What’s with the valve.

[Frank Mopar]

Check out the pic of the valve. What’s your thought of it being so clean and the plug being so black.

Background. Race engine with a high speed backfire at the top of rpm range. Usually one pop.

Everything electrical was addressed and that is not the cause.

512 with 240 trick flow heads. 600 cam. 1050 carb with annular boosters. 10.5-1, pump gas

 

[w.Hudson]

I have found that in most highspeed misfires are caused by the carb being too rich.

Maybe try going down on the jet size a little.

Just my opinion

 

[493 Mike]

Check out the pic of the valve. What’s your thought of it being so clean and the plug being so black.

Background. Race engine with a high speed backfire at the top of rpm range. Usually one pop.

Everything electrical was addressed and that is not the cause.

512 with 240 trick flow heads. 600 cam. 1050 carb with annular boosters. 10.5-1, pump gas



View attachment 1774624View attachment 1774625

It appears you have been mixing vanilla pudding with your gasoline.
Mike

 

[lewtot184]

I think the plugs too cold.

 

[Frank Mopar]

I think the plugs too cold.

That’s the only plug that looks like that. All others are ok. 86 jet.

More background. The car ran perfect for year and a half. On one run it backfired 1x and this issue persists.

Swapping the carb and the problem went away. So the question is, why only one plug? #2 cylinder.

 

[Lefty71]

Does seem like a manifold distribution mystery??

 

[w.Hudson]

That’s the only plug that looks like that. All others are ok. 86 jet.

More background. The car ran perfect for year and a half. On one run it backfired 1x and this issue persists.

Swapping the carb and the problem went away. So the question is, why only one plug? #2 cylinder.

It could have something floating around or stuck in the metering block on that corner of the carb.

Crazy as it sounds but it doesn't explain the plug condition.

OR

That corner is too rich

 

[Frank Mopar]

At the top of rpm it will backfire once. Could do it in every gear at the high rpm. One time. Never backfires twice.

Right now the heads are at the engine shop for checkup. The carb people tell me the front and rear metering blocks are the same to swap so that’s the plan. Unfortunately next season.

 

[Geoff 2]

Might be the pic, but the valve edge looks like it could be burnt.

 

[BSB67]

I think a lean cylinder can cause this condition. The plug gets black because of mis-fire due to the lean condition.

But it does seem like other things are going on too. What cylinder? 5/7? Maybe something gone wrong in the intake tract causing the lean condition?

 

[Geoff 2]

Spark plugs could be too cold, a common mistake made by many. Should be a 5 heat range in NGK.

 

[kiwigtx]

Check out the pic of the valve. What’s your thought of it being so clean and the plug being so black.

Background. Race engine with a high speed backfire at the top of rpm range. Usually one pop.

Everything electrical was addressed and that is not the cause.

512 with 240 trick flow heads. 600 cam. 1050 carb with annular boosters. 10.5-1, pump gas



View attachment 1774624View attachment 1774625

Is that leaded fuel you are using?

 

[Frank Mopar]

Is that leaded fuel you are using?

Yes Sunoco racing gas. Leaded.

It’s the correct heat range. #2 cylinder is the only one black like that.

This started out of the blue. Running a different carburetor fixes it.

 

[lewtot184]

Really need a picture of all 8 plugs to determine fuel distribution issues. It's possible for one plug to be funny with a single plane intake but I think more likely there would be 2 or 4. If distribution is the case it may be possible to help the problem with air bleed changes. If that's not possible then manifold work will be needed. I'm not sure it's an ignition problem but you can easily cover that base with a multimeter.

 

[Frank Mopar]

Here is #4 intake. All the rest look like this.

 

[Frank Mopar]

I'm not sure it's an ignition problem but you can easily cover that base with a multimeter.

Everything electrical has been addressed. Why? Because that’s what would make that plug black but it’s not the cause.

I even (but not for the fouling plug reason) rather a three step function swapped out my MSD for a Pertronix ignition.