[jhein]

I've shut the car off at full operating temperature at least a dozen times. Twice it has run on for a couple seconds. I talked to Archie (the builder) and he said first thing is to set the timing back a couple degrees. Is the theory behind that too much advance makes it run hotter?? Can't be spark because there is no spark. But my water temp is right at 180 so that's not hot at all. I guess the plugs could be too hot.

Other ideas? Too rich at idle. Too fast at idle? Wrong plugs?

I'm running fresh 92 octane non-ethanol gas.

[Sirrotica]

Idle is usually too high in these situations, the reason that GM, as well as some other manufacturers used idle stop solenoids. Those cars had 2 idle settings, a curb idle which was just above 600 RPM, and a solenoid engaged idle, usually 700-750 RPM.

This is how the system worked, the solenoid energized when the ignition was on, keeping idle at the higher level. When de-activated it dropped the idle down to the near 600 idle stop, stopping the dieseling/run on. When properly set, the system worked very well.

Some mechanics didn't fully understand the system, and set the idle with the carb stop screw, rendering the solenoid useless.

On some cars with A/C they used the same type of setup to raise idle speed while the A/C was activated, to minimize stalling under compressor load.

Small engines have a fuel shutoff solenoid in the carb to shut off fuel when the ignition is turned off, and also some mostly off shore made cars use the same system. With the advent of fuel injection, the same thing happens, fuel is cut off stopping the engine fuel flow with the ignition switch.

[Kenth]

Run-on (or dieseling) occurs when throttle blades are too much opened.
Too much opened throttle blades may be due to a lean idle circuit or late ignition timing, or a combination of the two.
Check for nozzle drip at idle.
If you have a functional TCS, bypass this for full manifold vacuum to the vacuum advance.
Also, you may need larger idle tubes and more idle bypass air than you have now?

[jhein]

Originally Posted by Kenth

Run-on (or dieseling) occurs when throttle blades are too much opened.
Too much opened throttle blades may be due to a lean idle circuit or late ignition timing, or a combination of the two.
Check for nozzle drip at idle.
If you have a functional TCS, bypass this for full manifold vacuum to the vacuum advance.
Also, you may need larger idle tubes and more idle bypass air than you have now?

No TCS. I'm on full manifold vacuum.

When Archie had it on the dyno it was idling at around 950. When he shut it down it had no dieseling at all. When I picked it up it was idling around 550 when hot and I had to keeping giving it some throttle to keep it from stalling. So I bumped the idle up to 850. But, the fast idle was backed all the way out, so I basically had no fast idle. I did turn that up to the first step on the cam. That made the fast idle around 1100. I noticed that when hot, it still wanted to idle high. But if I blipped the throttle it will kick down. I never shut it off at high idle.

[25stevem]

100% timing has nothing to do with the issue since you have turned off the spark, I can’t believe your builder even said that!

Your chamber or your plug is running a tad too hot.
Your chamber is either to sensitive from being a hair too lean at idle or your plugs too hot.

First try opening up your mixture screws maybe 1/2 a turn more or until your idle rpm comes down by 50.

If this makes the idle too rpm too iffy then try one heat range colder of a plug

I have also seen unused threads of the plug that stick out into the chamber make for a hot spot which was causing this same issue until the offending threads where rounded over.

Of course another resolve if the car has a auto trans is to just shut it off in gear.

[78w72]

my 10.75:1 467 will run on occasionally on very hot days with the engine fully heat soaked. lower octane seems to make it worse, it will run OK on mid grade 89 for normal easy street driving but i use 91 straight gas whenever possible. youre lucky to have a 92 no eth fuel option, have never seen 92 around the midwest, its 87 with or without eth, mid grade 89 with eth, straight 91, or 93 with 10% ethanol.

i mostly fixed this by finally tuning the cliff built q-jet that i didnt touch out of the box, bolted on & it runs great aside from the rare run on. couple years ago i did the idle mixture tuning which raised the rpm about 100-150rpm, that allowed me to turn the idle screw down some & keep the same ~750-800 idle in park could go lower but in gear its a tad too low for my liking.

the idle speed is the main issue as mentioned above, adjust the idle mixture if you havent & maybe bump the timing up 1-2 degrees if theres no pinging, both will raise the idle, then you can lower the idle screw.

too hot of plugs or carbon build up can contribute to dieseling too, but a new motor shouldn't have carbon issues.

last resort if you cant seem to fix it, is to just shut the car off in gear if its an auto trans. not ideal but it works.

edit: tree'd me on the shutting off in gear while typing.

[jhein]

The carb is a Ray Klemm tuned Qjet (school of C. Ruggles). The only "tuning" I have done to it so far is what I mentioned above, adjusting the fast idle and idle set screws. Based on the above advice, I think the first thing I'm gonna do is turn the fast idle back out all the way, then adjust the idle set screw to the lowest I can get it without stalling and see what happens. If that doesn't work then I'll look at the idle mixture and spark plugs next.

Thanks guys.

