minor piston siezure cause?

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Jun 6, 2001
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#1
I recently had a kx-85 cylinder overbored 2mm and ported to improve the power and powerband. I then installed a new topend using a wiesco topend kit.On the first ride after break-in, the bike ran o.k. but seemed to be running rich on the topend.On the next trip to the track i went from a 135 main to a 132 main based on what several other's were running in thier kx-85's but i was using a br8eg plug and it was a very hot day , we experinced a minor siezure after letting the bike cool down it was fine and it was fine upon inspection. The next time at the track i went to a br9eg plug and the bike ran extremely well for several hours of riding although it was a cooler day.Today we went back in the heat of the day with the same setup and although it didnot sieze it sounded and acted like it was going to before we stoped running it.The topend is still in good shape but, it seemed to be makeing a grinding noise just before we shut it off. I will probable try to call Eric tomorrow but thought maybe someone may have an idea what to look for ,try , or do .
sorry to be so long,THANKS!!
 
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#2
first off, on your first time out after putting in a new piston do not go abouve 1/2 throttle, that is how major seizures happen. second of all, i am not awesome with carbuerators, but i do knwo that that was a big difference to make at once in your jetting. try goig none jet at a time, starting with the 135 and going leaner. try the 134 and then the 133, and finally the 132, go back to the first plug you ran. i think what you did was lean out the micture to much, that causes more heat and also means less oil to lubricate internal parts which doubles the problem...so try going back to what you had adn work from there.
tim
 
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#3
Originally posted by Kawierider
i am not awesome with carbuerators, but i do knwo that that was a big difference to make at once in your jetting. try goig none jet at a time, starting with the 135 and going leaner. try the 134 and then the 133, and finally the 132
Kawierider, 132 is the next size down from 135.

tcmx, don't jet your bike based on what others are running. If the 132 causes seizure problems, then it has to be too lean. Go back to the 135. I would also run 26:1 oil ratio in that 85.
 
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#4
Spanky250, thanks for the advice, i check the plug when we ran it with the smaller jet and it looked to be jetted correctly. When we had the 135 main in it seemed to be running rich, it would not run good on the topend and the plug was wet and black.As far as the premix ratio i am a little confused , i thought if you add more oil it leans out the mixture. Although the plug matches what Eric Gore's diagnostic pictures show can it still be to lean?
 
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#5
Perhaps the main is not your problem. Have you tried the smaller main with the needle clip lowered one notch to richen the middle a little? Most riding is done from 1/4 to 3/4 throttle, you don't ride wide open around the entire track. There are also other factors that influence the plug color, the chief one being heat range. I only use plug color as a reference for the main jet, it is useless as a diagnostic for the other carb circuits.

As for the pre-mix ratio, I suggested 26:1 simply because I would not run less oil than that in a small-bore engine, because they have to be revved so high to make any power. The pre-mix has a small influence on the jetting, but it is more fine-tuning, not nearly like changing jet sizes. Try the 135 main with 26:1 ratio, and see how it runs. If it seems rich, you need to determine if it is actually the main that is off, or if it is the needle. To properly do a plug reading for the main jet, warm the bike to full operating temperature, change the plug, and do a full-throttle run through the gears without letting off. As soon as the bike tops out in top gear, pull the clutch in, kill the engine, and coast to a stop. Remove the plug and check it all the way down at the bottom of the insulator. This area is hard to see without removing the threaded portion of the plug, but it is the only accurate way to do it. If the plug color looks good after this test, then the main is ok, so you need to be looking at the needle. If the bike doesn't run cleanly with the 135, then go to the 132 and raise the needle one notch (lower the clip) to see if this cures your problem.

Rebuilding with a Wiseco can be a source of problems if the clearances are too tight, a forged piston runs slightly larger tolerances than a cast piston. If the cylinder was bored and replated to stock clearances for a stock cast piston, there is a good chance that the bore is simply too tight for the Wiseco. If this is the case, send it back to the place that did the cylinder job and have them replate it to the proper clearance for the Wiseco.
 
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#6
[breaking the new top-end is very critical and if you dont break it in right this will lead to a seizure