I don't know the carburetor diameter either, but get a MotorSaver air filter that will fit it when you find out. They're great! You won't believe the increase in engine performance once you put it on.

Also, NEVER run your truck without the air filter on it! Trust me, one minute little particle of whatever will kill your engine. My dad and my grandfather were working on an old Glastron jet boat the other day, and the carburetor started leaking, and to see where it came from they ran the engine without the air filter (or air resistor, as it is called on this engine). I came outside and saw what they're doing and gave them some grief about it... Being only 14, they blew me off a bit, but I made them put it back on with force! Luckily, we had no more problems with it; turns out the engine was flooded.

What a familiar characteristic... Hah!