Misfire at cruise, popping/afterfire at 3000RPM

[halifaxhops]

Hey check the reluctor gap on all 8 positions really thinking the upper dist shaft is bent. Seen it on even NOS distributors. The blue boxes are good for 5500 rpm with all the testing I have done. Doubt it is ecu. Really leaning to pick up related. Get any worse say when it is hotter?

 

[Righty Tighty]

Hey check the reluctor gap on all 8 positions really thinking the upper dist shaft is bent. Seen it on even NOS distributors. The blue boxes are good for 5500 rpm with all the testing I have done. Doubt it is ecu. Really leaning to pick up related. Get any worse say when it is hotter?

Thanks for the tip, I’ll check the gap in all positions. The problem remains the same whether the engine is hot or cold. Does it both under load and in the driveway.

It’s been mentioned before that the pickup wires could be reversed, but how likely is it that that would happen with 2 separate distributors?

 

[halifaxhops]

Very unless they were cut which I doubt.

 

[Righty Tighty]

It's very likely they're reversed? Is there a way to test and/or correct the polarity without cutting them? The only way I've been told to troubleshoot this is to watch the timing strobe, see if it bounces around. It does a little bit, but I'm not certain it's bouncing around enough to be solid evidence that the wires are reversed.

I supposed I could cut them, switch them, and then see how the engine performs afterwards. And if it's worse I can just switch them back.

 

[halifaxhops]

If they are not cut frm the pick up plate they are right.

 

[512Stroker]

It's very likely they're reversed? Is there a way to test and/or correct the polarity without cutting them? The only way I've been told to troubleshoot this is to watch the timing strobe, see if it bounces around. It does a little bit, but I'm not certain it's bouncing around enough to be solid evidence that the wires are reversed.

I supposed I could cut them, switch them, and then see how the engine performs afterwards. And if it's worse I can just switch them back.

What plug wires are you using? Im sure you have checked the firing order correct? Cap and rotor?
Just spit balling, this has to be something basic.

 

[Righty Tighty]

If they are not cut frm the pick up plate they are right.

They’re not cut, so I think I’ll leave them as is.

What plug wires are you using? Im sure you have checked the firing order correct? Cap and rotor?
Just spit balling, this has to be something basic.

I’ve got new Taylor 8mm wires. I don’t remember if I mentioned in this thread or not, but I found one of the porcelain boots cracked, along with the porcelain on one of the plugs cracked. Fixed that and it solved the misfire at cruise.

I’ve double and triple checked the firing order, made sure the wires are parallel and separated, confirmed correct wiring of the coil. One thing I haven’t done is checked voltage.

The biggest improvement was when I had a valve job done and replaced all the springs and valve seals. That changed the RPM when this is happening from 2800 to a little over 3400 (if I remember correctly?). So that would make me think this is mechanical and not ignition related, or maybe there are several problems going on here.

 

[halifaxhops]

If you can find someone with a scope it would pinpoint if it is a ign issue. and also probably show you if a cylendar is not right. A cylendar kill meter would show a cylendar for sure.

 

[Righty Tighty]

Awesome, I've asked the first two shops that came to mind, but unfortunately neither has one. Are they easy to learn to use?

 

[skykeith]

Awesome, I've asked the first two shops that came to mind, but unfortunately neither has one. Are they easy to learn to use?

You need to find someone who has and can read an ignition scope. It will tell you everything. The secondary ignition patterns can be displayed in different formats. the cylinders are individually displayed and can be compared to each other. Everything that effects an ignition will be displayed from distributor wobble to whether every spoke of the reluctor is triggering. Low voltage, high voltage, low and high resistance can all be seen on the scope. analyzing the arc burn pattern can tell you mixture/fuel problems and a rising burn line means the cylinder is lean. An erratic burn line can mean a restricted exhaust. There are some really good videos on Youtube showing how to analyze secondary ignition patterns. someone with a scope could probably find your problem in 15 minutes.

 

[halifaxhops]

They are great. LOL right now I have 5 of them. I fix them though.

 

[Righty Tighty]

I picked up a Snap On MT1665 yesterday, so I’ll be learning how to use it over the next few days. Once the car is back together and running, I’ll be able to hook it up and play.

 

[MopaR&D]

They are great. LOL right now I have 5 of them. I fix them though.View attachment 1715894465 View attachment 1715894466

That's so cool, I didn't know I needed one of those until I saw this thread... LOL Seriously, that's DAMN COOL.

One issue I had in the past with an electronic distributor, the phasing of the reluctor wheel was off because the roll pin was in the wrong slot. Most of the reluctor wheels have 2 spots for the roll pin to go into. In my case when the vacuum advance can was "on" (pulling full advance) the reluctor teeth would be too far out of phase and it caused slight hesitation/surging at cruise. Pulled the reluctor wheel and reinstalled it in the other roll pin slot, problem solved.

