Can too little idle mixture cause dieseling? If I try to shut it down on the high idle step it will diesel. If I set the idle mixture close to maximum vacuum it wants to diesel as well.

In P/N I can achieve 20" vacuum. I can't leave it there tho. I have to lean it out to 17" so it won't diesel.

Maybe I just haven't learned enuf about carbs yet. I think I'm missing something...

My setup is in my sig. I run a 42 equivalent heat range plug.

 

You are missing alot here. I think you are confused some. Shuting the car off with a high idle 1500rpm or above will most likely cause run on.

what is your compression ratio? What is the idle speed when you turn off your car? What is your total timing? What are you running for a gasoline grade?
All of these things will effect engine run on. Too little Idle mixture will not cause run on. Too hot of a plug can though. I see you are running a 42 heat range which should not be a problem here.

 

Your problem is most likely the high idle. pretty much anything will want to diesel if the throttle blades are still partially open (high idle step or just a high set idle) when you try to shut it down.

 

Put some decent gas in the gas tank 92 octane +.

get a anti- run on idle stop solenoid for the carb.

Also likely too much total ignition advance under boost and not enought initial advance at idle.
The combustion chamber is overheating and the carb throttle is too far open at idle.

Your "42" heat range plugs should be non projected tip type.

 

I run 93 octane all the time. The compression ratio is 9.3:1. Total timing under 4.5 lbs of boost is 26°.

Tell me a bit about the anit run-on idle stop. I don't think my plugs are projected tip but I'll have to check.

 

The compression ratio is very high for a non intercooled blower. I'd consider water injection. will allow a lot more boost without detonation with more ignition timing.

The anti run on solenoid, by opening the throttle a little once you turn on the key (sets the idle speed, throttle opening at idle) allow so that when you shut the key off, the throttle closes off completely blocking all air and fuel flow and stopping "run on". many GM cars with airconditioning and Qjet carbs from mid 70's to mid 80's had this in the carb. needs only key switched 12v power.

 

That solenoid is called "the A/C kicker". It's meant to bump up your idle speed when you turn on the A/C to keep the car from stalling. But, like mentioned, you can rig it up differently to make it an anti-dieseling solenoid.

I don't really think you need to do that though, maybe call that plan B.

Always set your idle mixture for max vacuum. If the vacuum is "too high" and its dieseling, bring down your timing at idle.


First make sure you are at an 800RPM idle when you try to shut it off. If it still runs on, make sure you don't have a lot of timing at idle. IIRC you have a small cam right? Shouldn't need much timing at idle. Maybe 10*. If it's on manifold vacuum advance that might be causing too much timing, try using ported, (but bump up the base timing a tad, back up to 10* say).

Heat range on the plugs sounds ok for a 9.3:1 blown app. I'd keep a careful eye on those, reading the mixture, and the heat line on the strap. I'd be trying Iridium plugs in that beast if it were me though, they're great in a forced induction setup.

 

Improper vacuum advance while cruising at light throttle openings will cause overheated cylinder walls and combustion chamber as well. it will tend to run on when you shut it off.