My engine runs on after I shut it off. Temp is good, car runs 180 all day. Gauge is good. Checked engine with temp gun. Not over heating. Idles at 700. 355, edelbrock heads, intake and roller cam. Hooker headers.

 

Usually the timing is set too retarded and you fix that by turning the idle screw up so it will idle. Set the timing more advanced and turn the idle screw down.

 

I will try advancing it a little. Thanks.

 

How long has this been going on, and does it coincide with the engine upgrades you mention? If you haven't already, try running a higher grade gasoline in the engine. That may solve the problem.

After the cam was installed, did you adjust the idle speed screw to keep the car running? If so, you may have adjusted it in too far. When that happens, the throttle plates are opened too wide and when you shut the engine off, its inertia can still pull and air / fuel mixture into the cylinder. Heat build up in the warm engine is enough to randomly fire the mixture. This can also be solved with higher octane fuel.

I would check the throttle plate position when the engine is warm and shut off. If it's open too far, and you're using premium gasoline, then you may have to install an idle stop solenoid. Good luck and I hope this helps.

Maj D

 

I had that problem on a motor many years ago and it was cause by the wrong heat range of spark plugs. The glowing tip would keep the motor running

 

Off-the wall suggestion: Use a volt meter to make sure the distributor isn't receiving some voltage even with the key turned off (just in case there is a wiring problem).

 

 

New motor. It was all put together at the same time. I adjusted the idle and mixture screws when I started the motor. Also using high test.
Thanks.

 

I will check everything you guys suggested. Thanks.

 

Do you have an electric cooling fan? When I had my Cobra, some guys with electric fans had a strange problem where the freewheeling fan, after the motor was shut off, would feed electric current back into the harness, keeping the engine running.

 

Interesting, heard the same about some msd type systems
mine can run on no matter where my timing idle is or what carb i have on it

When i ran a simple ignition system and manual fan never happened wonder if theres some merit to that

 

Wow! You guys are good. I think about every possible thing has been covered.
If running a MSD on cars with external volt regulator you must put a diode in the number 4 wire at the reg. to stop power from feeding back from the alternator spinning even after the key is off, keeping the MSD powered. Sometimes the engine will slowly die, and sometimes it will just stay running if the diode is not installed. The diode only lets current flow in one direction.
As someone already mentioned the carb can only be turned up at idle to a certain point. On Holley there are two vertical slots just under the throttle plates. If the plates are opened so that more than .040 of those two slots is exposed then the carb starts to pull fuel from the main circuit as well as the idle circuit, and it can cause run on from this added fuel.
The solution would be to change the idle air bleed size (if possible) to a smaller size so it will pull more fuel with less throttle blade angle getting the transfer slots back within the .040 tolerance.
I have never dealt with a cooling fan causing this, but I suppose it is possible, and could be corrected with a diode.

 

Where's your vacuum advance hooked to?

 

No vacuum advance.

 

Have you considered an idle stop solenoid?;
http://www.google.com/url?sa=t&rct=j...e8PV7iar_pak9w

 

I know in the early 70s they had a solenoid that would engage the ac compressor when the key shut off to help drag the engine to a stop preventing run on.
Not sure how it worked exactly but some cars and trucks had them.

 

Why would I put that on and I( have a Holley 750 on the car. I have never used one.

 

Kind of an odd question. To control the dieseling of course. You can set your idle to what works well for you on this stop, then set the idle screw on the carb so the throttle blades close completely when the stop is not energized. Then when you turn off the motor it actually shuts off. Did you even read the thread?

 

That's probably 90% of the cause right there.

 

I think it may have been mentioned, but the most common cause of run-on is insufficient timing, usually followed by idle speed set too high (or a combination of the two). Often, they are both in play, as insufficient timing can lead to idle speed being too high.

The throttle blades in in your Holley may be set too far open, allowing more air in and encouraging run-on when you shut down. Make sure your primary transfer slots are set up correctly, and that the throttle blades between primary and secondary bores are set up to allow approximately the same amount of air through.

If advancing timing (which can also be achieved by running manifold vacuum advance) then lowering idle speed to the slowest acceptable idle does not cure it, at that point an idle stop solenoid may be required. Sometimes engine combinations are just prone to this.

Most often, it can be cured by finding the right initial timing and/or idle settings.

As an example of a more difficult case to solve, in one of my motors, I initially had pretty bad run-on. After adjusting timing to achieve enough advance at idle, it stopped most of it, but after a long run it may still happen occasionally.

I removed my carb, adjusted primaries and checked that the transfer slots were set correctly. Then I held it up to light, and adjusted the secondary openings to match the primaries. I used the light as a visual indicator to help me estimate the secondary blade position (I adjusted to allow an equal amount of light through all 4 bores).

Reinstalled carb and adjust idle slowly to 850-900 RPM by tweaking both primary and secondary blades equally. This helped and did improve the symptom, but still had an occasional run on.

At this point, I added an idle stop solenoid. To set this up, I removed carb and rechecked throttle blade settings. I adjusted the solenoid plunger to keep primary blades open to the correct position when energized (square transfer slots). Then I set the primary blades to close completely when the solenoid is not energized - in short, the solenoid sets my idle speed when on, but when off it completely shuts off the flow of air.

This solved my run on completely. Most don't need to go this far, but in this case, it was the best solution for my combination.

AS mentioned above, a milder motor like yours may only need timing the initial adjustments to resolve the problem. I just thought my experience may be useful to someone else out there.