the seat on the exhaust valve is dirty so the valve does not seal it.... not sure how to clean it
if it only backfires when shutting off, usually this means its getting to much gas!! if it backfires will running adjust carburater.
Most single cylinder failures on B&S twins are due to electrical failures. Check ignition diodes, coils, and key switch first.
Is it a 4 cylinder? Sometimes a 4 cylinder engine will shake a little extra when the AC is running and it's at an idle. You might be able to step up the idle a little extra but 4 cylinder engines are notorious for shaking when the AC is running.
If any car backfires it is usually due to retarded timing, running lean or a problem in the valve train.
running lean can cause your 400ex to backfire ..if it backfires on deceleration its lean ...if it pops on acceleration its running rich.====carb issues
check the battery with voltmeter without the engine running then check with it running if voltage is higher it is working
No, I recommend go to a mechanic, chances are your firing order is out or running bit to much fuel
Needs a tune up? Bad timing?
Take a look at an article in the Chalotte World about Jack Stratton - he's a Charlotte resident that lost custody of his ten children. Also running for Mayor.
Briggs and Stratton...ask if you need help getting it running!
It may just need a tuneup of Plugs and wires....Or your coil pack maybe Failing.
If it backfires after you shut it off then it is probably running too rich. Any unburned gas the that is exhausted out and into the muffler will explode with a backfire. With a 1990 model you won't have the anti-backfire solenoid on the bottom of the carb like the newer ones have. If it backfires while it is running it might be out of time. Check the keyway between the crank and flywheel to make sure it is not partially sheared.