Backfiring After Engine Turned Off

[4129R]

Sometime my engine runs fast on idle, and then the engine backfires about 3 seconds after the ignition is turned off and the engine has stopped rotating.

I have done all the usual things to get the engine to run at around 1000 rpm on tick-over, but the backfiring is something I am not used to, and it seems to happen noticeably long after the engine has stopped.

Any ideas?

[jbcollier]

What engine and what carbs?  Any modifications?

[4129R]

What engine and what carbs?  Any modifications?

1558 TC running on 40 DCOEs with 31mm chokes, 115 main jets, 150 air corrector jets, free flow stainless exhaust manifold and silencer (headers and muffler). Federal standard big valve cams, gas flowed inlet and exhaust ports.

[jbcollier]

My initial thought would be too richen up the idle mix.

[4129R]

My initial thought would be too richen up the idle mix.

I turned the idle jets 1/4 turn richer, and I cured an airlock in the cooling system which left air in the top of the cylinder head by filling through the thermostat housing.

I am not sure which was responsible for the backfire, but so far so good.

I would be interested to know the theory behind richening the mixture to cure the backfire.

[jbcollier]

A rich mixture also cools the intake tract and combustion chamber.