Could a weak battery or one that is going south be the cause of a rough idle or miss at a hot idle? Everything is smooth with the idle, revs nicely and idles great up until the temp reaches 180 deg and then it wants to idle down really rough to the point of a miss and surge situation.
Engine is a 451, solid cam 254/.520, MP electronic ign, MP distributor, chrome box, 800 Eddy carb, 20 deg initial timing. It will idle at 1000/1100 rpm when cold but as the temp comes up it will idle down to 600 or so. It runs really good except at idle. I'll have the battery tested here in the next day or so but just looking to see if this may be cause?

Possible? Yes. But the simple determinant is to measure the volts at the batt. Got 13.5 +?
While engine running.
If the batt is old and likely on its way out....replace it.

It could if it was drawing your system voltage down, like PacNorth says, what's the voltage?

I cant find my meter, so I'll take it out and have it tested.

That's not going to tell you the whole picture. If you get 12.5 give or take on the batt, great.

But the real story is the volts, while the engine is running rough.

OK, I see. If I cant put my hands on the one I have I'll get another one and test it running.

I'm thinking elsewhere. I would start by removing the idle mixture screws & shooting a 2 second blast of brake kleen in the (2) ports with the thin red straw then reinstall the screws. turn them in to lightly seated before removal & count the turns to easily return them to their original positions when done & they might need richening. carb might need cleaning. Keep us posted.

I went through the carb and rebuilt everything, it was spotless inside when I opened it. The screws are 1.5 turns out and have always stayed right I have them set.

this just happened correct? Something changed (dirt or vac leak I'm guessing). I would still blast out the idle mixture ports (easy to do & not invasive)

Its been in this situation for a while now but I noticed it after it had sat for an extended time. The car can sit for months before I drive it. I can try a blast of cleaner in the screw holes but it was doing the same thing before I rebuilt the carb.

Bad gas? The new stuff goes bad a lot quicker.

The gas here is pretty bad but I do put a few gallons of fresh gas in it when I get it out. I have ran it enough to use any old stuff that was in there in there and replace it with fresh gas and the problem is still there. The part I don't understand is why the rough idle at operating temp.

Low voltage to a Orange box will eventually kill it. It's struggling to draw amperage from a weak system. As it starts to go south the motor may start backfiring. Put a good battery in the car and change out the box.

Think it runs bad, try running an MSD with a bad battery.

I hope it hasn't started to wipe out the box. I do have a spare orange box that I ran for years with no issues. I just found the sticker on the battery and its much older than I thought, April 2010! I really don't know how it has lasted this long. Batteries here are good for about 2 years tops. Needless to say I will be getting a new battery.

Do you run a choke? Maybe it masks a vac leak. Check ground for your ECU, but likely that would just kill it and not run rough. While at it check to see if VR is getting a good ground too.

I would say that your box is toast. Doesn't take much for them to go. I've seen them loose their ground and pop. If you need a spare I have several laying around.

My experience has been that the Mopar electronic will just quit, not cause stumbling and crap at op temps. I think that this has to be fuel related.

Check fuel pump pressure.

Doubt vapor lock, but if fuel line is in contact with any hot engine components, all bets are off.

Fuel filter ok?

Is engine running on fast idle cam when first start because of a closed choke, choke opens engine off fast idle cam on regular idle screw setting which may be to slow?

Bad coil failing as it warms up?

Cracked or otherwise damaged distributor cap or rotor.

What does a vacuum gauge read at idle.

Do you have another carb to throw on it for testing?


Engines normally will run perfectly even without a battery if charging system is in order, doubt if it is failing battery.

Engine ground sound?

Run a jumper wire from battery positive to coil positive to rule out any ignition wiring issues.

Does engine have a ballast resistor in good condition.

Coil wire to dizzy sound?

Coil wires nice and snug?

After I replace the battery Ill swap in the old orange box and go for a drive.

I have a bracket I bought from Hergfest that bolts to the firewall. I have the ECU bolted to the bracket. I also run a separate ground from the box. The bracket also allows for an airgap between the the box and the firewall.

can you get it to idle any better by adjusting the idle mix after it is warmed up?

