What are the plugs? If they are too cold they might be overheating and causing this.

 

Idle speed is too high and maybe the idle fuel mixture is set too rich as well.

 

That's 230F and you recently changed jets and rods in an Edelbrock carb. I'm with Randy, check your idle mixture screws. They usually need to be leaned out (turned clockwise) slightly after a richer jet change in this carb. They should be roughly the same on both sides. The vacuum gauge method is most accurate, then reduce the idle speed screw to the best idle. Your timing may also be a bit too advanced. What are you running at idle? Could this have been the real cause of your stumble?

 

I think it could be pre ignition, higher octane fuel helps to eliminate it.

Dan

 

What is your idle set at? High idle means your throttle plates are open possibly to much. This will cause excess fuel from the boosters to be pulled into the intake during shut down and cause run on. You may have to advance initial timing to increase idle speed so you can back off the idle speed screw to close the throttle plates.
Rick.

 

You may need to retard the timing.

Trent

 

Originally posted by TRENT:
You may need to retard the timing.

Trent

I assume you're joking

Mike

 

quote:

Originally posted by TRENT:
You may need to retard the timing.

Trent

I assume you're joking

Mike

Ummm.... Nope, too much advance in the timing will increase the temperature in the combustion chamber and this can lead to run-on or dieseling.

Trent

 

I think Trent is right

 

Originally posted by TRENT:
</font><blockquote>quote:</font><hr /> quote:

Originally posted by TRENT:
You may need to retard the timing.

Trent

I assume you're joking

Mike

Ummm.... Nope, too much advance in the timing will increase the temperature in the combustion chamber and this can lead to run-on or dieseling.

Trent </font>[/QUOTE]Ok, I see what you are saying. I have read that it takes a full 10 degrees of advance to increase plug temperatures by about 85C. I think running a little lean could make much bigger changes in temperature. Either way, the plug will show signs of excessive heat and a colder plug might help.

Mike

 

Hey Mike!
Way back in the day (when I was a starving student) I used to set the timing on my street rods by simply advancing until it pinged and then retarding it a bit and if it dieseled, it was still to advanced... Now I have a real job (and a timing light)

Cheers,
Trent

 

Timing and idle as said. The timing could be making it run hotter. Hi idle speed will diesel.

 

Trent is right on!!! TIMING and idle speed.

 

My vote is timing...another reason they invented elctronic fuel injection and computerized ignition. The computer does your thinking for you....(Well , almost).

 

This is a late response but it may be useful to someone. I'm running a sbc 355 crate motor 425hp, aluminum heads, MSD ignition, etc. I had intermittent dieseling problem hindering me since I installed the engine. The advice I always got was to retard the timing in conjuction with other things. I spent hours reviewing my plugs, octane ratings, the ignition system, cooling system, and retarding the timing and still had dieseling. My solution was to bypass the vacuum advance from my 750 holley and go straight into the manifold for vacuum. Additionally, I ADVANCED the timing rather than retard it. The combination resulted in my idle speed to raise from it's current setting. This forced me to have to bring down my idle (further closing the primaries on my carb). With my timing advanced I was able to idle well down to 600rpm. The dieseling is gone and the engine runs smoother!!! This won't work for everyone but it did work for me.

 

My car would run on when the electric fan was kicking on. It's on a thermostat and there must be some electical feedback that caused dieseling. A diode will solve the problem if that is the cause. just diconnect the fan & see if it stops dieseling. I got my idel down to about 850 rpm with carb & it is a rare occurrence.