Missfire Diagnosis Challenge

Hi guys (and gals),

I have what seems to be a challenge but you may know the answer.

I bought my GF a good used 2002 XT225. It seems to run extremely well but with 1 fault, a high rev misfire which seems to be at a couple of different rev ranges. The engine pulls strongly and smoothly right up to mid to high rev, missfires a little then revs a litle more then hits a worse missfire which continues until backing off.

I have stripped and cleaned the carb, added a little richnes with the mixture screw, replaced the air filter, replaced spark plug, coil and lead and CDI unit, checked the valves, wiring all looks like new, no visable damage to generator or pickup ........ What the heck else is there ???

It could be the generator or pickup i guess but that is expensive and the units bolts have seized in solid. I also checked the fuel flow and it flows like crazy.

Is there a way to monitor the signal from the generator/pickup to the CDI unit ?

I guess the float could be stcking but it seems clean and smooth when i inspect it.

Any ideas ?????

Thanks for your help.

p.s. the carb is stock and on this model they are considered very lean from the factory.

@Flint - You have checked or replaced basically everything except for the stator and pickup. You will need to get the specs for both the source and pulser coils (if this unit has both). My XT350 was doing the same thing and finally died on me one day... the stator was bad and after I changed it voila... no more backfiring or hesitation. I chalked it up to it not having enough umph at the higher revs to keep the spark alive. Thus it would miss, etc...

p.s. the carb is stock and on this model they are considered very lean from the factory.

Is the rest of the bike stock?

The main jet is not considered very lean from the factory (on U.S. models).

Here is a post that i did about a month ago... as you can see I have been through the lean mixture woes and back. I thought that the issue with the hesitation and stuff was the carbs but turned out to be the stator. Again, this is not an XT225 that I have.. I have a 1986 Yamaha XT350... but the symptoms sound similar.

http://www.thumpertalk.com/forum/showthread.php?t=937817

Hi Guys,

thanks for your input......

Yes, the bike is totally stock from head to foot except decent bars and controls etc. I will try to find the link with the references to the lean carb. It centers more around pilot and needle but also the main jet.

I think that i will try to find a way to test the signal from the stator. It will be difficult due to the high RPM but if i can do it without spending a fortune then i will.

I will keep you all posted on results.

I would check

float level and float condition

float bowl vents

fuel screen in tank

I checked the carb at the very start but now i think that i will completely strip it and get it sonically cleaned. Then rebuild it very carefully.

The fuel from the tank flows like crazy but i will check the screen to be sure.

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