Check your timing and your vacuum advance.
it is the carb. my uncles did the same thing. you need to have them cleaned and have them put in sync
spark plug gap, gas/air mixture on carb.
try to dial Dow your your gas intake on your carb, if that dosent work maybe your timing is off
That is caused by too much gas getting into the engine after the key is turned off. If the mower is new enough to have a anti-backfire solenoid on the bottom of the carb then it must not be working. If it is an older mower and doesn't have one then the carb needs adjusting to back off the amount of gas.
if it backfires it could be timing or else you have it flooded from trying to start it. You didn't say if the engine is carb or fuel injected. If it is carbed, let it sit for awhile to let the gas drain out of the carb. If it is fuel injected then it is probably timing. year make and model of vehic and millage
bad gas,timing off,valve sticking,carb. need adj.,faulty spark plug, vacuum leak,dist. cap or rotor crossed plugs,bad egr.,
if it only backfires when shutting off, usually this means its getting to much gas!! if it backfires will running adjust carburater.
Time for a tune up.
You have no gas going to your carburator.
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.
could be vent hole in gas cap is plugged,but more likely is carb is clogged.gas with ethanol tends to return to starch stage and plugs carb . so open carb, clean with carb cleaner
It backfires because if you are going high speed and then suddenly let off the gas the extra gas that is flowing in has to come out some time another way it backfires is that something on the exhaust or carbeurator is loose or is missing something.