Hey all,

Lately I've been having a little trouble with a 390 in the F100 not shutting off when I turn off the ignition. I think it's called run-on or something. Shut off the key and it coughs and sputters for a second or two then finally dies. It is intermittent. Does it most of the time when it is warmed up.
First of all, what is going on with it? Second, what's the fix for it?

It is a recently rebuilt .020 over, nearly stock (little rv cam) 390. Pertronix ignition. timing set about 12 BTDC. Runs real good, just won't always shut off like it should.

Thanks
gw

 

It's called run-on. Essentially what's happening, is you've got a hot spot somewhere in the combustion chamber or on top of the piston, that is lighting off the mixture. It's probably carbon build-up. If it's a little rich it can do this by causing too much carbon build-up. Also too much timing can do this. I'd advise you to set your idle mixture with a vacuum gauge to where you get the highest reading, then read the plugs and jet your carburetor accordingly. You want the end of the grounding strap to be light tan; not black or white. Once you get that set right, go burn the carbon out of it with a couple full throttle runs. If that doesn't get it, pull a couple degrees of timing.

 

Thanks Rusty,
Interesting. I know that my idle is a little rich. I was getting a backfire under hard deceleration, so I richened up the idle mixture a bit and it solved the backfire prob. But I guess that I created another one. I'll give your suggestions a try & see how things turn out. WRT the timing, this old beast doesn't run too well below 10 deg. adv. I'm hoping that I don't have to back it off too much. We'll see how it goes...
gw

 

What Rusty is saying is right on the mark, but if that does not fix the problem check your vacumn lines. I had a small split in one of my vacumn lines on one of my trucks and cause the run-on.

 

Rusty is right on. Also, the more rich your idle, the more chance that some part of the mix will ignite under compression.

In my case, my 390 was pretty bad at doing it. Had a Duraspark I retro-fitted. I changed over to an MSD, w/the base MSD coil (stock size) and the Duraspark dizzy. Cleaned it up a lot.

Next, I did some funky things with vacuum advance and manifold vacuum (don't go there for your application), and that really solved the problem.

Really, never any more runon. And that was with 11:1 compression and a huge cam.

Your particular problem just too much rich idle.

There's also another thing to think about:

Are you running a 180 degree thermostat? And is the cooling system working correctly?

 

I have found that by simply dropping the idle by a "few" RPM usually stops the problem. Although I agree with Rusty on the combustion chamber hot spots, I would have to disaggree on the timing issue. Since the ignition is off when this is happening, the amount of timing advance should have no effect. I believe people get the wrong idea that retarding the timing is the cure; what they might not realize is that retarding the timing often lowers your idle RPM at the same time. If you like where your timing is set now, it is best to lower the RPM at the carb. It may actually be better, if possible, to increase the timing advance (raises the idle RPM) and then lower the RPM back again to where it was using the carb's idle screw. You will have the same idle RPM as before, but your throttle plates will be closed tighter--less fuel to feed the fire.

Many vehicles in the mid 70's-80's used a solenoid at the carb's idle screw activated by the ignition switch. When on, the solenoid would push the idle screw to its normal operating RPM. When the ignition was shut-off, the solenoid would retract, thus closing the throttle plates tighter; something you might want to consider if the problem persists.

In the meantime, try shutting the the truck off with the transmission in gear (automatic) or slight clutch engagement (manual in gear). The little extra load on the engine is often enough to keep it from "dieseling".

Kris

 

Reducing the timing will reduce the amount of heat transferred to the combustion chamber while the engine is running, preventing the hot spot from forming in the first place. But it is the last thing you should change, and I listed it as such.

 

I think it's a good point on the thermostat and cooling system. I'd make sure you aren't overheating....if you are getting too hot and you don't find anything else then your timing could be set too high. Assuming you are not overheating then I'd pull the plugs for a look at them. As everyone has noted, the root cause is hot spots in the chamber and it's usually carbon buildup. You can see the signs of this on the plugs. One thing that hasn't been mentioned yet is the possibility that the carbon buildup is from too much oil getting into the chambers...FE's have been known to pull oil in through the intake manifold gasket when they aren't sealed properly and also through the valve guides because of worn guides, damaged stem seals, and or oil pooling on top of the heads causing the oil to get above the top of the guides and then getting pulled into the cylinder...I know these things because I've had so much hands on experience with it....LOL. At any rate, if it is because of oil then the engine most likely would show signs of smoking.

Good Luck!

Tracy

 

Exactly. Too much timing WILL cause hot spots in the combustion chambers making it more likely to detonate and "run on".

Been there, done that, didn't get a t-shirt...

 

Morning Men,
I got a chance to tweak the motor this morning to address my run on issue. First I checked the idle, it was pretty close. I leaned it out 1/4 turn. Then I backed off the timing to 10 deg. Took it out for a spin, ran it up to about 3500 rpm (70-80 mph) then let off the gas and pop-boom-bang. Major backfire. Got home and shut er down and it seems, so far, that the detonation has been reduced, if not eliminated. I need to drive it some more to be sure, but looks promising.
Now to the backfire. I had richened up the idle to reduce the backfire and it seemed to work. but then the run-on showed up. Was I wrong to use the idle mixture to address the backfire? Will the backfire cause other issues? (I think that I already know the answer to that one)
All in all, this motor runs pretty strong. There are just some little tweaks that I'm sure that I'm over looking.
Thanks for all you guys' help already.

gw

 

What carb is it? If it's a Holley, you might want to check if the power-valve diaphram is blown. This would dump lots of gas into the motor and cause that backfire.

I assume when you say "backfire" you mean out the exhaust? Or out the carb?

 

It's backfire out the exhaust. It's a Holley carb but, I don't think the power-valve is blown.