04 400 won't rev up
A common reason for back firing is water in the gas. I agree with Williebee that the carb needs a good cleaning but before you tear half the machine apart I would try putting some dry gas in as well as carb/injector cleaner in the gas tank. Try spraying some carb cleaner directly in the air intake, also clean the air filter.
If that doesn't work then you have to do as Willibie said and take the carb off and clean it.
A new spark plug wouldn't hurt either.
Start with the simple free/inexpensive stuff first. If that doesn't get it then take it to the next level. Take the carb off and clean it and since you have the tank off and stuff might just as well do the valve adjustment at the same time and pop in a new spark plug. Also at this level dump the old gas out and if you want clean the inside of your gas tank.
If none of this helps then the next step is electrical (ground wire, spark plug wire, battery condition etc) , then mechanical failure (timing chain, magneto, CDI etc).
90% chance it is only bad gas or dirty gas line system, or carb related (needle, jet, float etc)