Hello Tom:
Backfiring on overrun usually implies a overly lean mixture and is often
due to vacuum leaks. It could result from a leaking exhaust manifold
gasket. The other possibility is the air pump. On many cars - I don't
know about smogged MGs - there is a device which prevents airpumping on
overrun. This could be the problem.
John McEwen
>My recently-acquired '77 Midget has a backfiring problem when the
>accelerator is released suddenly and the engine is winding down (is
>"overrunning" the correct term?) Not being much of a engine mechanic, I'm
>not real sure where to start, and the material I can find suggests about
>25 things to check. Guess what I'm looking for is, if not a short cut, at
>least some prioritization.
>
>The engine has a Weber downdraft carb and Mallory dual-point ignition. The
>air pump is still installed and functional. It starts readily and runs
>nicely, other than the backfiring.
>
>I assume backfiring is caused by an accumulation of unburned fuel mixture
>in the exhaust manifold which suddenly ignites? Wouldn't that suggest that
>the engine is getting more fuel than can be burned normally, and wouldn't
>that suggest too rich a fuel mixture? The only adjustment I can find on the
>Weber carb is what I assume to be the idle mixture screw at the bottom of
>the carb. How does one adjust the mixture?
>
>There's no smog check requirement here in SC and no vehicle inspection.
>Should I remove the air pump? Might it, by supplying nice fresh O2 to the
>exhaust manifold, be contributing to the backfiring?
>
>Sorry for the "rank newbie" questions.
>
>Tom
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