Car keeps running...

[Red73BB]

I purchased a 1969 Corvette from a out of state dealer in June. It has a 350 that has been modified for high performance. I dealer would not get me the past owners name and so I do not have contact to discuss the engine mod’s. It does run very well
My problem is, after a drive the engine sometimes, not always, it will try to keep running after I turn the car off. I Run 93 octane, but I have heard that if you run too low octane cars will do this. I’m not a mechanic, so any help here is appreciated.

Thanks, Gary

 

[smokestack]

I would think maybe the timing is off. And maybe try different fuel. But the other much smarter than me guys will chime in

 

[Red73BB]

Thanks!

I can try a different gas station, but my thinking is (probaby wrong) that 93 octane should be about the same form a nationwide dealer.
Check the timing, now that is sometime that wouldn't cost much and should be easy to do.

Gary

 

[smokestack]

That. And plugs could be the wrong heat range. And some carbs have whats called a run on valve. I think thats what its called. To shut off the fuel. But it could be broke. I have heard carbon build-up can cause it as well.. But like i said. The smart guys will be on later im sure. And good luck to ya. Hope you can fix it easily.


Oh and on the gas station topic. I am a truck driver. And have some friends that drive tankers. There are a few stations that want 87 put into the 93 tanks. So you never know i guess. Seems hit or miss if or when the gas stations actually get checked up on to make sure they are legit.

 

[Red73BB]

Carbon build up,,,sounds like I need to put the pedal to the metal

 

[smokestack]

Carbon build up,,,sounds like I need to put the pedal to the metal

Get a bottle of seafoam.. Pour some down the carb and let it sit for a few minutes. Put the rest in the tank. Then put the pedal to the metal

If its built like you said. I would imagine the timing is too advanced. Easy fix

 

[Red73BB]

Thanks again

I not familiar with seafoam, I imagine that I can pick one up at the local auto parts store and try it out.

 

[Vettehead Mikey]

What you're experiencing is called 'dieseling' and was very common on early 70s smog motors. Even though the ignition is turned off, there is enough residual heat (hot spots) in the combustion chambers to ignite the fuel anyway and the motor just chuffs and pops for a while. GM incorporated an electrical idle solenoid that held the carb at normal idle setting with the ignition on, but allowed the carb to shut almost completely with the ignition off. This starved the engine for air and it stopped immediately.

Pouring in some 'miracle in a bottle' might work for little while if at all, but will not be a permanent solution. Can you post some pics of your engine or at least describe what set up you have?

 

[Rod75]

Vettehead Mikey explained the idle stop solenoid operation, which leads to the very basic and first thing that should be checked when you have engine run-on (dieseling). And that's the base idle on the engine. If it's too high, run-on can occur.
There will be one spec/step for the base idle adjustment, and another for the idle stop solenoid (if applicable). After that, I would check the base engine timing to be sure. If you make timing adjustments, go back and check/reset idle.
Unless you have a diff prob and/or compound prob, that should fix it.
If it were mine, I would give it an entire tuneup @ this time.
Good Luck

 

[lars]

Rod75 is correct:

Run-on after engine shut-down is caused by excessive idle speed or excessive throttle blade opening. GM solved this problem by using the idle stop solenoids so that the throttles would close slightly after the ignition was turned off.

The key, as noted in my opening statement, is to reduce throttle blade opening (which may or may not cause high idle speed). If the timing is retarded, or if the vacuum advance is not working properly (or you have any other engine tuning issues), you will be running an excessive throttle blade opening to keep the engine idling. With the timing correctly advanced (run 36 total), the throttles can be closed down to a minimal opening while still maintaining reasonable idle. If this does not solve the problem, install the idle solenoid and set up the idle speed drop on the carb's idle speed screw as outlined in the GM Service manual to cause the throttles to close on shutdown.

Lars

 

[catbert]

If the engine has a lot of miles, it may be deposits on the valves that retain heat and cause run-on. It was more common before fuel additives became pretty effective.

 

[Red73BB]

Thanks for all of the advice.

I will take some motor pix and post. Once again, i don't know the story on the engine and how many miles it has on it since being rebuilt.

It does have a Holley, but I don't know which one or how to tell them apart. hopefully the Pictures will help. It has 64K miles on the odometer, but who know how accurate it is.

What should the RPM's be idling?

Gary

 

[lars]

If the engine has been modified, the idle rpm will be the lowest rpm that will keep the engine idling consistent and reliably. Depending on the size of the cam, this can be anywhere from 650 to 1000 rpm. Regardless, run the lowest comfortable rpm at idle in order to reduce or eliminate run-on.

 

[Red73BB]

I have a picture on the "mystery engine" but I'm not able to upload - HELP!

 

[Vettehead Mikey]

PM sent!

 

[Red73BB]

Mystery motor

Finally, I uploaded this picture. now to resize

 

[njlouc]

What you're experiencing is called 'dieseling' and was very common on early 70s smog motors. Even though the ignition is turned off, there is enough residual heat (hot spots) in the combustion chambers to ignite the fuel anyway and the motor just chuffs and pops for a while. GM incorporated an electrical idle solenoid that held the carb at normal idle setting with the ignition on, but allowed the carb to shut almost completely with the ignition off. This starved the engine for air and it stopped immediately.

this seems correct.
I think some called it an "anti-dieseling dashpot"??
You had to adjust your idle with it in the on position then when it shuts it lowers the throttle to a stall position.
I think that since your car was "modded" that new Holley doesnt have one at all. It probably has just an idle stop screw.

see if you can get one and look for the OE wire that was disconnected.
It should be on with ign.

good luck and enjoy.

BTW click 3 times to enlarge the picture.

 

[Vettehead Mikey]

Can't see an awful lot in the pic, kinda dark and blurry. Certainly not anywhere close to a stock '69 in any case. Do you have the distributor vacuum advance hooked up to a full time source?

 

[Rod75]

Finally, I uploaded this picture. now to resize

Not the clearest pic. Simple observations:
Small block Chevy engine. Aftermkt valve covers, intake manifold and thermostat hsg.
Carb looks to be Holley 4150 series with the choke horn ground off.
That's the 1st time I've seen a dual carb fuel inlet line facing the rear. Kinda funky. Looks like a rubber fuel hose connecting the fuel pump hard line to the carb inlet line @ 180 degrees. That wouldn't pass on my car. I just wonder where the fuel filter is? Can't see clear enough....

 

[Red73BB]

Larger pix's

I resized to get closer to the max size.
Please let me understand the statement choke horn ground off and fuel inlet line facing the rear. So normally they face the front of the car? does this impede fuel flow? I will look for the filter tonight

Thanks for the help!!!!