Expert: Brad Sears - 10/4/2009

Question
A rebuilt stock carburetor was installed 6 weeks ago in my 1966 Oldsmobile Delta 88 with 425ci engine.  For three weeks it ran well, no dieseling when I turned off the engine.  However, a Mechanic installed the carburetor bolts with way too much torque, claiming that was the correct way.  After the initial three weeks, the engine started dieseling almost every time when I went to turn it off. I recently had him reinstall the carburetor, with an extra gasket, to the correct torque for the carburetor bolts.  That improved the acceleration some, and reduced the dieseling about half as often.  I'm wondering what to check for now, to eliminate the dieseling?  I forgot to have the Mechanic check the base of the carburetor for warping due to the overtightening.

Answer
Dieseling is almost always, 99%, of the time caused by the idle speed being set too high or a vacuum leak that causes the idle to be erratic when the idle speed is adjusted properly. With the throttle plates opened a little too far fuel is drawn into the hot engine on shut down and ignited by the heat. With the throttle set at factory specifications, which is 500 RPM in drive with the air conditioning on, the fuel drawn in on shut down is limited and the dieseling stopped. If the mechanic working on  your car believes that overtightening is OK then he may not want to use a tachometer to set the idle. But on this big engine it is important. If the car does not have A/C or it does not work, then the idle speed should be 550 RPM in drive.'
Brad