Pre-ignition
 

My car has been running great but once in a while when I shut off, it pre-ignite?
There's no sign of pinging and what so ever. I only use chevron 92. I try to keep the fans running for cool down about a few minutes before I park to avoid the motor being too hot and my holley 770 set up to idle at 850ish.

Any idea why it does that sometime?

My idle is set fast (1200 RPM) to ensure sufficient oiling for the solid rollers and valvetrain. At that idle speed, it would always run on. So what I do is put it in 2nd, put the parking brake on, put my foot on the brake, and let out the clutch very slowly to drag down the engine speed. Once it slows enough I cut the key and let out the clutch. No run on this way. It sounds difficult but the first time you do it you will see how easy it is, and also that the car won't move. If you have a tuning problem you should fix it. But if it is because of a fast idle, try this.

Lippy is correct and that method (dragging the motor rpm down slightly with the clutch before turning off the key) is the easiest way to deal with it if you don't want to lower the idle.

You can also install an idle solenoid on the carb that increases the primary throttle blade opening with the key on but when the key is switched off, the solenoid closes the throttle. This was basically Detroits way of dealing with this issue back in the late 60s/early 70s. They are available for Holley and Edelbrock carbs including brackets but will probably run you about $100.

What Lippy says......

Check your initial timing too.

Generally referred to as 'run on' or 'dieseling'. I get it occasionally on my 460, so I was interested to read this - which seems to indicate too rich or too lean could both be causes: https://en.wikipedia.org/wiki/Diesel...tential_causes

To much timing can be the culprit. I have no choice but to use the method Lippy suggested as my distributor is locked out fully advanced and idles around 1200 rpm.

My engine did the same and I had the timing adjusted and it never did it again

Besides timing if you have a large cam and to compensate the carb primaries have to be opened way up to get it to idle, it will often diesel when shutting it off. Assuming you are carbureted, are your idle mixture screws responsive? If not the primary throttle plate may be opened above the idle fuel slots which is never good. Setting the idle speed up high can do the same thing. Sometimes the secondary plates can be adjusted to pass a bit more air at idle to compensate, allowing the front throttle plate to be closed to where the idle mixture screws are functional. The screw to adjust the secondaries is underneath (have to remove the carb). Just saying as sometimes a little careful tuning of timing. idle speed and the carb throttle plate can take care of it.

You suggest that this happens in frequently... just a slightly different line of thought if I may... I'm no expert.

Have you noticed any common denominator?

Like after a long spirited run where your engine is running a little hotter than normal?
Any chance your carb is oversupplying fuel ie: high fuel pressure or high float at this point (feeding the engine more fuel) causing the engine to run-on after you kill the electrics?

If so, its possible that this run-on may be preventing your engine from cylinder wash.


In any case, as others have mentioned, Lippy's method is a simple fix.

Thanks guys ;) should the timing be advanced or retarded? I've noticed the idle mixtures doesn't move much with the vacuum gauge but I'll check the throttle plate too. So when the engine is idling the throttle plate should be closed?

If you have a Holley or similar carb and look down the primary throats you should see a couple small retangular slots on the front just above the throttle plate shaft level. Those are the idle fuel slots and the idea is that when the throttle is resting closed on the idle adjustment screw - the thottle plate should be in about the center of the slot height. If your throttle plates are resting at the top of it, then that is why you aren't getting much adjustment out of your idle mixture screws.

There are a number of ways to deal with that (throttle setting is above fuel slots) and it could be that your timing is retarded at idle requiring that the idle setting is opened to much to compensate for it. But we don't know enough about your motor or how it's tuned to be sure. You may have a big cam, etc. But since you say you aren't getting much response to idle mixture adjustments on your vacuum gage, there is a fair chance this is part of the problem.

If you find this to be the case and you are otherwise pretty happy with how the engine is tuned (timing, idle speed) then removing the carb and slightly opening the secondary throttle stop a little so that the primary side can be closed a little, could help things out. A lot of people use to drill the primary throttle plates with small holes to pass air so the plate could be closed, but that shouldn't be necessary if the secondary side can be opened a little more.

I mention all of this as I have a fairly well built BB Corvette that used to diesel for this same reason. Once I got the carb all sorted out for the big cam it would shut off just fine.