Motorcycle Repair: 98 xr400, rubber diaphragm, fuel mixture

Question
i just got this bike and it is back firing. it was dogging out when i first gave it gas, i had the carb cleaned and it is not doing that ant more but seems to be backfiring worse

Answer
Shayne,

-usually backfiring is caused by a lean fuel
mixture. That means your idle or slow jet
is too small (try a #62) or you have an air leak around
the carb rubber where the carb attaches to
the engine.

-Also check the rubber diaphragm on the carb air valve for holes.

-Make sure your exhaust pipe is not loose
on the engine allowing air to get in.

-What happens is a lean fuel mixture
is hard for the engine to ignite so the
unburnt fuel builds up in the pipe and then fires
when air hits it in the hot pipe.