Thread: engine slightly running on

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When I run my engine in my garage and let it completely warm up and then go to shut it off it always ends up firing a couple more times maybe for 3-5 seconds. It is very crude combustion and only a few cylinders are firing,(so I don't think it's the ignition) but it is enough to keep it running for a couple seconds.
The only reason I'd like to fix it is because it kind of makes the engine seem like a piece of junk when I shut it off and it coughs a half a dozen times afterwards.
I haven't yet gotten the thing on the road and really gotten to heat it up good. . .is it possible that I just have some deposits I need to burn off that are igniting the mixture?

Make sure your throttle plates in the carb are closing all the way. Could be you've got the idle too high, the spring is placed wrong for a full pull, or might be too week, a bind in the linkage, something that holds it open just a bit (doesn't take much)

the throttle is returning to it's idle set properly.
When this is happening it is after I've finished warming it up and have the choke fully off, so it's not still on the fast idle cam.
just regular idling like you'd do at a stop sign.

Okay, let's try it a different way.

In you original post you were thinking that deposits (presumably glowing) were the problem. In order for them to be a contributor to run on they would need fuel to ignite. In order for fuel to get from the carb bowl to the combustion chamber you need air flow. If the throttle plates are completely closed there won't be air flow to siphon fuel from the carb bowl and through the rest of the intake system to the combustion chamber. There are the tools, you need to do the diagnosis.

My throttle is set for about an 800 rpm idle, so the plates are just slightly cracked when there is no input on the throttle cable.

Would it even idle if they were totally sealed?

Is there something wrong with the way I have this set up?

Now I'm kind of confused. The engine is turning over so when the ignition stops it is still pulling through the carb, but can't light it off. For some reason mine is still lighting it at idle with no ignition from the plugs.

This was a problem with the early 70's cars, so much so that the manufacturers (GM at least) installed a solenoid (called a dashpot) to operate the throttle blades. When the key was turned off, the dashpot was de-energized and allowed the throttle blades to go completely shut. When the key was turned on, the dashpot was activated and opened the throttle blades to the idle position.

What may be happening in your case is that there may be some carbon in the chambers that is glowing from the heat of combustion which is acting as a glow plug to ignite the little bit of air/fuel mixture that is still flowing past the blades.

but the idle speed screw is a screw that holds the throttle open a tiny amount. If I backed off that screw until it totally bottomed out the engine would slow until it stalled.
so mine is open just a little. that's how the carb book said to adjust idle speed.

Originally posted by DennyW
Would it even idle if they were totally sealed?

Answer: yes. The idle circuit is the idle circuit. That means with the throttle plates closed.

If the plates are cracked open, you are no longer just on the idle circuit.

The first crack open goes from (base) idle circuit, to transfer circuit, then to main circuit.
Decrease the idle speed 50 at a time until the throttle plates are closed, and at the same time, adjusting the idle mixture screws. That is there purpose to supply enough fuel/air mixture. Thus idle mixture screws.

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this is true, dennys just taking for granted everybody would know what hes talking about. when GM first put the elec. sol. on their cars it was because, everybody wanted ac, but you had to idle the crab. up so hi. with the ac on, the smog motors didn't won't to cut off, because you were to far into the carb. transfer circuit, so this was the fix. when you tuned a motor with the elec. sol. on it you disconnected the sol. and adj. the carb. with the idle screw and air screws down to 400 rpm's. then you pluged the sol. up, and adj. the sol. to 700 rpm's in gear and ac on. the 400 rpm's came by adj. the air screws with the betterflys closed. with the ac off, car in park, if everything is right the motor should idle. at 400 rpm.

Originally posted by lt1s10
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this is true, dennys just taking for granted everybody would know what hes talking about. when GM first put the elec. sol. on their cars it was because, everybody wanted ac, but you had to idle the crab. up so hi. with the ac on, the smog motors didn't won't to cut off, because you were to far into the carb. transfer circuit, so this was the fix. when you tuned a motor with the elec. sol. on it you disconnected the sol. and adj. the carb. with the idle screw and air screws down to 400 rpm's. then you pluged the sol. up, and adj. the sol. to 700 rpm's in gear and ac on. the 400 rpm's came by adj. the air screws with the betterflys closed. with the ac off, car in park, if everything is right the motor should idle. at 400 rpm.

thanks for all the replies guys. I just got back from vacation, that's why I hadn't thanked anyone yet (went to Hawaii).

I ran my engine last night for the first cold weather start up (40 degrees). It was a little tough to start and popped like all hell at first. Plus the idle was all screwed up since the last time I tuned it was when it was 85 out.
One things's for sure, my vacuum wiper motor is no good.

Originally posted by tcodi
thanks for all the replies guys. I just got back from vacation, that's why I hadn't thanked anyone yet (went to Hawaii).

I ran my engine last night for the first cold weather start up (40 degrees). It was a little tough to start and popped like all hell at first. Plus the idle was all screwed up since the last time I tuned it was when it was 85 out.
One things's for sure, my vacuum wiper motor is no good.

glade to see you had a good time. i dont know what, but it sounds to me you have more problems than just idling. do you have a choke? it should not pop like all hell no matter what. did you say what carb. you had. i believe your timing is off still. do you know for sure where TDC is?

yeah, I checked tdc back when I had the heads off and it was dead on with the balancer. I'm using an edelbrock 650 mechanical secondary and mechanical choke. I set the timing with a light and found it idled best at 15 degrees static. I'm using the manifold vacuum for the advance.

There was never any popping back when it was warm out. I just heard some popping coming out the exhaust when it fired up and then a little during warm up. I think my idle was super lean though since I had last set it in summer and it was even a little lean in summer too from when I once looked at the plugs.

It wouldn't even idle with no choke until I gave both idle mixture screws a full turn richer.