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Backfire 5-6 sec after turning the key off. Help???

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My 1976 BMW 2002 backfires, or it sounds like it, about 5-6 seconds after I turn the key off. That is a long time, I am about half way out of the car by then. By this time, the engine has stopped rotating for 3-4 seconds. The car is carbureted. It does it about 60 percent of the time. It looks like the previous owner has disconnected parts of the smog control stuff. I did not see an answer for this situation in a forum search. Any suggestions? Thanks in advance.

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is a device that cuts off gas flow to the idle jet electrically when you turn the ignition off to prevent dieseling...but a backfire that long after shutoff is unusual...

Is the backfire thru the carb, or is it in the exhaust manifold?

Wonder if you have a hot spot in one cylinder--or an intake/exhaust port that acts as a glow plug, and it takes several seconds for a bit of gas to waft into contact with it, causing the explosion...

Does it occur at every shutoff, or just occasionally? If the latter, the hot spot may be only exposed to gas fumes when the valves are in a certain position as the engine stops...

Just my guesses...

mike

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adjust point gap to 0.016" (dwell meter is best - 60degrees)

adjust the ignition timing - "BALL" mark in the inspection

hole at 1600 rpm, with NON VACUUM at the distributor while checking

compression test - to reveal a abnormal high reading

from carbon or other mechanical malidy

check operation of carb's fuel shut-off solinoid as Mike said above -

it maybe leaking when off also causing gas feed when not wanted

condition of gasoline ?

all (1) bottle of TECHRON to a tank fill up - drive 250 miles non-stop,

and report back

smell your oil dip stick

smell like gas ?- how old is the oil ?

change oil and filter - gas fouled oil can contribute to your

post running pfarting. The oil is fouled because of

carb sickness

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