Redneck intake or straight to carb?
 

I've had a Uni on my 300 xrx for a while now. I just attached it straight to the carb, like on a motorcycle. Is this wrong? Do I need an intake pipe? What size pvc would I need if I have to go that route?
I've seen filters attached to the carb, but I read somewhere that you need like c 6 inches of intake?

That length of pipe between the filter and carb is to reduce tubulance and organize the air flow resulting in a larger volume of air entering the carb. Without the pipe a rich, unstable condition occurs due to lack of air. I see no reason why the same principle won't work for the 300. Try it and see.

CN250 and CF250 engines don't need the intake plenum. Your xrx300 has the cylinder horizontally like a GY6 instead of vertically, but I don't think that should make a difference.