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I have a 1988 300 trx with a recently replaced carb (approx. 2 yrs) that has started to backfire more than normal while riding in semi rugged terrain,( a lot of holes and extremely uneven). Any ideas what would cause this?
 

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You replaced the carb 2 years ago? And now its backfiring more than normal? What's normal? It should not backfire at all unless there are other modifications. Has the exhaust been modified or a different exhaust installed? Has the intake been modified in any way? (Just installing a K&N filter is a modification)

Generally backfire in the exhaust is caused by the fuel mixture being lean during deceleration. You didn't specify if the backfire was during acceleration or deceleration, but most backfiring is during deceleration. Therefore, you need to enrichen the air/fuel mixture at closed throttle. This is accomplished by turning the fuel screw out about 1/2 turn in 1/4 turn increments to alleviate the backfire. Another thing that can be done is to turn the idle up. If the exhaust is a modified or a performance unit, it's gonna backfire. The best you can hope for is to reduce the backfire by enrichening the mixture and turning the idle up.

If this didn't help or if your backfire is during acceleration, let me know.
 
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