Misfire at cruise, popping/afterfire at 3000RPM

[AJ/FormS]

I would say both. It’ll rev to 3400 or so, start misfiring and hold that RPM, and feels like it’s losing power slightly.

Have you put a timing lite on it? Check the strobe action. Make sure the advancing is smooth on the balancer with NO missed strobes, No jumping back and forth of the marks, and No skipped strobes. If any of this happens, and you have a magnetic pick-up in the distributor, then the polarity of it is reversed. Flip the wires, reset your timing, verify you strobe-action is now correct, and roadtest it again.

 

[Righty Tighty]

Exhaust valve is open when that happens.
Does it mean...
A. the valve isn't closing
B.The spring pressures arent right, again, leading to A.
Again what camshaft and what spring part number and what pressures did they tell you they set them at?

If it was loading up on fuel it would eventually blow the Muffler case.. that wouldn't cut out like lean...it would blubber the tone would get real low

I have never known what cam I have. Previous owner didn't have any documentation on the engine build, so when I sent the heads off to the machine shop, I measured lift. It was something like .447 on both intake and exhaust -- it's written down somewhere -- so they did their best to match the springs to that. I'll try to dig up the spring p/n, but the pressures they gave me were 120 seat and 285 open. I know that probably doesn't help without knowing the spring part numbers.

One thing I didn't do once the heads were back and installed was to perform another leakdown. I need to do that.

Have you put a timing lite on it? Check the strobe action. Make sure the advancing is smooth on the balancer with NO missed strobes, No jumping back and forth of the marks, and No skipped strobes. If any of this happens, and you have a magnetic pick-up in the distributor, then the polarity of it is reversed. Flip the wires, reset your timing, verify you strobe-action is now correct, and roadtest it again.

I know you've mentioned reversed polarity before, and I've tried to pay attention to the light while checking timing. The light seems pretty consistent until it gets to the higher RPM, and then it will skip around a little in position, maybe miss a strobe here or there.

My question is, how would polarity become reversed? New distributor, I've tried 2 additional harnesses for a total of 3. Are you referring to having the coil wired backwards? Initially I thought you were referring to the distributor pickup wires, but how would I end up with 3 harnesses and 2 distributors, all with incorrect polarity?

 

[MOPAROFFICIAL]

I have never known what cam I have. Previous owner didn't have any documentation on the engine build, so when I sent the heads off to the machine shop, I measured lift. It was something like .447 on both intake and exhaust -- it's written down somewhere -- so they did their best to match the springs to that. I'll try to dig up the spring p/n, but the pressures they gave me were 120 seat and 285 open. I know that probably doesn't help without knowing the spring part numbers.

One thing I didn't do once the heads were back and installed was to perform another leakdown. I need to do that.


I know you've mentioned reversed polarity before, and I've tried to pay attention to the light while checking timing. The light seems pretty consistent until it gets to the higher RPM, and then it will skip around a little in position, maybe miss a strobe here or there.

My question is, how would polarity become reversed? New distributor, I've tried 2 additional harnesses for a total of 3. Are you referring to having the coil wired backwards? Initially I thought you were referring to the distributor pickup wires, but how would I end up with 3 harnesses and 2 distributors, all with incorrect polarity?

Assuming HYD cam 'though those pressures will run about a .500 lift solid according to mopar" Are your pushrods arent too long?
Measure preload.
Check pushrod length. Do the math, check all 4 corners 1, 2, 7, 8
You can cheaply buy some rocker shaft shims from Mancini racing. Assuming the valvetrain isn't adjustable the idea is to raise the rocker arm off off the pushrod and give up some preload ..the effect is like shortening the pushrods. I believe the shim kit has .030 and or .025 varieties.
Or measure and if so.. buy new push rods.

IF that's even your problem.
Long distance diagnosis can be a crap shoot.

 

[Righty Tighty]

IF that's even your problem.

Yes, hydraulic cam.

Agreed, that's assuming that's even the problem. And quite honestly, if I end up digging into the engine again for troubleshooting, I'd probably just rebuild the damn thing. It really bugs me not knowing any of the specs of the build or what it looks like inside. And although I'm obviously not an expert engine builder, when I built my 318, I had great peace of mind knowing what went into it, because I did it myself.

 

[512Stroker]

Popping out the exhaust or intake?

Assuming you have dual exhaust, is it popping out both pipes?
Is it an H pipe exhuast system?
Which ECU are you running? Have you tried swapping out the ignition box?

 

[Righty Tighty]

Yes, dual x-pipe exhaust. It's hard to tell whether it comes out of both or one, but I will say it did this with open headers as well.

