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2002 YZ250F/FMF Power Core 4

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I have a 2002 YZ250F, which I purchase used in 2003. I had the dealer retro fit a 2003 exhaust valve for ease of starting. This year I installed a FMF Power Core 4 S/A exhaust so I can ride the trails. I have no intention of racing. FMF recommends a #45 pilot jet, jet needle clip at the 5th position, and fuel mixture screw 2 1/2 turns out. The main jet is stock. The idle is set to about 2000 rpm. Obviously the changes give a richer mixture. Is this too rich for Western New York? I had to change the spark plug, since the thing refused to start when I was out riding. The other thing I noticed was when I pulled the clutch in and clicked into first gear, the bike stalls occasionally. It has real good accerleration and no popping or severe rev changes. I am temped to put it the jetting back to stock and go from there.

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The stalling you speak of is a sure sign of a too rich low-speed. You should try adjusting the fuel srew in a couple of turns, but I think you will need to go back to a leaner pilot.

I think that for summer, the #45 pilot jet is way to rich for this bike. I'm using a #40 right now and could probably go to a #38.

During the winter though, I need to use a #45 or #48 to get rid of a hunting idle that develops as the temperature drops.

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Thanks. I will go back to the stock pilot jet/leaner fuel mixture in the mean time and get a #40 also. It has been very warm here and I suspected a too rich condition. The other thing I forgot to mention was that the engine rpm drop when I click into gear. I didn't know if that is normal or not.

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