Though all backfires may seem similar, all are not alike, and a variety of problems can ultimately cause a backfire. Two common types of backfires exist; one that occurs inside of the engine, and one that occurs in the exhaust pipe itself.
Improper Fuel Balance
In order for your scooter to run correctly, the balance of fuel to air must be just right. The type of backfire that occurs within the engine is caused by the fuel mixture containing too high a proportion of air, while the exhaust pipe explosions are generally caused by fuel mixtures containing too little air.
Leaks in the air injection system are a common cause of exhaust pipe backfires. When this occurs, fuel that has not burnt will suddenly explode. These types of leaks can occur when there is a fault with the air intake valve, or if it sticks. Similar problems with the gulp valve can yield the same effect.
If the explosions in the scooter engine are improperly timed, it could result in backfiring. Each vehicle has a unique timing adjustment method, but you can usually correct these problems fairly easily. A good way to check if this is the problem with your scooter is that a vehicle with improperly timed explosions will not idle.
Fuel Filter and Injection System
A poorly maintained fuel filter can also cause backfires. If the filter is not changed enough, or if it becomes clogged, backfires can occur. A poorly operating filter can lead to low fuel pressure, which causes backfires. Likewise, a fuel injection system performing improperly can cause backfires by failing to burn fuel completely.