Hard to start after new piston.

Just put in a new hi comp (13/1) piston in and its verry hard to start especialy when the bike is warm

I have a 05 YZ450.

Jetting is from 175 to 172 main, 45 pilot.

Before the piston swap it was also hard to start when the bike was warm, if the bike was cold it was verry easy, blib the throttle twice and 1kick it would run.

I have a open airfilter system,mega bomb header and now a hi comp piston from wiseco.

Any sugestions?

Thanx in advance, Ron

I think your main jet is bigger than it should be unless you are using some kind of heavily oxygenated fuel. Other than that, hard to start warm, easy to start cold makes it sound overall rich.

Be sure the idle mixture is correct:


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