Infrequently, I get what can best be described as a backfire from inside the intake manifold upon start-up. It is a very infrequent occurrence, sometimes months or more between episodes, and only seems to happen on cold start-up, after the car has been sitting for a long period overnight. What can be causing this, and any remedies ?

Thanks.

Mike

What type of injection/ignition system do you have? What year?

My first impulse is to think it's improper valve timing - caused by the timing belt being a tooth or two off on the cam gear (or the cam gear being a tooth or two off the alignment of the other rotating parts).

I think that the only way a backfire can come out the intake is for the intake valves to be open when the spark goes off, but I think that it can happen otherwise under the right conditions.

A bit more info will help us guess at the problem.

Good Luck

The car is a 1980 242 with the B21F engine and Bosch K-Jet fuel injection, Bosch ignition. It is not a chronic problem, only happens very infrequently, perhaps one or twice every 5-6 months, or even less. It happens only on cold start after car has been sitting for a lengthy period. As far as I know the timing is OK, and the car runs and idles perfectly except for these very random outbursts.

Mike

You might want to check the vacuum advance in the distributor.

The sliding plate connected to the diaphragm pulling arm could be sticking or dragging and not returning to a full retard position.

Pull different amounts of vacuum and change up the release speeds to see if if works smoothly. Especially near the beginning of the stroke outwardly. The return spring in the diaphragm may have gotten weaker or there is wear in the linkage, let alone any gummy substances over the years.

Phil