My car makes a gurgling sound

Question:

My 1965 Mustang has a 1966 289 short-block that was installed in 1978. I never drove it much until now that I’m retired in a small town and the total mileage on this motor is 25,000 over the past 35 years. It starts very quick.

Going through the gears (a 4-speed), there is no hesitation or roughness. But when I let off the gas while in gear, the motor doesn’t actually backfire, which is a loud bang in my thinking, but it makes what sounds like a series of muffled backfires. (I’m having a hard time coming up with a clear description of this noise. It’s almost like a gurgling sound.)

I have a Pertronix system installed in the distributor. The plugs, plug wires, cap and rotor were changed about 3000 miles ago. I add one ounce of Alemite CD2 lead substitute to 10 gallons of gas and also add ZDDP to an oil change. I don’t know what other “clues” to give you and considering that I’m not a total gearhead, can you offer some insight to this?

Answer:

The long and the short of it is the engine is running too rich, or the timing is off, and as a result the engine is not burning all the fuel going into it. Consequently, excess unburned fuel is igniting in the exhaust system, and that is what is causing the muffled backfiring.

First, with a dwell meter or a timing light, make sure the ignition timing is correct. If that doesn’t solve the problem, make sure the carburetor mixture screws are correctly set, and if that doesn’t help, check the carburetor for dirt and wear. If need be, install a carburetor rebuild kit.