Have not checked fuel pressure. Pump is a Carter mechanical street pump. It will pull hard to 7K so I doubt fuel pressure is a problem but I can still check it.

Fuel line is clear of hot stuff and I run a spacer under the carb. I have avoided going to an electric pump but this maybe the year I change it.

New fuel filter

I gutted the choke housing, no choke mechanism.

3 year old MSD Blaster, mounted on the intake. I asked here before if it could be getting too hot but the consensus was no.

New dist. cap and rotor, reluctor gap set correctly

11" of vacuum by a gauge at idle

I do have an old AFB I could throw on there

I can try an new ballast resistor

Coil wires are tight and correct

Engine grounds are factory located and tight

I've tried it @ 2 and 2.5 turns out and no change to speak of.

Interesting, I'll bet standoffs would be and option too to get some airflow around the backside.

I have run many years with a mechanical fuel pump in extreme heat with no issues...

An electric pump is not necessary...


Can you say with certainty that you are not experiencing percolation???

What type of fuel do you run???


Pump 'fuel' ain't gonna get it...

Especially where you live...

It very well could be vapor locking at higher temps especially in the summer. We have crap for gas 91 alcohol infused octane is as good as it gets. I know guys will say to put it under pressure to keep the boiling point higher which is why I have entertained going electric in the past.

In a word-race fuel...


You've got to address temperature if yer gonna stay mechanical...

Together with proper supply and return...


I've been running a 'mix' going way back...

Make one change at a time. Replace battery and go from there.

Definitely battery first. The voltage regulator hasn't been replaced since I've owned the car (17 yrs), but it was replaced during the restoration so its not 40 years old either.

I would suggest plumbing in a gas can with several gallons of race gas or other appropriate fuel to the mech pump inlet & see if that cures it. that would confirm or elim percolation.

Interesting to see what the issue will be.
I ran a 160 degree thermostat in my 451 engine. Sounds more like a fuel level/vapor problem.
I can say that I have had problems with the Mopar Electronic ignitions, but not as you are describing. I have CD ignition systems (MSD, Crane) on my cars. The multi-spark at low RPM helps clean up the idle.

Can a bad battery cause a rough idle? NO, as long as the alternator is working.

As soon as the engine starts running the bulk of the loads are handled by the alternator. It's only when the alternator can't make the required amperage that the battery is called upon.

Get a digital voltmeter and measure across the battery terminals before starting (~12.7V) and then start the engine measure again. It should be between 13.5 AND 14.5 VDC.
Now switch the voltmeter to read AC voltage. Check across the terminals again. The voltage you may measure is called ripple and if it's more than, say, 0.2VAC you have a diode either going out or dead.

If turning the idle mixture screws from 1.5 turns to 2.5 turns out from lightly seated does not have an effect on the idle quality, then the engine is not running on the idle circuit or you have an air leak that is causing the power circuit to open, something like that. You could probably read a writeup or two about setting lean best idle.

Good Luck!

R.

WHat is your fuel tank situation? Ever been cleaned? I was dumb enough to rebuild my carb without checking tank and guess what. It got clogged again. Go figure!

The tank was new when I bought the car. I have put less than 5,000 on it since 1999.

I have the same ignition as you. I can remove the battery and it will keep running fine. I would think that it would need to be both the battery and alternator issue to drop the voltage low enough for the problem.

Add one more thing to look for - engine off - remove the metering rods caps (don't drop the screws in the engine) - start it up and see if the metering rods are bouncing up and down or are they sucked down and stay there or are they up and stay there. If they are bouncing around - hold them down and see if the idle smoothed out. If they stay up - push them down gently until they seat and see if the idle smoothed out while warm. Your comment about surging after warmed up - sounds like they rods are bouncing up and down. The other thing to do is watch the timing when this surging is happening - if vacuum advance equipped - could be contributing to this - and if you have one light and one missing centrifugal advance spring that could as well.

