I don't think Matt has had any issues running a 300 with the stock setup before. Not only is the red X-piece different to stock, but the Y-piece is too, so it can all effect the end results, and those parts were not only designed not to be used with a restrictive airbox or power jets, but also to be used for a 250. I'd personally eliminate them from the equation.

The integrity of the seals and inlet manifold can be checked with the motor in place. The most crude way to check the left seal is with carb cleaner and the motor running. If it's sucking in air there and you spray carb cleaner behind the flywheel it will soon tell you if you have a leak.

If there is a leak, but it is more subtle, then you will need a leakdown tester. It can still all be checked without removing the motor though; even the crank seals can be replaced with it still en situ.
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!