315C V8 run on when shut off

digithead

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I am really frustrated with this 351C run on. It was rebuilt by the PO so I dont have specifics whats in it. Does have a cam for sure. Has about 9"Vac at idle. I have read tons of posts on here about things to try and believe I have tried most all of them. I have leaned idle mixture out many times to no avail. I have gotten idle down super low like 500-600 rpm, just chugging. I have made idle screw speed to low to run then advanced timing (to 22, 20, 18, 14 10) etc to raise idle back up. Nothing has really helped much EXCEPT putting idle screw stupid low and then timing at like 22 to get about 700-800 rpm. THis helped the most but the car will ping when you accelerate. Just not good performance with distributor in this position. The DAMN thing runs too damn good and WONT STOP RUNNING! I am thinking of going to that idle cutoff solenoid, where 12V holds it on and when you kill car it will snap carb shut? Should I?

 

delawarebill

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sounds like that cam is a good size looper, with 9" vac is kinda low. all the info u provided is what i would have done too.. my 302 was camed also. it don't like cold starts. maybe the sol is the way to go.. but the low vac is due to valve over lap and ain't nuttin to do but pull cam to get that up..

 
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You don't want to idle the engine too low with a higher lift cam it will cause premature wear of the lobes and lifters, beats them to death and lack of lube from throwing from the crank turning slow. That is why when breaking one in you keep the rpm up. It might sound mean to lope but it will not last long.

I would put the solenoid on they all had one when new for that reason to cut off the air and stop run on.

You did not say mileage on the engine but sounds like it is fairly new so I don't think carbon build up on pistons is part of the issue. On older engines that only get short runs carbon builds up on the pistons raising the compression along with getting hot enough to keep firing the engine under compression or diesel.

We use to squirt kerosene in the carb while running and it was thought to knock the carbon loose and clear some of it out. Don't know if it did or not really. Good long run on interstate will burn it off also.

 

c9zx

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What carb and intake do you have?

 

Don C

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I would check for a vacuum leak. A cam that just pulls 9" typically won't idle at 500 rpm. Check all of the spark plugs for anything that doesn't look right.

 

72HCODE

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also you want to retard timing when dieseling (run on) not advance timing.

seems like you have a massive vaccum leak. or something is wrong with your steering column ignition switch.

with engine running stable and normal idle rpms place hands over the carb intake and choke the air the engine should die if it keeps running listen for the sound of air sucking into the intake from the leak.

remove all vacuum accessories and plug the intake manifold vacuum ports.

9" vac is REALLY low. use a vacuum gauge and see if vac rising with all accessories removed and capped off.

besides leaks at mounting points like carb and intake manifold base the leak can occur at:

vacuum brake booster(blown diaphragm)

vacuum brake booster check valve(reed valve blown)

Vacuum advance on distributor (blown diaphragm).

vacuum modulator on automatic transmission if equipped, usually the hose rots off from the intake manifold.

PCV stuck open or incorrect pcv for engine.

EGR if equipped.

Vacuum reserve for A/C system if car equipped with A/C

Vacuum reserve for A/C system blown one way reed valve.

Carburetor secondaries open too far if you have a 4 barrel.

carb idle air bleeds open too far.

possible broken ignition switch.

does the engine run on when cold? you start it up and try to shutdown after 30 seconds of running.

does it only do it when hot?

 

barnett468

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hot pulugs and too high of a float level will contribute to this problem and so will an engine that runs on the hot side like maybe 200 degrees or more.

a compression test would also give us more info.

 

TommyK

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What carb and intake do you have?
+1?

Also have you verified TDC on the balancer. Perhaps the timing isn't what you think it is.

Some other good advice in this thread.

 
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Jeff73Mach1

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I run a solenoid to help mine shut off cleanly. It works very well, but when really hot I'll occasionally get a second of run on. Slight retardation of timing helped, as does using some 110 octane gas mixed in with every tankful. On 93 octane it will rarely do it, on 91 it will do it with regularity. My engine is at 11.5-1 compression.

My recommendation would be to add the solenoid, check the float levels and consider timing your engine by vacuum to see what the max you can obtain is. A vacuum leak will also add to the problem.

 

AOCUSNRET

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Don't know if this would help or not but might be worth looking into. On one of my older cars, 80 vette, it had a HEI ignition with a MSD module and a 1 wire alternator. I had the same problem with a run-on with the ignition off. Turned out that I needed to install a diode on the alt wiring. Fixed my problem. Hope you get fixed. Let us know what it turned out to be.

 

digithead

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also you want to retard timing when dieseling (run on) not advance timing.

seems like you have a massive vaccum leak. or something is wrong with your steering column ignition switch.

with engine running stable and normal idle rpms place hands over the carb intake and choke the air the engine should die if it keeps running listen for the sound of air sucking into the intake from the leak.

remove all vacuum accessories and plug the intake manifold vacuum ports.

9" vac is REALLY low. use a vacuum gauge and see if vac rising with all accessories removed and capped off.

besides leaks at mounting points like carb and intake manifold base the leak can occur at:

vacuum brake booster(blown diaphragm)

vacuum brake booster check valve(reed valve blown)

Vacuum advance on distributor (blown diaphragm).

vacuum modulator on automatic transmission if equipped, usually the hose rots off from the intake manifold.

PCV stuck open or incorrect pcv for engine.

EGR if equipped.

Vacuum reserve for A/C system if car equipped with A/C

Vacuum reserve for A/C system blown one way reed valve.

Carburetor secondaries open too far if you have a 4 barrel.

carb idle air bleeds open too far.

possible broken ignition switch.

does the engine run on when cold? you start it up and try to shutdown after 30 seconds of running.

does it only do it when hot?
Thanks for tips. The engine only does it hot. I can start it for 20 seconds and shut it off fine. Does that help?

