How do I cure run-on?
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How do I cure run-on?
I'm kind of stumped. All I did was replace my distributer and add a 1" carb spacer. I thought run on was caused by too much advance, but my old distributer I had set on 14btdc initial and this one I dropped in at 12btdc. I also backed it off to 10btdc but she still diesels like crazy for a good minute or two after I shut it off. Double checked the timing, fuel mixture, idle, and general tune of things, all good. What else can cause run on and how can I fix it? Thank you.
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Just for fun, next time you fill up, try premium.
Run-on is often caused by hot spots of carbon in the combustion chamber.
You could remove that with a trickle of water in the carb at high idle.
Just make sure not to hydro-lock it.
Also, if idle is set to high that could make things worse.
Run-on is often caused by hot spots of carbon in the combustion chamber.
You could remove that with a trickle of water in the carb at high idle.
Just make sure not to hydro-lock it.
Also, if idle is set to high that could make things worse.
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Thats true. The weird thing is that this just started after the swaps I mentioned above. Idle is at 750rpm like it always is. I just had the heads off, reconditioned, and new valves put in last summer. Went from never dieseling to hard core run on. Since I've actually retarted the timing slightly I'm confused ???
Posting Guru
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From: Evanston, WY
Since you have retarded the timing a significant degree (or degrees, ha ha) from where you had it before, I would say it is not a timing issue. Retarding the timing would cool the cylinder a little, which might eventually help, but once the key is turned off timing is irrelevant. The only other thing that you said you did is add a carb spacer. Could this be the problem? A vacuum leak potentially causing elevated cylinder temps?
Just a thought.
-Scouder
Just a thought.
-Scouder
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From: Montana
Don't the Ford distributor vacuum advances have an adjustment to set the amount of advance? Use an allen wrench in the vacuum fitting. Try unplugging the vacuum hose at the distributor, plug the hose by sticking it on top of the air cleaner stud 
See if this helps.
Did you correctly set the idle mixture? Find fastest idle speed with mixture screw, then slow down just a little. Do this with each screw. Slow down engine idle speed if necessary as you set mixture.
Is your throttle linkage letting the carb throttle plate close completely if you back the idle speed down to nothing?
See if this helps. Did you correctly set the idle mixture? Find fastest idle speed with mixture screw, then slow down just a little. Do this with each screw. Slow down engine idle speed if necessary as you set mixture.
Is your throttle linkage letting the carb throttle plate close completely if you back the idle speed down to nothing?
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Originally Posted by Scouder
Since you have retarded the timing a significant degree (or degrees, ha ha) from where you had it before, I would say it is not a timing issue. Retarding the timing would cool the cylinder a little, which might eventually help, but once the key is turned off timing is irrelevant. The only other thing that you said you did is add a carb spacer. Could this be the problem? A vacuum leak potentially causing elevated cylinder temps?
Just a thought.
-Scouder
Just a thought.
-Scouder
And a great thought at that. A vacuum leak just might raise the idle speed, with that air bypassing the throttle plates. A few years ago when I rebuild my old 2bbl I tried to reuse the old phenolic spacer. It was pretty trashed. When I started it up it had a higher idle and thing sounded like a cross between a tea kettle and a train whistle. A new spacer and better gaskets solved that.
Last edited by mlf72f250; Mar 28, 2004 at 11:12 PM.
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I'm thinking it has to be something that changed with the spacer, rather than the distributer too. Allthough idle is steady at 750 rpm and the vac gauge reports fuel/air mixture dead on I'm thinking that the problem must lie in those areas. Funny thing is, engine isn't running any hotter, but I guess cylinder temps can be different. I'll explore for vac leaks and fatten up the mixture some and let you fellows know what I find. Thanks for the tips.
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Elder User
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For there to be run-on, there has to be gas getting into the manifold.
Is the anti-diesel pot installed and working? Idle set at 750 RPM is too high. You're dumping too much gas into the manifold after shutoff.
You are supposed to adjust the idle with the set screw on the anti-diesel pot, not the set screw behind the carb. Then, when you turn the key off, the carb plates close - no more run on.
Is the anti-diesel pot installed and working? Idle set at 750 RPM is too high. You're dumping too much gas into the manifold after shutoff.
You are supposed to adjust the idle with the set screw on the anti-diesel pot, not the set screw behind the carb. Then, when you turn the key off, the carb plates close - no more run on.
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From: LAT 46.55501° & LON -122
Like Scouder said, once the ignition is off, timing is not an issue. You might actually want to try bumping it back up to 14°. This will probably cause your RPM to go up a little also; bring it back down at the carb to 700-750 RPM (the lower the better for run-in problems). The more your throttle plates are closed=less fuel to feed the fire.
Also, someone else mentioned making sure your linkage is not binding or sticky. Moving your carb up higher with the plate may have changed the linkage geometry enough to cause this to happen. Just before you shut it off the next time after a drive, try lifting up the gas pedal with your hand with about the same force it takes to push it down with your foot. If you notice a drop in RPM, then something is sticky.
If all else fails, and you are forced to live with the problem, try shutting off the engine while in gear (automatic tranny) or with a little clutch drag (manual tranny).
Also, someone else mentioned making sure your linkage is not binding or sticky. Moving your carb up higher with the plate may have changed the linkage geometry enough to cause this to happen. Just before you shut it off the next time after a drive, try lifting up the gas pedal with your hand with about the same force it takes to push it down with your foot. If you notice a drop in RPM, then something is sticky.
If all else fails, and you are forced to live with the problem, try shutting off the engine while in gear (automatic tranny) or with a little clutch drag (manual tranny).
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From: LAT 46.55501° & LON -122
The Anti-Diesel Pot scroob mentioned was not used on most FE's. Probably just the later ones with more emmision control stuff or some of the California engines? I know I have seen them on vehicles with air conditioning to bump the idle up a bit when it is on. They are basically a simple solenoid and could probably be easily retro-fitted. You will need to run a wire to an ignition power source and fab-up a mount to your carb. Should work!
Laughing Gas
Joined: Jan 2003
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From: Georgetown, Ky
I had a older local hot rodder suggest upping the octane in the fuel.
MY FE is also suffering from run on, only it isnt a consistant thing, Im wondering if the pertronix ignitor setup that was installed on the motor is going bad?
MY FE is also suffering from run on, only it isnt a consistant thing, Im wondering if the pertronix ignitor setup that was installed on the motor is going bad?
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Well, I know having your timing too advanced can cause or worsen run on, despite the ignition being turned off. But I agree that I don't think timing is the culprit here. I fattened up the mixture somewhat and while I'm still experiencing run on its signigicantly less, this tells me I'm fiddeling in the right area. I still need to check for vacuum leaks. Thanks for the continuing input. I don't have one of those anti-diesel pot thingies by the way.
Post Fiend
Joined: Feb 2001
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From: Missouri
Higher ignition timing will by nature more likely to cause detonation and run on due to higher cylinder temps BUT, if your timing is so retarded that you have to open the throttle blades past the point of the idle circuitry working then you will get run on.