Greetings All,

I have been going simultaneously broke and insane trying to diagnose a misfire between 3000-3500 RPM on my 440-3. The 440 is powering a restored '76 Winnebago Brave ("Pandora" - photo atached) which had a complete engine rebuild last year/~12K miles ago. At that time the stock carb and ignition were replaced with the aftermarket components listed below (MSD, Edelbrock etc). It all ran brilliantly until a few months ago when I began to notice this misfire. Ever since I have been in diagnosis hell. I have obtained about as much info and assistance as the engine builder and the MSD techs can provide, and spent countless hours over the past month trying out different theories, settings, and configurations by trial and error. Most recently MSD sent me a new dizzy under warranty, thinking that was the culprit, but I installed it last week and the misfire still persists.

More on the condition: It is a smooth, almost rev-limiter feeling misfire that occurs only under a load between ~3000-3500 RPM. It isn't constant or exact, as sometime the misfire won't begin until ~3500, while other times I can sense it as low as ~3000. The engine starts and idles perfectly, and pulls smooth and strong up to the point of misfire.

Here are the relevant engine and component specs:

440-3 w/ 12K low-RPM motorhome miles since stock rebuild with rv cam grind.
Edelbrock #1411 750cfm (calibrated up to #1407 750cfm specs.. I am 99.9% sure carb/fuel issue/s have nothing to do with this after trying several different compensatory carb configs/adjustments...larger mains/secs/needles/seats, stronger step-up springs, etc. No difference)
Edelbrock Performer 440 intake
Hedman Elite Headers/Flowmaster 50
Accel 0576S shorty plugs @ .040 (same result with fresh Champion RV12YC @ .035)
MSD PB/RTR Dizzy #8387 (Second one. 99.9% sure it's not the dizzy)
MSD 8.5mm wires w/ 90 deg boots (om out perfectly @~40-50/ft. This is the second new diagnostic set… it is not the wires or boots)
MSD Blaster 2 #8202 (second one... threw still more money at the problem and now I have a spare... it isn't the coil)
MSD 6AL (MOPAR version #P4876728) no rev pill. The same condition happens witout the 6AL box, so it isn't the 6AL box or wiring: all of MSD's rules for power, ground, and EMI separation have been followed religiously.

I sum, I have no idea where to go from here. I'm thinking perhaps weak or broken valve spring/s or a cam issue or timing chain/assy/gears, but I have no reason to suspect these beyond lack of any other possible explanation. Based on the above description of the misfire, can we rule any of these in or out? If it is something mechanical like springs I am going to need genuine Mopar guru assistance. I am in Southern California (Inland Empire) if anyone has any references/leads.

TIA for any responses!

Mosin

Sounds like a vaccum leak,possibly intake gasket leak. Try running an unlit propane torch around the intake to head mating surface. Are the power brakes working normally? Above 3500rpm or under heavy acceleration the miss is not notacable?

Had a similar situation a few years back with a 440 in a motorhome, after isolating it to one cylinder found that while a cranking compression test showed the cylinder within 10% of the others a leakdown test showed 30% leakage on that cylinder throught the exhaust valve... pulled that head & had it repaired by the machine shop...fired it up & the engine ran great... for another 4k miles than it started losing the same cylinder, leakage tested at around 18%, pulled the head again & I was there when the machist tried touching up the seat.. it had a low area, that once the seat was cut sealed well by when inspected with a magnifing glass looked "odd/off" found that the factory induction hardened valve seat apparently was very thin & when it was cut left an area of soft cast iron exposed...Installed a hardened seat & it's been fine ever since...

BTW click on the flashing envelope at the top of your screen...

I guess my reading comp tonight is of a 1st grader.LOL Lots of things could be the problem,but the vaccum leak I'd doubt now that I read more carefully. It very well could be valvetrain related,weak or broken valvespring would be possible. You could drive it at the missfire point for several miles and pull the plugs checking them to find the cylinder with the problem.Leakdown testing might also reveal the troubled cylinder.Using a propane torch to locate a vaccum leak will cause the idle speed to pick up rpm as the leak is passed over,no explosions needed.

Long shot-

Looks like #5 and #7 plug wires are close or touching right as they leave the dizzy.

The engine might be able to handle driving under that misfire for miles, but my nerves can't. Sparing everyone the brutal story of the original engine blowing a rear main on the freeway last year after ~$3K of investment to smog it and make it roadworthy... I am totally paranoid about damaging this rebuild. I think a propane test is next, and if that doesn't yield anything I need to find a Mopar specialist to assist with a comp and leakdown test and any subsequent repairs. I have invested so much time and $ into attempting to diagnose this that at this rate Pandora will be a glorified paperweight all summer unless I get a pro involved. Of course, at that point I just might have enough doubles/spares on the shelf to build another one!

FWIW on the one I dealt with the miss started intermitantly at 3-3500 early on, we kept testing it, feeling the miss but it was hard to pinpoint, but the guy I built it for spent a whole summer chasing Nascar Cup races around the country & racking up miles... As the miles added up the miss got worse cause the soft portion of the valve seat sank increasing the miss.. Obviously after we found it the first time the second time around we knew right where to look.. The engine was a 78 440 & originally the valve seats were induction hardened... Well apparently they figured out just how little induction hardening they could get away with...

