QuestionQUESTION: 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
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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?
AnswerVacuum 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
