Expert: Brad Sears - 7/29/2009

Question
QUESTION: Hello. I have a 1966 Oldsmobile Delta 88, 425ci, with 2-barrel carburetor.  I had replaced the carburetor last month with a remanufacturered matching carburetor.  After that, the engine ran smoothly, turned off well, and accelerated well.  However, I was concerned when the mechanic over-tightened the carburetor mounting bolts to maybe 3X normal torque, and I have to redo them.  Today, after I've driven a few miles, when I turn off the engine the engine diesels, has small backfires, and/or spews exhaust back through the carburetor (the air filter is a little darker now).  When it diesels, I switch the key over so the engine runs again, and then try turning it off again.  It often takes 2-4 times before the engine goes off smoothly.  After driving for more than 20 minutes, the engine stops smoothly.  The jets in the carburetor haven't been adjusted.  Only adjustment I made to the carbuertor was the idle speed, which had the engine running to slow and hesitantly when cold.  I turned the idle up just enough to smooth out the idling when cold.  I can start the engine fine.

ANSWER: It sounds like you have a vacuum leak perhaps between the carburetor and the intake manifold. The dieseling is usually caused by an idle speed set too high or a vacuum leak letting too much fuel and air into the engine during shut down. I would also address the cold idle and running problem as a choke problem and readjust the choke and the fast idle adjustment and then get the idle speed turned down for the hot idle.
Brad

---------- FOLLOW-UP ----------

QUESTION: First thing I did on the Olds today was replace the dirty air filter.  I suspected it was choking the engine too much, creating a richer than usual air/fuel mixture.  I also have been driving the car on only many short trips (2-5 miles) this month, so I suspect the spark plugs are fouled and need cleaning.  I took a short drive today, and turned off the engine in gear.  The first time the engine turned off smoothly, and the second stop it very mildly dieseled for 2 seconds.  Third stop- engine turned off smooth, and back at home 2 second dieseling.  I'll check the spark plugs first, and test drive the car after (engine cold).  If it still has dieseling I may bring it back to the shop that overtightened the carburetor bolts.  I'm wondering if the over-torquing of carburetor bolts could damage/warp the carburetor base, and how they could test for a vacuum leak between the carburetor base and manifold?  Soap bubble film around gasket, or vacuum line pressure reading?

Answer
Vacuum leak testing is simple. With the engine running use a can of spray carburetor/choke cleaner. Spray the cleaner on the area that you suspect as having a leak. If the engine speed changes, there is a leak. And yes the carburetor can be distorted by overtightening or the gasket spacer between the carburetor and the manifoled can be damaged.
Brad