kx 125 powerband problems

[jdager]

Hi,
I just bought a kx125 last week. I just got done doing a full top end replacement on it, including piston, all new gaskets, rings, the works. it fired up on the first kick, but still had the same problem that I had before the rebuild. riding, in first or second, it will go good up to about 1/3 to 1/2 power, and then totally drop off from there and bog down. with full throttle, it will not go any higher, just sit there, half bogged down. I know the first response will be to jet the bike, and I plan to try that tomorrow, but I dont feel like that will solve all of the problems. I looked at the jetting specs on the fmf website for my bike and pipe, and put it to that, so I think they should at least be close, but it runs horrible. what else can I check or try? it seems like the boot between the carb and the engine has a lot of cracks in it, I dont know if any of them go all the way through, but could those cause the problem? could it be bad reeds, bad, or improperly set floats? anything else for me to check? thanks in advance
Jim Dager

[gaz]

I just went through all this with a kx250. Yes it started ok, ran 1st and 2nd ok as long as you didn't push to hard (like wot) into the power band otherwise it would run erratically and foul plugs and it seemed to be excessivly rich. I leaned of the main and pilot and tried two different needles and clip positions as the problem seemed to effect the mid range of the bike. It made no difference. From reading other posts I suspected the right hand side main bearing seal to be leaking. (this really makes a bike run rich, left hand side makes it run lean and both don't respond much to rejetting). The best way to determine this is to do a crank leakdown test as it doesn't cost much and confirms the leak. This can be done with the motor in the bike. I did thid after I checked verything else like reeds, float etc. Drove me crazy! If you google "kx250 crank seal leak test" you will see the test results on youtube.

[jdager]

ok, well, I am not sure how to post pictures on here, so I will try my best to explain it. the old piston that came out after the rebuild seemed to look a little funny. there was carbon buildup on the top of it, but only in the shape of a triangle. the wide base of the triangle was along where the exhaust exits, and it came to a point in the center of the piston. could this be due to a sticky KIPS valve, as that would seem like it may explain away my problems. how do these normally function, and how should I check it/clean it/fix it? thanks so much

[jdager]

thanks gaz for your reply, it was nice and detailed. I will definatly be looking at that if I cant get this figured out, I just want to try the easy things first. how hard was replacing the seal, or how much labor did a shop charge you? thanks

[jdager]

here is a link to a picture of the piston. the bottom of the picture, where the carbon triangle is widest against the edge of the cylinder, is towards the front, where the exhaust exits. also notice the symetrical black spots on either side of midline, could this be where the power valve is hitting?
thanks
JIm
http://i394.photobucket.com/albums/p...er/piston2.jpg

[gaz]

Replacing a seal requires the crankshaft to be removed so if you don't know how much work its done replace it as well includung the main bearings, costly but you will $$ in the long run. If the piston is really carboned up then there is a good chance the kips are as well so give them a clean. I use a wire brush on angle grider to get them spotless. The piston (top only) I clean it with metal polish/cleaner. Rips the carbon right off and shines it up real good. If the power valve is hitting the piston it will leave big scratches on the piston and can also hear it when idling. Costing at a shop needs you to go to them and negotiate a price. I ordered the crank/bearings/seals and stripped the motor to the stage of splitting the motor let the shop do the rest. Cost me Aus$100 labour on top of the cranks etc. There are other costs like new piston ring, gaskets, seals etc.