The rotor I am running does have a longer blade.
All of the electronic vacuum advance firecore distributors come with a longer blade rotor.
The length of the blade on the firecore rotor is the same length as the napa M03000 rotor.
I have a napa M03000 rotor in my tool box and I have compared it to the rotornthat cane with my firecore distributor they are the same length on the rotor blade.
Awhile back I had done a gear swap on the 8 3/4 that I put in that truck, went from 3.55s to 4.56s and I never got around to re-gearing the front Dana 44 axle to match.
So now the truck is at a shop waiting to have the front Dana 44 axle geared to match the rear axle.
We have over a foot of snow on the ground this morning otherwise I would have ran over and grabbed it today to do a couple more quick tests on the issue and then brought it back.
But without 4wd I’m not sure I could even drive it back to the house from the shop.

And with the timing giving 32-33 degrees at 2500 RPM with the hose capped, it is NOT missing? EDIT If NOT missing, can you plug the can in & twist the dist CW to get the total back down to 32/33.

Yes once I get the truck back from the shop I will perform all the tests you are suggesting.
I will verify where timing is for a second time with vacuum advance connected at 2500 where the missing occurs.
Then I’ll plug the vacuum advance and verify that the missfiring is gone when the vacuum is capped off.
Then I’ll bring distributor timing only up to the same amount of timing when vacuum advance is connected and it’s missfiring and bring rpms up and see if it misses without vacuum advance but at the same timing setting at 2500.
Then I’ll turn the distributor down and plug vacuum advance back inn to get the timing around 32-33 degrees where it is without vacuum advance at 2500.
I’ll report back what I find.
My truck is at the shop waiting to get the front axle grated to 4.56s to match the rear axle.
It is near by but with over a foot of snow on the ground I don’t want to try and drive it with no 4wd, not to mention I don’t have a shop or garage at the moment.
I am moving into a new house at the beginning of January that does have a small garage.
So I’ll be able to mess around with it and not be out in the elements.
I’ve got a lot going on in the next week, we’re moving on the 29th and 30th of the month.
I’m starting a new job on January 2nd.
Just had to get Christmas out of the way with the wife and 3 kids.
And the truck is in the shop waiting to be re-geared up front.
And I still have to figure out this strange missfiring bull crap.
So it’s been pretty stressful to say the least.
Plus I had to give notice to my current job which I do like a lot but this new job opportunity pays 3 dollars more an hour with better benefits and paid holidays and more paid vacation.
So it’s just been a hectic month and I really haven’t had much time to mess with the truck and specially now since it’s sitting at a shop without 4wd.

You got a lot on your plate, If we cant get this dizzy straightened out I will blueprint a regular one (if it will work with your setup/I will read back thru all the posts to ans that Q) & send it to you.

I just need to get the truck back and get in the new garage and have some time to perform a couple more tests.
I’m going to start by bringing distributor timing only up to 42-43 degrees at 2500 and see if it’s still missing.
Then I’m going to retard the distributor timing down to around 22-23 degrees at 2500 and plug the vacuum advance back in to bring it up around 32-33 at 2500 and see what happens.
That should be able to pin point wether it’s soley a vacuum advance problem or if it’s a problem without vacuum advance.
Then I should have a better idea on what direction to go inn from there.
It’s just gonna take a week or two before I’m in the position to have the time to play around with it some more.
But I will report back on this thread after I perform the foloweing tests.

I’m not quite done moving, I got the truck back from the shop yesterday.
It is geared up front with 4.56s now so I have 4wd back.
Tonight I decided to turn the rev limiter all the way up to 9k rpm on both dials.
I pull the light out and check every cylinder with and without vacuum advance with engine refer to 2500 rpm.
Now the only cylinders that were showing a break in the light tonight are cylinder 6 and cylinder 8.
So I plugged the vacuum advance back in and checked cylinder 1 at 2500 and it’s not missing anymore like it was a couple of weeks ago.
The only change made was I turned up the rev limiter from 6k to 9k.
I don’t know why 6 and 8 are missing but I’m gonna bet money that the plugs in those two cylinders are gonna he fuel fouled or some what fuel fouled.
Or maybe just cylinder 8 will be and I was picking up the miss with the light from cylinder 8 while i was on cylinder 6 since the plug wires are so close together.
I’m going to pick up a couple new plugs tonight and play with it a little more in the morning.
I’m going to set the rev limiter at different settings and see what happens.
Since turning it up from 6 to 9 seemed to take away the missing from the other cylinders.
I’m still not 100 percent sure on what to think of this but I still have a gut feeling something is wrong with the crane ignition box.

