Hi I have a stock 70 Z M21,when I turn my motor off it wants to keep running, I think it is called dieseling ? I have set the timing to the factory spec. after my engine gets gets up to operating temp and the choke goes off, my rpm drops to about 900. What should I check ?
Thanks
John

Hi John, this sub forum is more of an original parts forum, your post will probably get an answer in the engine forum here:

http://www.nastyz28.com/forum/forumdisplay.php?f=12

Good Luck!

Rich

Start with quality of gas. An 11:1 engine needs lots of octane, and it should have lead in it. Lack of octane will cause dieseling all by itself.

Dieseling is often caused by idling the engine with the primary throttle plates partialy open instead of with them closed and idling on the idle circuit.

Make sure the idle screws are set for correct idle. Then check the timing and make sure it isn't retarded. Retarded timing causes slower idle speed, and usually people will compensate for the slow idle by turning up the idle screw to partially open the primary circuit.

Your idle speed seems a little high. It should be about 600-650 rpm with a manual trans. Having this too high will cause run-on. Also, you might want to set the initial timing a little higher than the factory spec for better performance. Something in the 10-12* range usually works well for an sbc. Good luck.

Thanks for all your help guys.
John

My 70 Service Manual calls for idle speed of 800 rpms for the Z28 with manual tranny. They call for 8 degrees initial timing, but if your gas has enough octane, you should be able to add a few more degrees over that. The 10-12 degrees warped suggested should be a good place to start.
The radical cam of the Z28 isn't going to idle right below that factory setting of 800, so I'd go no lower than that.

How many miles are on the engine? Its possible to that there is carbon buildup in the combustion chamber, and its glowing hot when you shut down, causing your problem. We own a few old tractors that would do this (gasoline powered ones) with 4 cyclinder engines. It was kinda funny, it would just keep going, timed it once, thing ran for 30 minutes just on dieseling alone. Just when you think it was going to shut off it would get a big boost and keep chugging, with the occasional backfire through the muffler due to excess fuel not being burned.

I've seen a lot of old trucks due this as well. Try running some spray cleaner through it, usually involves warming up the car and spraying it into the carb until it chokes it and dies. Let it sit for whatever the can says, 30 minutes or so. Start it up, take it for a good run, work it a bit, should clean it right out. New plugs would probably help to. As everyone else says, also turn down your idle with a flat screwdriver to a lower setting, that will help some as well.

If that doesn't help, you can always put the car in 4th gear and when you shut it off, just let the clutch out easy, resistance will slow the motor to a stall.