I recently acquired a 1983 F150 with a rebuilt 351 windsor. I runs great, drives down the road fine, and if its cold, fires up within the first crank. But when it gets to temperature and i go to shut it down, i turn the key off and it sounds like the engine is still turning over, it doesnt fire, but the truck will shake a little and you can hear the engine clunking over. Sometimes it will do this for a good 10 seconds before it stops, sometimes it doesnt do it at all. Also, when it is hot, it is a lot harder to start, and takes a few cranks. Could the the problem shutting down be something with timing? it is a rebuild and i am unaware of weather it was a shop rebuild or just some kid in the garage. Thanks, Zac.

 

I have always heard that running-on and hard starting when hot are symptoms of the timing being too far advanced. If you have a timing light you really should check it because that running-on can be hard on an engine.

 

Run on can also be caused by a fast idle. That, combined with hard hot starts, makes me think you should check your choke to make sure it's full open at operating temperature.

 

Plugs too hot, mixture too lean, compression too high, fuel octane too low, and/or timing to retarded.

 

Another thing i forgot to add, it has msd ignition box on it. could that cause the plugs to get too hot? also when it is in park,it idles around 1100 rpm, and when its in drive it only idles around 700. could that surge in rpm when i switch it to park and shut it off cause it? Thanks guys i appreciate all the advice!

 

How many miles on this 'rebuild'?

Your idle speed is way too high unless you have a hot cam.
Try closing the curb idle stop a little.

What plugs do you have?
I hate those MSD boxes on street vehicles with a passion so I can offer no advice there.

 

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BruteFord had a lot of good suggestions (but I have found that it's usually timing too advanced that causes run on issues. Check all of the things he said.

MSD has a publication for dealing with "run on" due to the way their box is wired with vehicles using an external voltage regulator (like most of our trucks). See MSD Ignition

However, more likely the problem is due to your high idle speed in neutral. If you have a performance cam or carb installed you may need to keep the idle speed high in neutral to avoid stalling the engine when you put it in gear. If you shut the truck off while it's in gear, does it stop running immediately? If so, you just need to install a throttle position solenoid that maintains your higher idle speed when the key is On (so the truck won't stall when you put it in gear) and when you turn the ignition key Off the throttle will close slightly preventing your run on condition.

See: http://www.summitracing.com/search/P...-and-Brackets/

 

Run on issues are tough.

The MSD thing is a wiring issue common to MSD boxes caused by electrical bypass, the power isn't shut off so it will run like you didn't turn the ignition off, not what is described.

Timing, the root cause is something hot in the combustion chamber acting like a glow plug, the engine is quite literally dieseling. Very advanced timing can potentially cause this but it's more likely to cause hard start(early fire/kickback) and knock. Late timing however can tend to overheat the exhaust valve so it acts as a glow plug, this combined with lean mixture and a 3 angle valve job makes it worse.

Next is a question for the OP, when it's hard to start hot, does it crank slower? Or normal speed cranking but just doesn't fire? Does it ever backfire?

Lean can make it both hard to start when hot cause well it's lean, it will crank fast but not fire, however will start easier with a little throttle and tend to make it diesel as it runs hotter causing the chamber to be hotter.

Too much timing can tend to make it harder to start when hot and can contribute to dieseling. But in this case the hard start will be cause the spark is too early pushing the piston backwards against the starter so it will crank noticeably slower.

That there is so much difference between in gear and park idle speed tells me it's more of a tuning issue, that or a vacuum leak.

My first suggestion on the tuning issue, did you adjust the idle mixture with it in gear, few do? With an automatic transmission to get the correct idle mixture it must be adjusted in gear, be careful you don't run over yourself. Set the brake, block the tires, and/or have a friend sit in the vehicle and hold the brakes. Then lean it out until it slows, richen until you have max RPM/Vacuum, adjust idle speed and repeat, then in most cases another half to 3/4 turn(with AT) to rich will give additional throttle response if it stumbles.

 

it does have an edelbrock performance carb on it. And when it starts hard, it does occasionally crank slower, but most of the time it cranks at normal speed. ive tried shutting it off in gear and it does turn off immediately, the truck has never backfired for me, it runs perfect, its only starting and shuting it off im having problems. Its hard to test these, because like i said, it doesnt happen every time i shut it off. But i definitely have some more tests to do. Thank you so much everyone thats helped.