 

[halifaxhops]

They have two positions one for CW rotation (LA) and CCW rotation B/RB)

 

[halifaxhops]

I picked up a Snap On MT1665 yesterday, so I’ll be learning how to use it over the next few days. Once the car is back together and running, I’ll be able to hook it up and play.

Good meter make sure you find a instruction manual on it has to be set up to the application.

 

[Righty Tighty]

Thanks, it did come with an instruction manual, as well as some other literature for interpreting waveforms, etc.

 

[halifaxhops]

Once you get proficient with it you will never be without one. Subtle changes to the waveforms are the key, easy to miss them.

 

[skykeith]

Once you get proficient with it you will never be without one. Subtle changes to the waveforms are the key, easy to miss them.

You are absolutely right. Once you troubleshoot with a scope, you will never go without one.

 

[halifaxhops]

I have some Heathkits scopes I went through right now in the for sale area if any one is looking for one.

 

[Righty Tighty]

Pulled the carbs today so I could send the base plates to Harold Demes -- he's going to replace the throttle shaft bushings. I noticed that the intake bores on the outboards looked significantly richer than the center carb, and secondly, the front outboard was richer than the rear. Why would that be?

Didn't have time to pull the plugs, but I suspect they're all going to be rich as previously mentioned.

 

[Righty Tighty]

Getting back to this thread after the transmission rebuild. When I pulled the carbs to have the throttle shaft bushings replaced, I noticed the front throttle plates were catching on the gasket. I widened the bore so the plates move freely, and I also noticed the front plates were hanging open slightly, which explains why I couldn't get it to idle down.

 

[Righty Tighty]

Finally got around to pulling the plugs today, and they are all rich. I also noticed #6 and 7 headers are dull, while the rest still have a polished look. I think these two cylinders are running hot, but I'll have to wait until the engine is back in to put a thermometer on them.

 

[Phreakish]

Darkness in a manifold isn't necessarily rich/lean. It's gas reversion. The clean areas are where fuel has washed the deposits away. That's why one side of the bores in the center are clean, but the other side is still dark. I wouldn't make guesses about rich/lean based on the color inside the manifold. The sparkplugs are present for combustion, so rely on them instead.

I can't remember, have you pulled the valve covers and looked over your preload/lash to rule out a cam failure?

Dark spark plugs can indicate rich, but can also be impacted by timing and plug heat range. With a misfire situation, you're looking mostly for differences. The observation about the dull header pipes is a good one. I'd be looking there for signs of a cam failure (by pulling the valve covers and looking at lift/lash/preload) or for a bent/missing pushrod, or looking for stuff that could have caused excess fuel delivery. The sticking throttle plates might be a partial contributor there somehow, but I'd be looking deeper into the carbs to rule out other issues (float height, power valve condition, etc).

 

[Bewy]

Looks like a case of Holleyitis. There is a vaccine, it is called Carter.....

Some cyls maybe misfiring because of the fouled plugs, which would account for the difference in header temps.
Plugs should be heat range 5 in NGK, no colder.
I would remove the centre carb & make sure you have 0.040" or less of T slot showing. More than 080 would probably account for the rich idle & indicate bypass air is needed.

 

[Righty Tighty]

Darkness in a manifold isn't necessarily rich/lean. It's gas reversion. The clean areas are where fuel has washed the deposits away. That's why one side of the bores in the center are clean, but the other side is still dark. I wouldn't make guesses about rich/lean based on the color inside the manifold. The sparkplugs are present for combustion, so rely on them instead.

I can't remember, have you pulled the valve covers and looked over your preload/lash to rule out a cam failure?

Dark spark plugs can indicate rich, but can also be impacted by timing and plug heat range. With a misfire situation, you're looking mostly for differences. The observation about the dull header pipes is a good one. I'd be looking there for signs of a cam failure (by pulling the valve covers and looking at lift/lash/preload) or for a bent/missing pushrod, or looking for stuff that could have caused excess fuel delivery. The sticking throttle plates might be a partial contributor there somehow, but I'd be looking deeper into the carbs to rule out other issues (float height, power valve condition, etc).

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.

Looks like a case of Holleyitis. There is a vaccine, it is called Carter.....

Some cyls maybe misfiring because of the fouled plugs, which would account for the difference in header temps.
Plugs should be heat range 5 in NGK, no colder.
I would remove the centre carb & make sure you have 0.040" or less of T slot showing. More than 080 would probably account for the rich idle & indicate bypass air is needed.

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.