Run on and backfire through carb

[PolaraFury]

Hello,

I have a new problem and I’m looking for some help

It’s a 500 low deck stroker, Holley 950 DP (no issue last summer on the same motor).

I added an MSD 6AL plus with MSD distributor. The car fires right up when you turn the key. When I shut it off at operating temperature it will run on and backfire out the carb.

Where do you think I should start looking?

Thanks

 

[jimbosride]

My guess timing? Don't go by the timing marks ,go by how it runs .... But hey what do I know

 

[R413]

What is idle speed?
Is carb throttle plates closing at idle?
Do you have a vacuum leak?
What is the camshaft?

 

[Geoff 2]

High idle speed?

 

[sw70rr]

You may want to try adding some static timing. Doing so will increase idle RPM and allow the carb idle speed screw to be backed off to achieve the same RPM. The smaller throttle opening limits air flow through the carb which reduces/eliminates run on. My 440 used to like to run on during hot summer days. This solved my problem. As a side benefit additional static timing tends to reduce engine idle temps.

 

[PolaraFury]

The engine ran just fine last year. The only thing I have changed is the MSD ignition system

The cam is .544/.541 hyd roller. I run 15 degrees at idle

I will mess with the timing and see if I can get this figured out