I'm running a light blue Mopar ECU from Halifax and when I bought it, I bought two from him. I've swapped this one out for the other, and the same problem. The stock coil that came with the car swapped for an MSD Blaster 2, same problem. New distributor, same problem.

For a while I was thinking it could be getting too much advance, but I checked my total and was only getting 24 degrees all in at 2800. Initial is 14BTDC, so for some strange reason I'm only getting 10 degrees mechanical with this distributor (??). I talked about this in another thread, but I felt like we were getting sidetracked, so I wanted to come back to this thread and not have a hundred different threads about the same car/problem.

 

[Righty Tighty]

Also, prior to having the heads done, #6 and 7 plugs were consistently carbon fouled. Badly. I haven't pulled the plugs since re-installing the heads to see what they look like, but I have a sneaky suspicion I'll find the same thing. But I've been told that's likely a separate issue with fuel delivery.

 

[K.O. SWINGER]

Well I understand you're frustration but you need to work out the problems before you rebuild or you might just be bolting your problems right back on. I didn't have a chance to read all the posts but I do run a six pack and have had similar problems in my ignition. Two things that I would do very first would be make sure that my outboard carbs were working correctly because of leaning problem would be coming into effect around the RPM range you're talking about, but the first thing I would do replace that distributor for another working distributor to see if it eliminates your problems. My guess is it's in the distributor after isolating that you can move on.

 

[Righty Tighty]

Well I understand you're frustration but you need to work out the problems before you rebuild or you might just be bolting your problems right back on. I didn't have a chance to read all the posts but I do run a six pack and have had similar problems in my ignition. Two things that I would do very first would be make sure that my outboard carbs were working correctly because of leaning problem would be coming into effect around the RPM range you're talking about, but the first thing I would do replace that distributor for another working distributor to see if it eliminates your problems. My guess is it's in the distributor after isolating that you can move on.

Yeah, it’s pretty frustrating. I’m on my second distributor, second coil, two separate ECUs, three different engine harnesses, valve job on the heads. I’ve checked and double checked timing.

One thing AJ suggested was reversed polarity at the distributor pickup, but I don’t understand how that could happen with all those different harnesses and both distributors. I’m not doubting him, I just don’t understand.

 

[Righty Tighty]

I have verified the front outboard is opening, but the rear is sticky. I temporarily disconnected the rear, so I’m just running on the center and front. I would think with essentially a 4 barrel I would be getting enough fuel, right?

One thing I plan to do while the transmission is out is change out the tank to pump line for a 3/8”. From what I understand the 440 six pack should have a 3/8” and not 5/16” line, which is what I have.

 

[K.O. SWINGER]

Yeah, it’s pretty frustrating. I’m on my second distributor, second coil, two separate ECUs, three different engine harnesses, valve job on the heads. I’ve checked and double checked timing.

One thing AJ suggested was reversed polarity at the distributor pickup, but I don’t understand how that could happen with all those different harnesses and both distributors. I’m not doubting him, I just don’t understand.

Wow that's a lot of changes. just spitballing here but you know for a fact that you're out bored carbs are full according to site bowls? 2And are working properly I think you could still be looking rich on your plugs if you're out bored carburetors aren't opening and letting air in under load. Have you unhooked your linkage and pulled vacuum on your outboard carb pots to see if they're opening freely and properly?

 

[Righty Tighty]

You must have been typing when I posted that second reply, sorry about that. Yes, I’ve pulled a vacuum on both outboards and the rear is sticky, front opens wonderfully.

 

[K.O. SWINGER]

Yes even with my outboards disconnected my big cam 340 will still pull up to 5 grand seamlessly using only the center carb.

 

[K.O. SWINGER]

Is the rear carb sticky when being engaged by vacuum or can you feel it hanging up manually. I would do whatever I could to correct that problem because you know it's going to be a problem.

 

[K.O. SWINGER]

One more thing that comes to mind and again excuse me if it's been covered have you checked top dead center number one cylinder in conjunction with your timing mark to make sure that something weird is not going on there.

 

[Righty Tighty]

Yes even with my outboards disconnected my big cam 340 will still pull up to 5 grand seamlessly using only the center carb.

I've also run off of just the center carb in the past when doing work on the outboards, same problem. Outboards completely removed and the intake sealed off, so I don't think it's an outboard issue.

Is the rear carb sticky when being engaged by vacuum or can you feel it hanging up manually. I would do whatever I could to correct that problem because you know it's going to be a problem.

It's sticky both under vacuum and when it's being moved by hand. Doesn't open freely, and doesn't close on its own. Although I do understand that they don't necessarily close on their own -- that's what the center carb linkage is for -- the front outboard closes on its own when disconnected from the linkage.