After playing around with springs I ended up going back to the silver springs out of the strip kit. They were staying put when I checked them. The idle is OK in Park or N after it gets warm but in Drive is when it wants to idle down and surge and act like its missing. The vacuum advance is not hooked up. Both springs are connected in the dist.

sure sounds lean. Keep after it, we'll work till we fix it.

i had something very similar happen to me
if there is enough juice in the batt to start it up
that is your highest demand out of it so it is most likely ok.
heat and cold will effect the diodes when they are weak
try changing out the alt with a known good one

Did you check the rods when warm in gear or just in neutral and park. If this unit drives down the road when warm and doesn't miss under load how can the spark be suspect - under load down the road you place more demand on the ignition. Something is not adding up.

I'll check the numbers on the battery using this test and see what it shows. The in gear idle is fine until it gets to 180 and then the problems set in. I'll check the position of the rods after its get warm in gear too. It will be Saturday now before I can get to it but I will keep you all posted with what I find.

i'm really interested to hear the outcome
my loot is on the alt giving you the trouble
i just sort of went thru this problem
with a little different set of circumstances.

when the car was started in the morning below 30 degrees it would run like crap with a real bad idle until it warmed up, the lights were also dimming.
once warm, it would run great for the rest of the day.(100 plus miles a day)
after some head scratching and thinking of many reasons like
computer,grounds,coil,bad wiring harness, ect..
for some reason i had a extra alt on the shelf and my gut said what the hay to change it out..
and guess what?
it hasn't happened since..

I stopped at Costco for a battery and they didn't have any group 27s which is what I am finding is the correct battery for the car. Could a 24 be used in place of the 27 without issue?

i believe it will work,but it might not last as long..

If I remember correctly the 24 is simply shorter than a 27 so you will have extra room on the long side of the battery box, post are in the correct position. Typically back in the day a 24 was a Chevy battery, 24 F was a Ford (swapped post position) and 27 was for Chryslers if memory serves.

Ford used the 27F and the 24F, Mopar the 27 and 24.
Top post GM batteries were 22F, 24 and 24T(tall 24), AKA Delco Y55, R59, R69.
The 27 isn't hard to find, it's the 27F that's not as common.

I've been using a 27F as a replacement for the 31 that came in my tractor. Last week, the Walmart 27F that I had in it died, so I went to get another. Neither Waldoworld or Sams Club had a 27F, only 27's. My cables are just long enough I could get by with a 27 so I bought a Duracell 27 from Sams. Cost was $101+tax.
If I couldn't have been able to make the 27 work, I was going to use the Duracell 24F. The reason I was going to try the smaller 24 is the Duracell 24 has a CCA rating higher than many 27's of other brands.

Go with a good group 24 and forget the heavier 27, it will start a 451, especially if you have a modern starter.

If you think all Walmart or other box store batteries are inferior, I've used Walmarts batteries for 25 years. Most last 50% longer than the warranty period. The best one I had was in my '68 V200. Bought it in 2001, it died last summer, 15 year life span.

I picked up a 27 tonight @ O'Reilly's. Its slightly larger but not much and has another 170 cranking amps over a 24. I had the old one tested and it was bad, no surprise there. Going to tackle it tomorrow and do the test for the alternator and swap the boxes and see what happens. I'll send out the updates.

all the wallyworld battery's i have bought over the years were made by johnson control. they have lasted way past the warantee as was stated. a couple things worth mentioning though, in my opinion. get the biggest CCA and reserve cranking amps you can in the size you need regardless of what brand and place you purchase from, and go through the rack and pick out the newest manufacture date that is stamped [usually on a separate sticker, but not always] on the battery you choose.

OK, so I put the new battery in, the diode test shows the alternator is fine. I tried both a chrome box and orange box, same idle conditions using both boxes. I got the best idle highest vacuum with the mixture screws out 2 turns. The idle is better but not great still. I didn't have a helper today but I opened the rod covers and for the most part the rods stay down and don't bounce while in Park but when the idle takes a dip and idles down they bounce a little. I can rev it a little and they instantly come up as you would expect. I stretched the springs a little to make them longer and it did help some by doing that but I feel the idle could be more consistent. What do you all think now?

I would blast the idle mixture ports with starting fluid & retry it then I would block the metering rods up with something under the springs & retry it.

I gave it a blast of carb cleaner. So you are saying block the rods from coming up, thus keeping them in the down position with no chance for upward movement?

