Different size jets in the X and Y pieces.

Again, applications vary, and you really need a dyno to tell whether they are helping or not. I can't remember if the jets are larger or smaller than stock to be honest, but they effect mixture pretty much across the entire rev-range.

I would imagine (and it's speculation) that you would be best off with the stock Y piece at least, if you are running the PJ's.

Andy.
NSR-WORLD.COM