 

digithead

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hot pulugs and too high of a float level will contribute to this problem and so will an engine that runs on the hot side like maybe 200 degrees or more.

a compression test would also give us more info.
I have checked float per Edelbrocks specs and thats spot on. Engine runs dead on at 180. New thermostat and new gauge and new temp sensor too from AUTOMETER. I forget plug part number. I did put in all new ones when I got it this winter. AUto parts store looked it up and gave me stock ones.

 

digithead

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What carb and intake do you have?
+1?

Also have you verified TDC on the balancer. Perhaps the timing isn't what you think it is.

Some other good advice in this thread.
Edelbrock 4BBL carb 650 CFM and weiand intake. Have not verified balancer dont know how to that, so yes I am trusting the number on it.

 

digithead

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Don't know if this would help or not but might be worth looking into. On one of my older cars, 80 vette, it had a HEI ignition with a MSD module and a 1 wire alternator. I had the same problem with a run-on with the ignition off. Turned out that I needed to install a diode on the alt wiring. Fixed my problem. Hope you get fixed. Let us know what it turned out to be.
thanks. I understand why that can be a problem. I think its only on single wire alternators. Mines not upgraded.

 

72HCODE

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what is current initial timing?

Are you running ported or full manifold vacuum on the distributor advance?

when the engine is warmed up and diesels on shutdown is it hard to start also?

a vacuum leak is still high on the list to check for, due to low vacuum and run on.

i would still go down everything i suggested to rule it out.

get a vacuum gauge and hook it up to full manifold vacuum and get a base reading. then plug all vacuum accessories and compare readings, choke the intake with your hands and see if you can get the engine to stall out, if it keeps running find the leak either by sound or spraying some carb cleaner around and listen for the engine rpms to pick up.

another issue can be your spark plug and ignition wire choice. after market wires can cause artificial timing advance and massive carbon build up inside the heads that would increase compression and make run on even worse. it can also cause very low vacuum and lots of carbon build up on plugs.

one question would be, did your engine ever run correctly while you owned it and what changed since then? you might need to go back to basics strip off any funky aftermarket stuff and install Ford oem ignition to go back to a known baseline and rule out issues.

but before all that you want to 100% sure you don't have a vacuum leak somewhere.

 

TommyK

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What carb and intake do you have?
+1?

Also have you verified TDC on the balancer. Perhaps the timing isn't what you think it is.

Some other good advice in this thread.
Edelbrock 4BBL carb 650 CFM and weiand intake. Have not verified balancer dont know how to that, so yes I am trusting the number on it.
To verify the balancer you need to put #1 cylinder at TDC on the compression stroke (both valves closed) and see if the timing pointer is indicating 0 on the balancer.

If that looks good I would suspect that you have a vacuum leak as others have suggested, and in order to get it to idle you are over exposing the transition slot which often is the cause of run on. To test this theory, set the initial timing to something reasonable like 14 degrees BTDC and the idle speed to around 800rpm. shut it down and remove the carb and see how much t slot is exposed under the primary throttle blades. If it is more than about .040" that is probably the problem.

Are you running a pcv valve?

 

digithead

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what is current initial timing?

Are you running ported or full manifold vacuum on the distributor advance?

when the engine is warmed up and diesels on shutdown is it hard to start also?

a vacuum leak is still high on the list to check for, due to low vacuum and run on.

i would still go down everything i suggested to rule it out.

get a vacuum gauge and hook it up to full manifold vacuum and get a base reading. then plug all vacuum accessories and compare readings, choke the intake with your hands and see if you can get the engine to stall out, if it keeps running find the leak either by sound or spraying some carb cleaner around and listen for the engine rpms to pick up.

another issue can be your spark plug and ignition wire choice. after market wires can cause artificial timing advance and massive carbon build up inside the heads that would increase compression and make run on even worse. it can also cause very low vacuum and lots of carbon build up on plugs.

one question would be, did your engine ever run correctly while you owned it and what changed since then? you might need to go back to basics strip off any funky aftermarket stuff and install Ford oem ignition to go back to a known baseline and rule out issues.

but before all that you want to 100% sure you don't have a vacuum leak somewhere.
Thanks man, I'm on it!

 

digithead

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I did recheck vac at idle yesterday. About 11". I tried both ports on carb. Made diff as far as RUN ON problem goes. I checked for leaks couldnt really find anything. Used carb cleaner spritz as test as welll. Cupped both hands over carb and it stalled pretty quick when starved for air. See pixs. I put on the AC idle solenoid from EDelbrock. Kit is #8059. Pretty pricey for little solenoid. Runs about $160. Simple 12V deal. Apslaply 12V and it extends. Does not have the oomph power I thought it would. I thought when +12 applied it would extend (with engine off key on) and bump up idle. Doesnt work that way. It has enough balls to hold throttle "up" (eg not close all the way) but it cant bump up on its own. So basically you go to key on, usually pump 2-3 times anyways. once you've "pumped' you lifted all pressure off it and its full extended. Then your foots off pedal and throttle arm is in its "idle rest spot" if you will. SUPER EASY to set idle now with 1/2" wrench from drivers side. Can easily do it with air filter on. Much easier then before with screwdriver. When I turn car off solenoid closes throttle slams completey shut and she dies. I got my +12 power for solenoid right of +12 hot of coil nearby. FYI, BEFORE I INSTALLED solenoid, I got bright light and backed the idle stop screw fully out so it was not making contact. So I KNEW that throttle body was fully closed and not getting any air via that path and only mechanical opening movement would ALL be from this new solenoid.

carb1.JPG

carb2.JPG

 
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