Big trouble guys...
I've got a relatively new 347 stroker... lately it's been dieseling to the point that it blows the passenger side valve cover gasket spewing oil everywhere!! The idle is about right and the timing is at 12 degrees. Seems to run fine until I shut it off. Any ideas?
Thanks!!

I would check and make sure the balancer hasn`t spun as it sure sounds like the timing is off. It also could be the fuel mixture is excessively rich which has caused carbon build up. Need more build info such as the compression ratio, intended usage, etc.

Street use...10.5 to 1 Edelbrock 800, Performer RPM manifold, Trick Flow heads.

Takin' the balancer off as we speak... thanks for the idea!!!

GM added an anti-dieseling solenoid to the carburetor linkage on early 70's cars when car makers were forced to lower compression ratios and run the motors hotter to meet Federal emissions laws due to the Arab Oil Embargo and pressure from "green" people. The idea is to close the throttle blades completely when the ignition is killed, so that no fuel/air can enter the motor, therefore no dieseling is possible. When the ignition is re-activated, the solenoid pushes the throttle blades to the idle position and all is good. A google search for "anti-dieseling solenoid" will yield sources for the solenoid and bracketry to mount it.

Thanks, but it ran fine before... it's not a stock motor anyway, but thanks!!

Here's a diagram of operation....
http://www.tpub.com/content/constru...s/14264_137.htm

800 cfm is way too big for 347 cubic inches. A 650 or 700 would have served you better. The compression ratio is also on the high side. With that much compression plus too big a carb it`s quite possible you have a carbon build up issue. I would run some water out of a bottle into the carb while giving it throttle or run it low on gas and pour in a can of seafoam, wait 15 minutes then do some wide open throttle blasts.

I thought it was big too, but the techs at Edelbrock said that's what I need... anything over 400HP they said to use the 800. They said it draws differently than Holley. If I went with a Holley a 600 or 750 would be appropriate. Took out the plugs and they looked fine... took off the balancer, and it's it's not spun, took off the distributer, the gear is fine... I'm stumped!!

Check the mechanical advance- clean it real good and be sure it's free to move- it sounds like it's sticking advanced.

Be sure the inside of the cap isn't carbon tracked or cracked. Check in the dark under the hood for ignition wires cross firing. Are the plugs' heat range too hot?

Does the engine diesel every time, or sometimes?

Does temporarily retarding the ignition have any effect?

Brand new wires, new distributer (HEI), using the recommended plugs, retarding helped a little, still diesels, but at least it's not real bad... I'm stumped... like the other guy said, it's a big carb (800), but that's what Edelbrock recommended. I've got a 650 laying around... maybe I'll try that tomorrow. Seems like it's running rich though (really smoked up the garage), but the mixture is lean... put a vacuum gage on it, leaned it out till vacuum drop, then richened it up a quarter turn or so. It's basically a brand new engine, so it's not carboned up I don't think. It runs REAL hot when it's not moving also. I'm still stumped!!

Brand new or not- check that the centrifugal advance is freely operating, that there's no way it can hang up at all.

If you start it up cold and immediately cut the engine- no warm up- does it diesel? Or does it need to run for a while (how long?) before it diesels when shut off?

I'm wondering if there's a hot-spot that's formed in a combustion chamber. Might be a good time to throw a can of your favorite combustion chamber cleaner at it. People say Seafoam is good...

Also, despite the plugs being the recommended ones, the fuel blends have just changed around the nation- so there can be a fuel related issue that didn't exist just weeks ago. Something to consider.

Another thing to try, is to completely back off the curb idle screw until the blades are shut tightly. Start the engine (with it in whatever condition that it needs to be in to diesel- warmed up, whatever) and let fully off the gas and turn off the key. This mimics the way an anti-dieseling solenoid works, w/o having to actually install one. Does this help or does it still diesel?