It is probably red hot carbon deposits in the combustion chamber that continue to apply an ignition source after the key is off. Try some "Top Tier" fuel for a while plus any fuel detergent cleanser additive with "polyetheramine" in it. I like "44K Platinum" from BG Products but there are others.

This carbon did not develop overnight so don't expect an overnight remedy.

Rick

15 hours ago, Pete Wortley said:

Overall Rick is correct a good PEA product added to the fuel system will help, but in general those products are more favorable for injection systems & specifically direct injection. I could go into a longer dissertation, but in short PEA is more of a vapor contact additive solution for carbon deposits where liquid fuel never goes. Such as I.E. plenums & the back of valves found in D.I. systems. A quicker fix for any carbon deposits in a combustion chamber is water! Yes I said water! Presuming you do not have a timing issue, which can also cause that issue with mild deposits. Run the engine up to about 1500 - 2000 rpm. Take a squeeze bottle filled with water. Squeeze the bottle and direct the water down the throat of the carb as you do this the engine will start to slow as the rpms drop lower, down to around 1000 stop adding water. Let the rpms come back up to the previous 1500 or 2000. Than do it again. Repeat this til you have gone through 8 Oz or 237ml. If it has improved but is still there repeat again. If it's gone your done.

Why water? You can not compress water! As such when added to a compression chamber it acts like a little sonic blaster or hammer if you will. This in turn blasts the carbon deposits away. & no it does not hurt the engine. It is actually one of the oldest forms of increasing power in an engine. Used during WW I!

Edited December 21, 2025Dec 21 by Last Indian

I can testify to what Last Indian said, it works. It was explained to me as "steam cleaning" the inside of your engine. I did it to a 66' Impala with a 396 I had in Texas. The way I did it was to fill a 20oz Aquafina water bottle, poked a small hole in the top where it would barley trickle out. Get the engine up to temp and start trickling the water in, SLOWLY. If the engine drops RPM's to much or sounds like it's going to die the stroke the throttle on the carb to raise the RPM's. After your bottle is empty pull the car forward about 5 feet and you will see a wet spot under your tail pipe. It'll dry in justa few min and you will be left with a black spot of carbon that came out of your engine. I did it to that 396 three times before there was almost no carbon spot left.

By the way, welcome to the forum. You have a beautiful ride!

Edited December 22, 2025Dec 22 by Wrongway

Welcome to the site, thankz for the pic's. Amazing Pontiac.

Thanks to everyone who offered advice. I’ll see how it goes

Another thought is to double check your ignition timing. Your timing maybe too advanced or too retarded.

Hi mate, thanks for that. That was my first go to……I think it is carbon related. Going to take it to a mechanic early new year.

I was thinking the timing may be a bit advanced myself, also check that the throttle plates of the carb are actually fully closing when the engine is at idle, I have seen them partially hang open with some build up, or with wear to the throttle shaft holes in the carb body