FMF Ti Powercore

I'm running the FMF Ti Powercore on my YZ450F. With the spark arrestor removed the bikes runs good. I ran the bike for the first time today with the spark arrestor in and it had a lot of low end hesitation. The hesitation would disappear when I got on the gas, but at a steady speed it would reappear. I'm not familiar with jetting, but it appears I may need to rejet. What can I do to smooth out the engine with the arrestor in, while protecting how it runs with it removed?


You can't have it both ways. Jet for each configuration or live with the poor performance with the arrestor in if you only use it once in a while.

If the hesitation is right off an idle then it more than likely can be fixed by changing the pilot circuit. You need to turn the fuel screw one way or the other until the hesitation disappears. You may need to change your pilot jet if you get too far in or out with the fuel screw. Do yourself a favor and buy an extended fuel screw, it's the best 20 bucks you can spend on a bike. This way you can adjust it with your fingers, and since the fuel screw should be adjusted every time you ride for optimal performance, adjusting with your fingers is a good thing.

We switch back and forth between running a spark arrestor to running without all the time, and it makes no difference. I doubt it's the source of your problem.

You reported having hot starting problems in another thread. Such difficulty is almost always related to improper pilot circuit settings. Review this thread and see where it leads:

Once again Gray, thanks for pointing me in the right direction. I will do the test recommended by Eddie Sisneros.

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