Misfire at cruise, popping/afterfire at 3000RPM

[Phreakish]

The valve covers have been off and I manually checked the rockers for preload, but I didn't actually measure. I sent the heads in for a valve job, their cylinder head guy said virtually none of the valves were sealing. With the heads (and intake) off, I was able to get eyes on the cam, and all lobes looked good. I also pulled and checked all of the pushrods, none bent.

Heads back on, car runs much better, except the misfire appears now around 3400 instead of 3000RPM.

I forgot to mention that I suspected a lean condition causing the misfire, so I richened the center carb just a bit, which may be the cause of the dark plugs.



Haha, I thought the cure was fuel injection...

I will say that the misfire happens even with all new plugs. I'm running Autolite 85 plugs, which are heat range 5.

I'd been following the thread for a while and didn't realize it was a sixpack until yesterday. Have you tried disabling the outer carbs to see if you can isolate it to fuel? Does the misfire only happen under load? Or can you replicate it in the driveway? If you can replicate it in the driveway, I'd hook up your timing light just to verify a spark and go from cylinder to cylinder and also at the coil to verify everything is firing - especially during the pop/backfire.

This is a strange one...

 

[Righty Tighty]

Yes, I ran the car off of the center carb while I had the outboards removed to freshen the gaskets, etc., same problem. I've tried different springs in the secondaries, and recently I ran on just the center carb and front outboard because the rear outboard was sticky.

The misfire happens both under load and revving in the driveway. Getting back to timing -- I don't know if you've seen the posts where we talk about how the distributor only gives about 10 degrees advance by 2800, so with 14 initial, I end up all in at 24 @ 2800 RPM. I didn't think to check each cylinder with the timing light, I'll do that once it's back in the car.

This has to be a simple problem, and I'm sure someone with better troubleshooting skills would have it licked by now. It's really tempting to take it to a shop to see what they think, but I really don't know of any good Mopar shops in my area. I'm also stubborn as hell and want to figure this out.

 

[Mattax]

I'd been following the thread for a while and didn't realize it was a sixpack until yesterday. Have you tried disabling the outer carbs to see if you can isolate it to fuel? Does the misfire only happen under load? Or can you replicate it in the driveway? If you can replicate it in the driveway, I'd hook up your timing light just to verify a spark and go from cylinder to cylinder and also at the coil to verify everything is firing - especially during the pop/backfire.

This is a strange one...

its taken many turns but all for good reasons.
Go back to the very first post. The issue was described as misfires at moderate to full throttle in the range of 2800 rpm up while driving.

The timing curve was never fully mapped out as far as I can recall. I'm not convinced this is the cause as the description doesnt match, but is still something critical to do early on. IMO the mechanical timing ought to be ballpark correct before trying to hunt down issue tied to moderate to full throttle.

 

[Mattax]

Getting back to timing -- I don't know if you've seen the posts where we talk about how the distributor only gives about 10 degrees advance by 2800, so with 14 initial, I end up all in at 24 @ 2800 RPM.

I either missed or forgot that.
Remind me why is it all in at 2800?

 

[Phreakish]

its taken many turns but all for good reasons.
Go back to the very first post. The issue was described as misfires at moderate to full throttle in the range of 2800 rpm up while driving.

The timing curve was never fully mapped out as far as I can recall. I'm not convinced this is the cause as the description doesnt match, but is still something critical to do early on. IMO the mechanical timing ought to be ballpark correct before trying to hunt down issue tied to moderate to full throttle.

Agreed - many twists. That's why I asked a few questions since some stuff has seemed to change as OP has learned and found more. Sometimes the original post becomes a little less relevant over time - that's the only reason I'm asking redundant questions.

Also agree about the timing. Poor ignition seems to cause 90% of 'carb' problems.

 

[George Jets]

Guessing that center carb is running Lean, outboard carbs running rich.

Just as a test in the driveway, can you block your outboard carbs on the manifold, and bolt on one of the outboard Carbs in the center?

Will it bolt up for the test or are the bolt patterns different?

 

[Mattax]

I dunno George. Not seeing that looking at the plugs but ??

I either missed or forgot that.
Remind me why is it all in at 2800?

Nevermind. I found it it.
Why are my distributor cap terminals burnt?

if this is the curve that's in there now, its a little retarded to a lot retarded. That would make a sluggish response. Might also leave a fuel partially unburned -

 

[Righty Tighty]

I either missed or forgot that.
Remind me why is it all in at 2800?

This was a while back, but it was when I was trying to measure the curve. I started at 14 advanced, idle speed was around 850 IIRC, then revved until it stopped advancing, which was at 2800 RPM, and the advance was then at 24. New distributor from Summit. We had talked about changing springs to see if I could get more total advance, but the transmission took the front seat for a bit.

Edit: I was typing this response while you posted. I forgot I had a separate thread where we talked about that.

Guessing that center carb is running Lean, outboard carbs running rich.

Just as a test in the driveway, can you block your outboard carbs on the manifold, and bolt on one of the outboard Carbs in the center?

Will it bolt up for the test or are the bolt patterns different?

