hmm... you still have the end cap on? if the gas is going thru a pipe then enters into an open chamber then has to squish back thru the endcap that would cause restriction i would think. either put a solid pipe in there or just take the endcap off. idk just my 2 cents.
and lbdw, blasters are not crap, you just don't know how to ride. lol. j/k. they are kinda small but i still love mine. i have ridden a couple 4 strokes tho and i absolutely love yfz 450's. i know one kid that has one that is stock with exception of a slip on muffler, it is pretty quick, good power, and a good buddy of mine has one that was a race quad before he bought it, it has everything done, bored, ported, like $600 exhaust. that thing flies. push on the thumb throttle and it scares ya but you can't help but give it more lol.