Engine running on

[preaction]

I have a new to me engine that appears to be a 390 with CJ heads a 80's Comp 268H cam and unknown compression  but I believe it may be somewhat high the timing is at 15* initial and the vacuum advance is working. This is being called a 2000 mile motor the problem is its running on after turning off the ignition could high compression be the cause of this ?

[fastback 427]

Could be too rich. What plugs are you running?

[preaction]

Carb is a rebuilt Holley 600 for a 67 390GT this is its first use, I will check the plugs shortly. I only mention high compression as  the only info that came with the motor was it was run with "race" gas. Plugs are a Motorcraft BRF 42

[blykins]

What's the idle speed?

Are you running an electric fan?

[My427stang]

As Brent pointed out, make sure idle is low enough.  Having the throttle plates too far open allows air in, combined with heat, it will autoignite (diesel).  Sometimes it's as easy as turning the idle down 50-100 rpm

Additionally, although 15 should be plenty, going UP to 16-18 if total allows, may let you close the primaries more and get idle under control.

[preaction]

Brent, no electric fan all factory set up although the fan blade is a tad smaller than a factory piece without a clutch. When I first discovered this happening it was after checking and setting the timing and idle speed in this case only had to check as it was at 15* and this is a stick car so I was able to get the idle down to 550 which is the factory setting.

[preaction]

I got the idle down to 500 and may be able to go lower but when I turn off the key it still runs on and has the feel of not stopping unless I intervene.  I looked at the secondary butterfly and its looser than I would like (rotationally) if the vacuum diaphragm doesn't hold the butterfly tight (closed) is there a spring that handles this on a constant basis ? Im not seeing anything obvious that would be supplying fuel the float levels are spot on new and correct return spring on the primaries, could bad bushings on the throttle shafts contribute to something like this ?

[My427stang]

Does this thing have an MSD box that needs a diode?  500 rpm and closed throttle blades doesn't seem like a dieseling situation. 

Did you happen to check TDC on the balancer?

[Lenz]

Also, have you read the plugs?  Know they've been mentioned but if you're running a little "fat" then dieseling is possible even with the low RPM setting, it doesn't take long to put down some carbon deposits.

[preaction]

No MSD it has a points distributor and I didn't check TDC I will do that today and a new set of plugs.