No, block them up so it is on the power circuit. EDIT When working in that area I take off the AP rod to keep from kicking raw shots of fuel into the intake while I am manipulating the throttle, not a dealbreaker just my OCD talking. its gonna be interesting to see what it ends up being.

OK, I see.

You sprayed into both of the idle mixture ports with the thin red straw?

Yes. I'm not sure I follow you on what taking it off the idle circuit by raising the rods is going to tell us. My other thought is to bump the timing by 2 deg and see what that does. If it doesn't detonate I don't see what it will hurt. I guess its a chicken or egg situation. Is it idling down because the rods are bouncing or are the rods bouncing because the idle drops?

I was thinking that it might be lean cuz of a partial blockage and or vac leak & being on the power circuit might temporarily straighten it out if that be the case.. bumping the timing wouldn't hurt either & I would go more than 2 deg.

The rods are fairly consistent with little to no movement when in Park or neutral or but will bounce when in Drive with no chance of staying put at all. I have not bumped the timing yet so that will be next. I raised the rods and held them up with no change in he idle quality. I am going with the assumption that in no way is it lean at idle. Just got back from a 45 mile drive and the car runs absolutely great other than the idle. No flat spots or laying down in any gear at all past 6500.

Unless I missed a prior post from you - no mention of timing fluctuations when in gear, and no mention of vacuum fluctuations when in gear. Hold the rods down when hot and idling in gear and see if things smooth out. ( take the springs out for this test ). Rods up = more fuel ( you don't want that at idle) - you state it runs down the road great with no surging or misfire that means you should be close on your mixture while driving - at idling the surge could be the rods popping up and or the timing fluctuations from the change in engine speed. Putting more timing in it may help too as long as you don't get detonation.

what is your initial? you might bump it up to 18 or so & see how it does.(I run 18 on a dead stock 85 318). I would blast out the carb idle ports. we're getting closer to a fix.

I gave another blast of carb cleaner again in the idle ports. The timing is now at 20 so there isn't much room left if any to raise it so I thought I would try it @ 22 and see if it detonates. I've seen different people on here go to 24 even with larger cams but I'll see how it responds. Its a 10.5 to 1 alum head motor with an ST-21 solid cam, 254 duration. Everything else with the way it drives is great. I'll try it again with the springs out when I check the timing. It will be Wed before I look it all again. At this point I'm at a loss if the raising the timing doesn't help.

if you are at 20 that is in the ballpark & plenty high for now, I didn't know it was already that high.

Here's a comment on a reply in this thread.

It is not recommended to run the car with the battery removed.

The sensing circuit will tell the voltage regulator to kick up to the max, say 19 - 20 volts. Any light or electronic gear on the car may fry due to overvoltage.

I once fixed a car that had been on a railcar and had to be driven off, but the battery was dead. They borrowed a battery from a passing taxi (not in the US) and used it to fire up the car. They turned on the lights, got it off the railcar, and then with engine running pulled the battery and put it back in the taxi. As they were removing the battery, they noticed all the lights burning out. The alternator also failed, developed a short in the windings. So we got to replace all the bulbs and the alternator on a brand new car.

Profit from my misadventure, don't do it.

R.

and for those guys who like to check the charging system on our old heaps with the 35 or 45 amp alternators by removing one or the other battery cables, take heed of the above. same thing can happen, especially when you re-install the removed cable and the charging system surges. as said : "don't do it". use the proper meter[s] to diagnose the problem the correct way.

Definitely on the right track with carb and fuel system. Try to get it to idle in gear even if it's at 1,100 or above. My suspicion is that the carb is losing it's vacuum signal at the lower rpm. The bouncing rods say to me that the carb is sensing an open throttle do to the low vacuum signal and trying to go into the power circuit. My roller cam 400 does the same thing if the idle falls below about 1,000 in gear. With that cam and heads you might be better served with a Holley.

I hear you on going to a Holley and will make a few more adjustments and see if I can get the Edelbrock to work with what I have. I looked at some 850's on the Summit site earlier today. I'm not ready to go that route just yet but will make the change if necessary.