[jhein]

Originally Posted by 25stevem

100% timing has nothing to do with the issue since you have turned off the spark, I can’t believe your builder even said that!

In his defense, I have to correct my initial post. The first thing he said was to turn the idle down and let it low idle for a minute before shut off. The timing thing I think was referring to effects that may have on chamber and plug temp.

[Jay S]

Your builder was correct on pulling the timing back. It is a crutch, lower the idle first, but if the engine is trying to run on, do not run anymore timing than it needs. Why it helps really has nothing to do with the ignition being shut off. I think it has more to do with less combustion heat under no load making if less able to run on.

You hardly have an issues. High compression 455 Oldsmobiles loved to run on, even with the switched idle circuit shut down. Those shuff offs were needed for the LOW compression 455s, the high compression the shut off barely touches the run on. I have shut them off, pulled the keys, got out of the car, shut the door, then walked away on my 70 455 Olds with it still running on. Only to get back in the car, putting the keys back in, turn it back to on, put it in gear, then get it to shut off. On a bad tank of gas in may even take a couple more times to get in to stop!

[jhein]

OK, so I backed off on the fast idle all the way (that's how it was when I got it). Then I set the idle set screw to 775 RPM. Idles fine there. Took it for a short ride to make sure it was fully at temp. Pulled in, let it idle for about 20 seconds and it shut right down. So hopefully that's that.

Thanks for all the advice.

[jhein]

For anyone who's thinking about that cam, this is what it sounds like at 750-775 RPM:

https://youtube.com/shorts/-wf39h1Vdso?feature=share

[Tom Vaught]

Was the video made after you did the latest mods to the carb?
Engine/Exhaust sounds really smooth.

Tom V.

[Schurkey]

Originally Posted by jhein

OK, so I backed off on the fast idle all the way (that's how it was when I got it).

Why does the fast-idle screw make any difference when the engine is fully-warm? It shouldn't be touching the fast idle cam at that point, because the fast idle cam should be rotated out of the way by the choke coil.

Originally Posted by jhein

Then I set the idle set screw to 775 RPM. Idles fine there. Took it for a short ride to make sure it was fully at temp. Pulled in, let it idle for about 20 seconds and it shut right down. So hopefully that's that.

Shouldn't need to "idle for about 20 seconds" before getting shut off.

[jhein]

Originally Posted by Tom Vaught

Was the video made after you did the latest mods to the carb?
Engine/Exhaust sounds really smooth.

Tom V.

Yep

[jhein]

Originally Posted by Schurkey

Why does the fast-idle screw make any difference when the engine is fully-warm? It shouldn't be touching the fast idle cam at that point, because the fast idle cam should be rotated out of the way by the choke coil.


Shouldn't need to "idle for about 20 seconds" before getting shut off.

Well, as for the fast idle that's a good question. When I had it turned up, when I would go into neutral at a stop, the idle was staying up and I would have to tap the throttle to get it down and it took a few seconds for it to settle in at idle. Don't know if something needs to be adjusted there or not.

[Goatracer1]

In almost 60 years of working on cars a TOO FAST idle is the cause of after running. If you are running a choke make sure it is opening all the way. If it is not, the fast idle cam is holding the throttle open. If your engine is so radical it won't idle at a low enough speed add an idle solenoid. You didn't block the heat cross over in the intake manifold did you? If you did the choke won't open correctly either.

[jhein]

Originally Posted by Goatracer1

In almost 60 years of working on cars a TOO FAST idle is the cause of after running. If you are running a choke make sure it is opening all the way. If it is not, the fast idle cam is holding the throttle open. If your engine is so radical it won't idle at a low enough speed add an idle solenoid. You didn't block the heat cross over in the intake manifold did you? If you did the choke won't open correctly either.

That's a very interesting post since I don't think this started until I adjusted the fast idle. I also don't know for sure about the heat crossover, but I think it is open. Supporting that idea is that this happens only/even when the engine is fully heat soaked. So I think that would argue against a crossover heat problem but it is a good question

[Singleton]

My GTO idles at 950-1000 and will run-on, so my procedure for shutoff is to put trans in 1st gear, hard on the brake, ease out on the clutch to drop rpm to around 500, then quickly turn key off. Engine shuts down immediately

[gtorich]

I do the same as Singelton, im at about 900-950 at idle, car only acts up here or there, i can go days without a issue, but when it starts to act up , i just let the clutch up a hair and turn the key, no problems then.

Rich

[mgarblik]

Just remember, it takes fuel and HEAT to keep an engine running. Spark is not needed. So, removing either will stop the engine. 90% of the time, (estimate), the issue is continued small amount of fuel into a hot combustion chamber by engine momentum, (fast idle speed), and continued fuel feed from the idle circuit. 10% of the time, it can be extremely volatile fuel, (low octane, high vapor pressure), or a hot insulated combustion chamber, (carbon build up, hot spot) BTW, timing can not be part of this equation because the ignition is turned off. But timing does effect idle speed if it's not readjusted after setting the proper timing. Retarding timing drops idle speed. This is the wrong way to address a run-on situation, (dieseling).