Its very likely the spark plugs are now carbon-fuel fouled and will need to be replaced.
Blower motors tend to like a spark plug that is slightly cooler heat range than
what you would using in a stock motor. 2 to 3 heat ranges cooler.
Racing plugs are not used on the street. Too cool .. Hot plugs are not used. Too hot (detonation, priignition)
Ignition faults and vacuum leaks must be eliminated. blower intake gaskets.
When the timing curve and carbs are set up right the plugs will no longer foul and the
backfireing and poor drivability will be cured.