Annoying problem. I have a 1969 Coupe with the original motor crated up and a ZZ4 motor installed. With this setup the motor ran perfectly and everything was fine.

This past summer I decided to do some "enhancements"- namely some AFR 195 Street Eliminator heads angled milled to the ZZ4's stock 58cc for around 10.0 compression. Also included was a Magnum 280HR single pattern cam from CompCams, a Barry Grant double pumper, and a dual electric fan setup from SPAL to keep things cool on the stock aluminum radiator.

So I get everything set up, timing curve dialed in (MSD) 14 degrees initial 36 degrees total. Set the idle to 700-800 rpm (I have A/C with no throttle solenoid) and used an Innovative motorsports wideband air/fuel sensor to tune the carb.

Motor runs great (idles at 11 in. vacuum), however, if the motor has warmed up it has a tendancy to "Run-On" or Diesal after shutdown. (It does it about 90% of the time) Even runs backwards sometimes for several strokes... Only if the motor is warm though... Only does it about 5-10% of the time on a cold motor.

Heres what I've checked: timing, electric fan "noise" coming through the ignition, and carb tuning. I've also verified the engine runs around 180 degrees with a thermal gun. I've even re-done the valve lash twice.

So, I'm kind of at a loss... I'm currently running spark plugs from NGK with a heat range of 5. Will going a step colder help, maybe pull more heat from the chamber? Any other solutions? I could pull the RPM down a hair to around 600 and ultimately get a throttle solenoid for the A/C, but I really dont think it should be doing this at 800 rpm.

How bad is this really for the engine? Should this problem be priority one.. as in don't drive it until its solved, or just keep playing with it until its fixed?

Thanks for your help,
Trace

 

Your primary thottle blades are likely too open at idle which uncover the transition slots. Run your vacuum advance to a manifold source which will increase your idle speed. Now you can lower your idle speed at the carb, which will close up the TB's and prevent fuel from being sucked into the engine as it shuts down.

Good luck!

 

I did a search and found this on dieseling.

The problem was that there were enough carbon deposits built up in the cylinders that they continued to ignite the fuel / air mixture after the ignition was shut off. They behaved similar to what you describe.

The cure was checking the timing, fuel cleaner and driving them hard enough to clear out the carbon hot spots.

Also make sure the throttle plates are closing all the way down on the idle screw when at idle, no binding, etc.

 

A8skyhawk:Look at this thread maybe it will solved your problem
http://forums.corvetteforum.com/show....php?t=1872133
Avner

 

I appreciate all the help, idle stop solenoid it is...

Trace