Maintenance & Technical > KX500 Original

KX Jetting... AGAIN!

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gowen:
Hey Guys,

I seem to be having some jetting issues again. So, here goes, I have been trying my best, but can't seem to get the jetting good. I tried the jetting from the first page, and the bike runs like absolute trash. :-( And I'm unsure why. The low-end is strong, but can't seem to get the RPM's past the mid-range. So, I dropped the neidle a notch, (2nd), it cleared most of the mid up, but the top-end was still rough and very smoky. So, I dropped the neidle another position, (1st notch), it helped the mid-range good. :-) But, the top-end is still way too rich, and I've been working with it, all the way till I'm at a 160 mainjet! What is up with that? Anyway, all is good, except the mid-range is still having issues! It can't seem to get past the mid-range, kinda like fouling out unless you pin the throttle for about 20 seconds, it will break past the mid-range and slam in the top. What is the issue? I have nothing special done, why so little mainjet? What's up with the midrange. Anyway, frustrated. :-) Sorry. Anyway, if anybody has some advice, please let me know, I'm desprate. I'm tapped out on the mid.. I have the air screw at 1 1/2 turns out. I am in 70-80 degree weather, sea level, and not too much humidity, maybe 50%-70%. MAX. Anyway, thanks alot.

Paul:
When was the last time the crank case was pressure checked?

If - I can never remember which one - one of the seals in the bottom is leaking you get a rich condition and if the other is leaking you get a lean condition.

Rick:
Dry side is lean, and will make the crank go bye-bye.  This is especially a problem on the 125's.  So much so, that when my oldest son was consistently finishing in the top 3 in the 125 Expert class in the National Hare & Hound series, I replaced the seal at least once per month just to keep the motor together.  I have also helped a friend resolve a continuing motor failure on a YZ125 with the same fix.  Weird thing.  No mater what we did (new cranks and bearings every time, jugs, pistons, seals and gaskets), the only solution was to keep a fresh seal in place every month.

Fortunately, I have not experienced anything this dramatic on either the 250 or 500.  Good thing, as while you can change the dry side seal on a 125 from the outside, you must split the case on a 250 and 500 to perform the same surgery (on older models, I think you can get to the dry side seal from the outside as well.  Newer models, nope).  Hmmm, kind of makes you wonder why the 125 seal is so easy to change...

If the wet side seal is bad, you can tell pretty easily, as you will have way too much oil in the crank valley.  While this, too, will cause the crank to take a hike, it happens much more gradually.  But, hey, since you have to spit the cases to change the seal, might as well throw in fresh bearing on the crank and rod, along with that nasty dry side seal.

Good luck.

Rick

gowen:
Well, I could test that, but I have about 3 hours on a brand new bottom end, new bearings, tranny gears, rods. Everything mechanical is new down there, except the cases.. :-( I had a professional do it. Just from lack of time. But, anyway, I'd be frustrated if that was an issue. Because it is brandnew. :-/ So, anyway, thanks for the input. I will go about and test it. Thanks, any other suggestions if that isn't the case?

gwcrim:
What size slow jet are you using?

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