Semi-fould spark plugs will cause a back fire.
Install new spark plugs.
Dirty carbs will foul spark plugs.
Untreated gas goes bad.
Drain the gas tank and put in fresh gas.
Every time you attempt to start the bike, the bad gas clogs the carburetors again.
After changing the gas, bring the carbs to a shop and have them professionally cleaned.
Cleaning at home will not clean all of the passageways in the carb.
The mixture screws are plugged from the factory to prevent tampering by owners.
The mixture screws must be removed to adequately clean the carbs.
The pilot jet openings are as thin as a pin head and are easily clogged with bad gas.
The throttle slide diaphragms get worn out over time and don't seal properly.
Carb cleaning is not a job for beginners.
There are many factors to take into account to get the job done properly.
Bring them to a shop.