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I have a 2002 honda 400ex and when I am going full throttle in 5th gear for some reason the engine cuts out at around 50 MPH every time. I have checked timing and valve gap and everything is correct. I have also tried putting a new main jet in and it did not help anything. All that is done to it is a fmf slip on exhaust. Could the pilot jet need to be cleaned or what could it be? I am from Wisconsin also
 

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The pilot jet is for idle only. The valve timing and gap has nothing to do with cutting out at a specific RPM. If the engine is cutting out, then it's an ignition issue. If it simply loses power at a specific RPM of a certain gear, then it is generally either an intake or exhaust issue. Did it run fine before the installation of the FMF slip on? If yes, does it run better or worse with the top off the air box? If it runs better, then it needs more air and if it runs worse, then it needs more fuel. You said you tried a new main jet. What size was the new main jet in relation to the old main jet?
 
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