dieseling

[surfersmurf]

car is a 79 trans am 400 4 speed
pontiac 670 heads
comp cam 280H
cam specs (http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1196&sb=2)

rebuilt the carb with the recommended parts for my engine. at first i was not able to get the car down below 1250 rpm so i went back and fallowed recipe 3 and chapter sixed my carb. now she starts good and runs good. i was able to get the idle down to about 750-800. cliff recommended 950 with the cam i have. it sounds nice deep and lopey (is that even a word) at 750. now i am dieseling when i shut the car off. the engine is not getting overly hot (about 190) but it was kinda creepy when i pulled the key and she kept on running. i cant remember what the timing was set to when i got the car but i will have to check. any other things i should look at?

[73 Z28]

The three things I'd check would be the base timing of the engine, the number of turns out from fully seated on the idle mixture screws and the thickness of the base gasket. Any one of these can contribute to dieseling (engine run-on).  Post back with what you find.

Paul

[surfersmurf]

the turns out on the idle screws are 4.5 out. the inlet hole for them has been opened up to .090 so i dont need as many turns out. the base gasket is 1/8. i dont have a timing light but im gonna go to my buddies house on tuesday weather permitting and get it checked out. 

[73 Z28]

To verify that your idle mixture screws are not set too lean, install a vacuum guage to one of the carbs vacuum ports and adjust both screws the same amount until you get the highest vacuum reading, (note: once they are adjusted, you'll have to readjust your curb idle speed). To help prevent fuel percolation in the fuel bowl,which can lead to dieseling, a thicker 1/4 base gasket will help. Lastly, once you check the base timing, try retarding the timing setting by about 2 degrees. These things should help to eliminate the dieseling.

Paul

[surfersmurf]

i replaced the the throttle return springs today, there was a nice hillbilly spring on there when i got the car. now the problem has stopped. but i did call down to cliffs and they said i need more advance. my timing right now is 15 at idle and 30 total. if i go to the recommended 36 total then i end up at 21 on idle. i feel that the car could still shut down better but if i cant get the timing better than what it is now should i replace the distributor?

[73 Z28]

If your idle quality is good at 15 degreees advanced, but total timing is only 30 degrees, then I would seriously consider an aftermarket performance distributor. Increasing your base timing to 21 degrees to get 36 total probably will cause the dieseling to return when you shut down the engine. A performance distributor such as a pertronix will allow 24 degrees of mechanical advance allowing you to set your base timing to 12-15 degrees advanced and your total timing will come in at 36-38 degrees. This is just one example as there are plenty of good companies making performance distribs that will do the same.

Paul