ok.. the 15mm manifold is too big for that engine... it dumps too much fuel into the piston... I found that when I went from the 15 down the 14 or even the 12 it has alot more pull and power! Save the 15 for your kit! The 15mm carb is fine though! you need the airbox.. with or without the right jet... Those engines dont like unrestricted air-flow to the carb I tried the K&L filter too and the stock airbox will give you more power!