If your issue is the same driving it some more won't hurt it..

If I was experiencing that same condition (a soft or otherwise defective seat) the leakdown test would show it, but a visual inspection of the springs would not, correct? And the remedy would be pulling the head to have a new, hardened seat installed (?). Sorry but I am essentially ignorant to what is really going on under those stylish valve covers.

I can keep it under 3000 RPM to shuttle it back and forth from diagnosis/repair without breaking into a sweat, but it is essentially worthless right now for the trips we take which involve mountain passes and long straightaways. To paraphrase the 20th c. philosopher S. Hagar, I can not drive 55 on Hwy 5!

Just a wild shot here, but I have seen camshafts with a wiped lobe create a symptom similar to what you describe. Perhaps the others here can elaborate on whether they agree if this could be the culprit.

To test for this, a dial indicator (on a magnetic base) would be used to check the lift of each valve while rotating the engine by hand.

Good luck. Cool retro motorhome, BTW! You see 45' Prevosts every day, but not 70's Winnebagos!


Jim

In this order I would #1 advance the initial timing 5-10 degrees from where it is now & take her out then retard it 5-10 degrees from where it was originally & try it(rotor phasing). #2 I would hook a can of gas to the pump inlet & take it out.(fuel starvation) #3 I would do a compression and especially a leakdown test(as mentioned). and what is your initial timing at?

If it never recovers over 3500, and continues it's struggle, I would also suspect fuel issue possibly. (lack of it)

hi
the 750 edel carbs are knowen to have some issues,might try a holley .

week fuel psi under load install a fuell psi gauge and drive 4lbs minium under load 7 psi max at idle .

week valvespring ?

bad cam lobe , remove valvecovers and watch for i valve not moving as much as others at idle, of its kind intakes should all move app same amount also all ex same amount ?

partialy cloged fuell filter , restricting flow at speed ?

bent pushrod ?


bad fuel pump pushrod , there are some defective ones out there wearing short early .

fuel pump cam lobe woren off ?

hi
750 edes have problems sometimes try a holley ?


badd ignition switch , bypass it and test ?

fuel filter partialy cloged restricting flow ,
install a psi gauge ,and drive 4lb min 7 lb max !

bad cam lobe , remove valve covers start and watch for movement on all similar distance ?

bent pushrod ?
weak valvespring ?

badd fuelpump pushrod ,woren off there are some soft ones out there ?

bad fuel pump cam lobe ?

Did they install a NEW oil pump drive or re-use it on a new bushing? What about the fuel pump push rod? I've had bad distributor drives do that at different RPMS. Pull the distributor and using a large flat blade screwdriver, try and wiggle the oil pump drive side to side and back and forth while it's installed. If it moves at all, replace it and the bushing.

Hi..

I doubt seriously its the routing on the plug wires. I can lterally wrap my MSD wires together and no miss at 7500rpm in my race car.

Cant see it being mechanical, a valve spring doesnt know if its undfer load or not, all it knows is RPM,right?..If you can rev it in neutral, and no misfire i would think its got to be spark or fuel related.

my first thought would have been the 6al, but you SEEM to have eliminated that.

My second thought would be fuel pump,filter,or some sort of undervoltage issue

I can't thank you guys enough for the responses.


Prioritizing from the simplest to the most complex, I am formulating my next offensive in Operation Mopar Misfire.

Re. FUEL: Many great points above that got me thinking about the system from tank to intake. I have noticed that after the engine warms up the fuel press drops from ~6.5 to around 4 or fluctuates therein, but I read in a few places that this was a common effect of the gauge heating up. (?) I will run from a gas can to eliminate the tanks and dual-tank switch (it's original, and might be only opening/switching partially?), bypass my Holley press regulator, and flow straight to the carb.

And speaking of the carb, I neglected to mention the carb history, since I didn't think it was relevant before. In short, I drove the first ~11K miles on this engine with an Eddy AVS 650, which gave great gas mileage, but always produced a lean stumble off idle. When I swapped the AVS 650 (#1806) out for the Performer 750 (#1411) the condition improved somewhat, but the stumble was still noticeable. Then I saw that the #1411 is jetted way lean compared to the non-electric choke #1407, so I matched the 1407 main rods/jets, went up to .110 secondaries (from .107) and the thing ran like a bat out of hell. I don't remember any 3000-3500 misfires immediately after making this swap and adjustments, but they definitely began soon after. So, if the steps to eliminate fuel pressure/flow to the carb don't work, I am going to throw on the AVS 650 again and see if that changes the condition.

I will also check the dist. drive gear, and do a propane test around the intake. I will do this all sequentially (probably these two first, then the fuel/carb) and take good notes. I might even strap a vid cam to the passenger seat so I don't crash watching the fuel press gauge.

If those steps don't produce any results, I will go back to my #1411/750cfm and ponder the tools and time I will need to learn how to do comp and leakdown tests OR locate a Mopar specialist in SoCal that is willing to lay hands on Pandora. Does anyone know of such a shop/individual?

Many thanks again!