No matter what it usually turns out to be something simple

That it the truth for sure and is what keeps me from throwing parts at it until I rule out some more things. Timing, vacuum leak, carb tuning, etc. I feel I am closer than I was a week ago.

Really interesting problem.

I'm inclined to also think you're running lean, or on the ragged edge at idle. (hate those step up rods) I've had problems with throttle shafts leaking, and screwing up idle quality. Other than that, you're obviously competent enough to know if the base of the carburetor is warped, or the gasket not sealing well. If you spray carb cleaner around the base of the carb while idling does anything happen?

The air/fuel mixture density in the combustion chamber will be quite low compared to when the throttle is opened appreciably. That type of mixture tends to burn more slowly, and in my experience was a primary reason for the vacuum advance, which you say you don't have connected.

I know I struggled with a warmed over 331 SBC years ago that had a Mallory dual point in it, with no vacuum advance. Played with springs, weights, on and on, and things really only got better when I installed a distributor with a vacuum advance. It really cleaned up with idle/off idle characteristics of the engine.

Update: I blocked off the PCV port with a vacuum plug, no change to speak of doing that. Started playing with step up springs from the strip kit. I went down to orange spring and it cleaned up the idle a nice little bit. I tried to yellow, the next lowest rated spring and the idle got a little worse from the orange. I went back to the orange spring and idle is back to be better than it was but I still feel there is some tuning left in it. I raised the idle speed and a little better yet even. I left the timing at 20 deg. I cant say for sure how the part throttle cruise is or more performance spirited driving is with the spring change but I'll exercise it on Saturday and report back. Overall, I feel progress was made.

Peabody: I have sprayed around the carb base and is well sealed. When I rebuilt the carb a replaced the thick spacer gasket.

We're getting closer!

Yes, slowly but surely. I appreciate the help from everyone.

I'm back to addressing the situation and have tried every step up spring in the kit, set the timing from 15 to 21, vacuum advance hooked up and I still have basically the same problem as before. I near the end of my patience here. It runs great everywhere else except the idle.

[/quote] I near the end of my patience here [quote] At this point just me I would (1) plumb a can of race gas or pump gas with no ethanol to the pump inlet & if no change then (2) borrow another carb.

I guess I'm not understanding how race gas can help but I can look into getting a few gallons. Can you explain how it can help? Thanks Robert.

the only reason I suggested race gas is that alot of areas of the US of A do not have the option of non ethanol gas at the pumps so that test would elim or confirm that the problem is the ethanol & it is infamous for causing idle problems, however obviously if you have non ethanol in there right now then we can bypass that test.

No its 10% ethanol 91 oct. That is the highest octane and only pump option for this area unfortunately. Would avgas be a reasonable thing to try?

I'm just curious too if I would run into this same situation with a 4150 Holley, like an 850 mech secondary carb.

No matter what I have done the metering rods wont stop bouncing. If I could get that corrected I would have progress I feel like. A previous post on this thread mentioned the vacuum signal being the cause and the carb is "confused" as to whether its on the idle or main circuit.

(1) I believe so. it does have differenct characteristics than race gas I'm sure but we'd at least have the gasahol out of there for a test (2) if the ethanol is the cause then likely yes, it'd act pretty much the same with another carb, the heat vaporizes it easier (not a problem in modern FI engines but problematic for us). if non ethanol does not fix it then I would borrow/try another carburetor as it might be strickly carb related/not fuel related. However when E10 was intro'd here in the 80's, cars dieing at stoplights from it were epidemic. EDIT on your 2nd post yes they should not be bouncing, can we block them all the way down (on cruise) for a test

Read up on vacuum gauge readings, might help point you.

http://www.secondchancegarage.com/public/186.cfm

I have used a vacuum gauge when setting idle mixture screws. I can 10 out of it which is as good as it gets. It does stay at steady 10 in Park at idle of 1100 RPM. In gear, the RPM will drop to around 850 and then dance around a little from there.

I haven't read this all but will throw out there you may need to drill small holes in the primaries to get it down on the idle circuit. I wouldn't be surprised at 1100 if you were on the main circuit.