The bolt patterns are the same, so I think it would be possible.

 

[Phreakish]

I dunno George. Not seeing that looking at the plugs but ??


Nevermind. I found it it.
Why are my distributor cap terminals burnt?

if this is the curve that's in there now, its a little retarded to a lot retarded. That would make a sluggish response. Might also leave a fuel partially unburned -

I would expect 24 total to cause a misfire. It also sounds like when he disconnected the rear carb, the car ran better - but still broke up.

I think you're right on the money with this being an ignition timing issue. It comes in late, and doesn't go far enough.

 

[Mattax]

I started at 14 advanced, idle speed was around 850 IIRC, then revved until it stopped advancing, which was at 2800 RPM, and the advance was then at 24. New distributor from Summit. We had talked about changing springs to see if I could get more total advance, but the transmission took the front seat for a bit.

Edit: I was typing this response while you posted. I forgot I had a separate thread where we talked about that.

Yep. Post 60 were my suggestions.
Assuming its a factory cam, initial is in the ballpark, from 900 to 2600 is way too late and from 3000 up is wayyy too late.
I think you have to expirment with the other springs and the 'limit' set somewhere around 18* rather than 10*. Because the limit also effects the initial spring tension, I'd make that change first.

 

[Phreakish]

This was a while back, but it was when I was trying to measure the curve. I started at 14 advanced, idle speed was around 850 IIRC, then revved until it stopped advancing, which was at 2800 RPM, and the advance was then at 24. New distributor from Summit. We had talked about changing springs to see if I could get more total advance, but the transmission took the front seat for a bit.

Edit: I was typing this response while you posted. I forgot I had a separate thread where we talked about that.


The bolt patterns are the same, so I think it would be possible.

That distributor can be adjusted for the amount of advance it allows (by loosening the two screws and adjusting the window width), and how quickly (with springs).
But, if the billet dist behaves/misfires the 'same' as the factory distributor, I'd go back to the factory one again and check the idle/all-in timing and get it close to what Mattax has suggested. Focus on getting the max timing correct and let the rest land where it does - it's far less important than figuring out the misfire, and misfire isn't going to be affected by what the idle timing happens to be.

No reason for a stock distributor to not work, and it would work with the limiter plate you bought (if it becomes necessary).

I think the outer carbs are your likely culprit for the breakup, but your ignition is so out of sorts there's no way to confirm that until the timing is correct.

 

[TT5.9mag]

@67Dart273 mentioned rotor phasing very early in this thread. I read the whole thing and never saw it addressed. Has rotor phasing been checked?

 

[Righty Tighty]

That distributor can be adjusted for the amount of advance it allows (by loosening the two screws and adjusting the window width), and how quickly (with springs).
But, if the billet dist behaves/misfires the 'same' as the factory distributor, I'd go back to the factory one again and check the idle/all-in timing and get it close to what Mattax has suggested. Focus on getting the max timing correct and let the rest land where it does - it's far less important than figuring out the misfire, and misfire isn't going to be affected by what the idle timing happens to be.

No reason for a stock distributor to not work, and it would work with the limiter plate you bought (if it becomes necessary).

I think the outer carbs are your likely culprit for the breakup, but your ignition is so out of sorts there's no way to confirm that until the timing is correct.

Unfortunately I don't have the old distributor, I donated it to a member here. Kinda wish I didn't now.

The engine didn't run any better or worse with the rear carb disconnected, nor any different with both outboards disconnected.

@67Dart273 mentioned rotor phasing very early in this thread. I read the whole thing and never saw it addressed. Has rotor phasing been checked?

I didn't check rotor phasing, because I ended up with a new distributor.

 

[TT5.9mag]

Unfortunately I don't have the old distributor, I donated it to a member here. Kinda wish I didn't now.

The engine didn't run any better or worse with the rear carb disconnected, nor any different with both outboards disconnected.


I didn't check rotor phasing, because I ended up with a new distributor.

As you know, new don’t mean good. I’d check the new one as well.

 

[Bewy]

Disconnecting the throttle linkage of the end carbs does not remove them from the action, if that is what you have done. If the fuel line is still connected, they are still getting fuel, which could be leaking into the engine. The fuel line need to be disconnected for complete isolation or the carbs removed.

 

[Righty Tighty]

As you know, new don’t mean good. I’d check the new one as well.

Sigh...I know. I honestly don't know why I decided to get a new distributor without checking my old one, but that's in the past.

Disconnecting the throttle linkage of the end carbs does not remove them from the action, if that is what you have done. If the fuel line is still connected, they are still getting fuel, which could be leaking into the engine. The fuel line need to be disconnected for complete isolation or the carbs removed.

I ran the car with the outboards completely removed from the car. They were sitting on my workbench as I drove away.

 

[Righty Tighty]

Today I'll be reassembling the front end and welding up the exhaust, hopefully I'll have time to crack into the transmission to swap converters. Once all that's done, I'll be dropping the car back onto the engine (feels weird saying it that way) and I'll be able to continue with troubleshooting. I'm going to start with timing and checking the distributor, and I'll probably readjust the idle mixture screw so it won't be so rich.