The Stovebolt Site Forums The Shop Area Engine Shop Dieseling (run on)

My 292 runs sweet but, Sometimes it will run on for about a two seconds after I turn the key off. Am I running to much advance? Its about 4 Degrees BTC. Idle speed is at 700 RPM. I did all my tuning using a vacum gauge. It does not do it all the time what gives?

If it,s a manual trans. slow thw idle down a little. Check for vacuum leaks. The Monojet carb. can wallow out the gasket between the throttle body & fuel bowl. You usually can see it from the outside. The timing doesn,t effect that unless it,s way off. You can usually run 6-8 advance on those & it will run better. If it pings set it back a little. If your spark plug heat range is to hot it can cause that. If you can,t fix it get a idle selenoid from a 6 cyl. car with air. Hook it up to the ignition switch. Set the initial idle to about 500 & set it with the selenoid to 6-700. When you shut off the key the selenoid will let the throttle almost close.

I'm sure someone much more knowledgeable than me will come along shortly, but can I ask a couple of questions?. What type gas are You using,(octane), and could it be possible You have some carbon buildup?. Maybe time for a little Marvel Mystery Oil.

You have something causing a hot spot. The timing makes no difference since the plugs aren't firing with the ignition off. acwhite is on the right track by suspecting a carbon build up. Setting you idle down can help sometimes but to FIX it you need to eliminate the hot spot that's acting like a spark plug when you turn the key off.

Engine just rebuilt. has about 600 miles on it. I use 87 Chevron or Shell. I lowered the idle speed down a tad and it seemed to cure the problem but,....it still deisels every so often maybe once a week. Its a daily driver. I checked for vacum leaks there was a little one at the base of the carb so I fixed that. I like the idea of adding an idle solenoid so I'll be looking for one. I run AC RT44 plugs. Maybe too hot?

No those plugs are not too hot.

How do you use the Marvel Mystery Oil? Do you just dump a bit in each of the cylinders and put the plugs back in and let it sit for a few hours?
Drew

Some people get out in the country where the smoke wont get out the fire department, and pour some down the carb while its running. It will smoke likes its on fire, so make sure Your not in downtown Burbank. :p

I have used Seafoam to clear carbon-take a well warmed up engine, put a can of Seafoam down the carb or thru a vacuum line, adding the last little bit quick enough to stall engine. Let sit 15-20 mins, then restart & rev engine, I like to drive them. they will smoke like Hades for a while, but it will stop. his is nearly identical to the "top end cleaning" done by GM dealers. brian

I like seafoam to. Used it on quite a few of the ol trucks. Seems to work well to. Just make sure you arent downwind when you fire her back up!

Jeff

If you just have a few miles on a rebuild I doubt you have a carbon hot spot. Sometimes a nick or burr in the piston or cylinder head can become hot enough to ignite the fuel, causing dieseling. Like a glow plug in a diesel. Unfortunalely the only way to check that is to pull the head unless you have access to a bore-a-scope.

I'm inclined to agree with Wrenchbender Ret.
Your throttle blades are simply too far open at idle. Recheck the dwell just to make sure it's OK. Set the timing up a couple of degrees as Wrenchbender said. That should increase the rpm a little. Reset the idle speed screw to bring it back down, then reset the idle mixture. Good chance you'll have to reset the idle speed down again.

An idle solenoid is for a smog equipement burdened '70s engine or a small 4-cyl with not enough power to handle a AC pump at idle.

While it is true that a hot spot can ignite fuel air mix and contribute to deiseling, if there were no fuel air mix it wouldn't matter. Long way of saying, get the idle speed down and the problem will be fixed. Even a normal engine can do this after a hard workout on a hot day IF the idle is too high. Run on was a big problem in the early days of smog equipment because of higher temps and high idle to run A/C without overload or overheat. The idle soleoics mentioned were and are the solution, if just dropping the idle to 550 or so doesn't do it.

Ok Ill bite,

Technically speaking when no power is to the plugs, as in ignition off, what ignites the fuel mixture if not a hot spot? No power means no spark. No spark means no igniting the fuel. Hence no dieseling. I would think 600 miles is way to soon on a rebuild for it to have carbon spots. Why does the idle speed affect this?

Please enlighten me. Jeff

Hy rustednuts, as the guys have said if the idle speed is down and hence throttle plate is closer to being closed little or no oxygen exists for combustion to take place no matter what the ignition source is, hope that helps.

Ignition with the ignition off is caused by latent heat of the head/block plus the heat of compression,(think how hot your air compressor head gets).. That's why it happens on engines that have been run hard, like towing on a hot day. The head temp is probably around 250 and coolant is 240 or so. Slowing the idle reduces the amount of air/fuel and reduces the heat of compression. If you have ever worked with diesels you know how hard they are to start with low batteries, same principal, slow revs lower heat of compression.

running on caused by,,,,idle speed to high,,late timing,,,carbon buildup in cyl...doc

Great news! first I want to say thanks to all for your help and input. I was under the hood today and bumped the timing up to 6 degrees BTDC kicked the idle speed down to 600 RPM and now I'm good to go. No more run on and it idles so smooth and quiet you can barely here it run while sitting in the cab. I guess the 292 likes the advance.