I'm assuming the rev limiter is in the Crane box, otherwise it could be disconnected (we gotta add/run one this season). it is sure looking highly likely.

Yes the rev limiter is inside the box but I can dis-connect the 12v source to the rev limiter.
Which I may very well try today.
I’m going to try a few other things first and I’ll post the results.
I need to pull the plug in cylinder 8 since it seems to be the one missing the worst at the moment.
And put a new plug inn and start some more testing with and without vacuum advance.
And with the rev liimter set at different rpm’s and with it dis connected.

pjc360 i thought of you the other day. i thought my msd streetfire died on me. it was just a funny mistake though.
i haven't driven my truck in a month or so, i needed to move it, so i moved my coronet first, the battery ran down so i had to get jumper cables out. i was in a bit of a rush too.

got it moved then went to start my truck, turned the key in the ignition and i could tell it wasn't firing. checked some stuff and then gave up. later i checked resistance on the pickup coil in my dist, seemed fine. i was thinking i would just swap out the dist real quick. then i got back in my truck and turned the key, got one turn out of it and it fired off once and then nothing with the key.
then i remembered the ignition switch in my truck is bad and it wasn't supposed to be turning over with the key, there is a button to bypass it. ign. switch doesn't give the ign 12v in the start position, only in run.
LOL.
i just forgot how to start my own truck...

also, it's wayyyy cold here now and i still have the summer tune on the carb which is actually just a box stock quick fuel HR650 with the transition leaned out. it's just a bit lean in the summer but totally driveable after it gets about 5 mins of heat in the motor. with it in the low 30's to 20's right now it's misses real bad until it's fully warm. it's mostly in the transition circuit. because i have the jets at stock baseline tune.
i thought about bumping the IFR's up to .029 from .028 but i just gave the idle mix screws the smallest turn i could give them (setting is about half turn from closed) and it smoothed it out a bit, driving better now. will prob drill the .028 IFR's to .029 when it warms up.

So the only thing I was able to get accomplished today was I decided to pull cylinder 8’s plug and it was completely fuel fouled.
Looked like it had been sitting in a chimney to a fire place.
So I threw a new plug in cylinder 8 and left the rev limiter on the crane box turned all the way up to 9k rpm.
Took it for a drive and it’s no better, still missing real bad, and it’s even missing at a lower rpm now then where it normally started missing.
So I’m basically back at square one not knowing what it is causing this or what to do to figure it out.
Back to basically wanting to push this dam truck off a cliff and walking away from it.

Maybe your issue is that number 8's injector is leaking not that the ignition is the problem?

It’s carbureted not efi.
Floats are half way up sight glasses, idle feed restrictors are .031’s, idle air bleeds are .065’s.
Main jets are 65s and secondary jets are 75s, high speed air bleeds are .033’s.
Timing is 18 initial 32 total, 10 degrees of vacuum advance @ 20hg of vacuum.
I have played with the carburetor settings a million different times, I’ve tried smaller ifr’s bigger ifr’s smaller and bigger main jets, I’ve tried raising and lowering the floats.
Nothing works the missing is still there regaurdless of carburetor tube and settings.
I’m convinced there is a specific issue with cylinder 8 most likely a broken valve spring or the crane cd ignition box is causing the issues.
I’m just not sure which one.
Normally I’d be blaming the crane ignition box but the fact that cylinder 8s plug always seems to be struggling to fire compared to the other 7 cylinders has me thinking that there is a chance that cylinder 8 has a specific issue that’s isolated to just cylinder 8.
But then again I hooked up my timing light to all cylinders and brought the rpm up to 2500 and I could see every cylinder missing at 2500 rpm with my timing light.
Then I decide to increase the rev limiter from 6k rpm to 9k rpm and now I’m no longer seeing the missfire on every cylinder at 2500 rpm.
But I am still having an issue with cylinder 8s plug becoming fuel fouled.
I pulled the passenger side valve cover and looked at the valve springs on cylinder 8 and didn’t see anything un-usual with them or anything un-usual with the rockers.
So I’m just confused and frustrated cause I have no idea what’s going on with this thing.

Could it be oil fouled? & I'm assuming the valve stem seal is looking good (pliable not stiff). might be a ring issue on that cyl