Why won't my 2000 Honda xr50 r run it will start but shuts right back off?

Why won't my 2000 Honda xr50 r run it will start but shuts right back off?

Needs fuel.

Its got fuel and it wouldn't fire up w out. My guess was float valve needle. But stock carb is 13mm i want to put an 18-20mm w aftermarket air filter no air box and put a fuel filter on it

Put an in line fuel filter on and take the float bowl off and remove the idle jet and clean it out with a wire from a wire brush. That should do it.