I don't know if you've seen any of Harold Demes' videos on YouTube, but it turns out he lives about 30 minutes from me. I've been in contact with him, and I'm trying to set it up so he can change the throttle shaft bushings on at least the rear outboard. Speaking with him on the phone, he doesn't think the shaft itself needs replacing, but the base might need to be align bored.

One more thing that comes to mind and again excuse me if it's been covered have you checked top dead center number one cylinder in conjunction with your timing mark to make sure that something weird is not going on there.

I don't remember if it's been covered here or not....after 4 pages you kind lose track. But yes, I verified the timing mark with #1 TDC. That was a thought early on, that the balancer slipped.

Any thoughts on the 5/16" tank to pump fuel line vs. 3/8"? I already have the 3/8" along with the 3/8" sender, just need to install them. I don't see how a larger line could hurt, but could 5/16" line cause a lean issue? I changed the pickup sock last year when I bough the car, but not the sender or line.

 

[K.O. SWINGER]

Yes 3/8 line and sending unit upgrade is a good one. One of the first things I did when I went to six pack induction. 5/16 is adequate if everything is working perfectly no kinks no obstructions etc but why? 3/8 goes right through all the holes in the frame . Six pack like volume and not pressure their little finicky if you start putting too much fuel line pressure to them, one 3/8 line helps and your All good. I just run a Carter muscle car pump and it seems golden.

 

[Righty Tighty]

Thanks. Speaking of pressure, I installed a pressure regulator between the pump and carbs, and I'm right at 6 PSI. It was my understanding that I should be between 3 and 7. Is that right?

 

[K.O. SWINGER]

Thanks. Speaking of pressure, I installed a pressure regulator between the pump and carbs, and I'm right at 6 PSI. It was my understanding that I should be between 3 and 7. Is that right?

 

[K.O. SWINGER]

Sounds good I would Target 5 psi or six

 

[512Stroker]

Yes, dual x-pipe exhaust. It's hard to tell whether it comes out of both or one, but I will say it did this with open headers as well.

I'm running a light blue Mopar ECU from Halifax and when I bought it, I bought two from him. I've swapped this one out for the other, and the same problem. The stock coil that came with the car swapped for an MSD Blaster 2, same problem. New distributor, same problem.

For a while I was thinking it could be getting too much advance, but I checked my total and was only getting 24 degrees all in at 2800. Initial is 14BTDC, so for some strange reason I'm only getting 10 degrees mechanical with this distributor (??). I talked about this in another thread, but I felt like we were getting sidetracked, so I wanted to come back to this thread and not have a hundred different threads about the same car/problem.

I dont trust the blue ECU boxes, they lose timing on the top end. You may want to consider an upgrade.
Please post close pic's of all 8 plugs make sure they are numbered by which cylinder they were in. The pic's need to show the ground strap so we can read the timing.
I have a suspension that they will be pig rich.
Have you run a compression test? If so, how much cylinder pressure did you record?

 

[Righty Tighty]

I dont trust the blue ECU boxes, they lose timing on the top end. You may want to consider an upgrade.
Please post close pic's of all 8 plugs make sure they are numbered by which cylinder they were in. The pic's need to show the ground strap so we can read the timing.
I have a suspension that they will be pig rich.
Have you run a compression test? If so, how much cylinder pressure did you record?

I apologize for not replying sooner, been busy with other things. The transmission is currently out and the engine tilted back, so I haven't been able to pull the plugs just yet, but I'll get after it when time permits and post pictures.

I haven't done a compression test since the heads came back, but previously I saw numbers between 150-165 psi. I'm not sure if the variances were due to my cheap Chinese compression gauge or user error, or just normal variances in the cylinders. I'll run another test when I pull the plugs.

 

[512Stroker]

I apologize for not replying sooner, been busy with other things. The transmission is currently out and the engine tilted back, so I haven't been able to pull the plugs just yet, but I'll get after it when time permits and post pictures.

I haven't done a compression test since the heads came back, but previously I saw numbers between 150-165 psi. I'm not sure if the variances were due to my cheap Chinese compression gauge or user error, or just normal variances in the cylinders. I'll run another test when I pull the plugs.

150-165 I would consider stock reading maybe a littlle on the lower side.
I myself ran into trouble using a chinese compression gauge, dont trust it.
Make sure you have all the plugs out and the carb wired open WOT when you take the new readings.

 

[Righty Tighty]

Thanks. I didn’t know to have the carb open, I’ll do that on my next test. As well as making sure all of the plugs are out. I might also shop around for a quality gauge.

 

[512Stroker]

Thanks. I didn’t know to have the carb open, I’ll do that on my next test. As well as making sure all of the plugs are out. I might also shop around for a quality gauge.

You can get one from your local auto parts store on thier tool program, and just return it when your done.