Perhaps I have missed it, where is our initial ignition timing? Are throttle butterflies at idle so far open to cause you carb to be running in transition circuit instead of idle circuit. A simple test is to adjust idle mixture screws,,,,does this cause significant difference in idle speed? Crank initial timing up to say 15 to 20 degrees which will likely increase idle speed, then adjust speed back to normal using carb idle speed adjuster, with screw idle mixture screws out from all the way in, out say to 1.5 turns...Any improvement?

Have you tried earlier recommendation to bypass ignition wiring by jumping directly to coil from battery.

Your idle vacuum reading is extremely low unless some crazy camshaft. ,,are you certain of no major vacuum leaks,,,,power brake booster,,,intake to head, cracks in intake manifold. I've even run a garden hose on low pressure on a running engine and been surprised by what I have found.

Any bad cylinders,,,pull plug wires one at a time with engine idling best it can with vacuum gauge connected. Any cylinders with zero change in vacuum reading?

Carb float levels ok?

Initial is at 20. Its a 254 @ 50 solid. Idle screws are 1.5 turns out. That is where is get best vacuum #'s and as steady of an idle as it will get.

I did not do the jump to the coil test. The brake booster is ran of an external pump. No card base leaks or intake leaks using the brake cleaner spray test. No dead cylinders, it runs way too good to have a dead hole. Float levels have been raised to 7/16". Like I said it pulls hard and clean to redline.

The first thing I would have done was throw that POS carb in the trash and put a good Holly on it. I like the ones with the down leg boosters instead of the straight leg boosters.

Meter voltage at plus side of coil when engine is cold, and again when warm. Should be at 12 volts unless you have a ballast resistor,,,about 9 volts then. Does it change between hot and cold, if not I would not suspect battery. Might suspect ballast if you have one,,,or other ignition components, wiring, connectors etc.,,,,hence recommendation to jump from battery plus to coil plus just to eliminate any possibility here. A defective coil could be at fault if begins to fail due to internal heat or as engine warms.

I might measure again when in park and when in drive. Change? If voltage decreases when in gear,,,could be battery if slowed down idle decreases a marginal alternator's output sufficiently.

Can we assume that non idle performance is as you expect,earlier post is a bit murky on this point. If so, I would lean more to a carb problem. Again ask where are butterflies in relation to transition circuit? 2 1/2 turns out on carb idle mixture screws sound excessive,,,what happens when screwed further in,,,,really close in or closed engine should die. If not,,,I am very curious as to butterfly position,,indicating if engine not running on idle circuit of carb at idle but on cruise circuit.

When running engine from fresh cold start and idling at what you assert of is about 1,200 rpm, I assume that you are not running on fast idle cam of carb. If not can you manually force carb butterflies toward closing. Spray a little wd 40 at carb and cable linkage to ensure no friction that may disappear as engine warms. I would likely disconnect accelerator linkage to eliminate this from equation.

I do have a ballast resistor. I even replaced with a spare as it was getting a few years on it. I'll do that test here in a day or two and see what I find. I'll try a spare coil on there too, currently using an MSD blaster.

I'm at 1.5 turns out on the mixture screws. Not running on fast idle cam, I gutted the choke mechanism quite a while back. The linkage is free and the throttle shafts are also good. My guess is that its right on the edge of idle/main circuit given the idle rpm. Again, in gear it fluctuates around 800 or so. The battery is new and the alternator tested fine also.

Has car ever run well at idle with current engine/torque converter configuration? If so, when did it cease to do so?

Have you changed to another stall speed convertor? You have indicated that idle is fine in park or neutral, but falls way back when transmission is placed into gear. Any chance that torque converter is too tight for your engine cam/carb/etc configuration? Perhaps forcing you to adjust idle speed higher, thereby placing butterflies at idle past transition zone into cruise where your mixture screws have little consequence.

I had seen earlier post where your mixture screws were more than 2 turns out.

I would

The convertor is a 9.5 PTC that was spec'd for car, cam, gear, etc. It has never had a good idle to be honest. Its been in there as long as I have had this motor combo together. At that time I did have the screws out 2 turns but have since went back to the 1.5 setting. I'm going to see where I can get some race gas and try that next along with electrical checks. Thanks, I appreciate the help.