1991 KX250: 155 and 42 pilot and top clip seem lean?

Smokes a bit much.

Doesn't spooge.

Runs alright.

Plug fouls every 6-8-10 hours of riding.

Plug is dark and covered in black.


So.  I dropped from the 158 main to a 155 I have....See if that helps.

Don't have a 40pilot so 42 is leanest I have in there.

Clip is in top position so lowest setting.


Doesn't gurgle to bad.  Doesn't load up to bad.  Ran at a track yesterday and it ran good with some hard/long 3rd 4th gear pulls.


That 155 and 42 doesn't sound to lean a set up?  Came stock with 160 and 58 back in the day.  Seems I pretty far removed from that setting.


Standard needle and slide.

ProCircuit pipe and 304.


Can't wait to try it and see if I notice any difference.  Dare I go to a 40pilot?  That seems way lean....or does it?  40pilot on a 250 is not unheard of?  155 main?


Guess just keep messing with it (and leaning it) and see what it likes.  Every bike is different.  Ran like crap when it had 160 and 58.


Thanks for any advice/experience.






Your motor is not new anymore.

You need to determine if you cranks seals are leaking, which they probably are, causing the black plug.

They are supposed to be replaced every 150 hours on a two stroke!

Trying to fix mechaincal problems by changing you jetting can get you into trouble (boom!)

Do a leak down test.

Feels factory fresh :)


Ran some laps around my house and 155 main seemed better.


Rips pretty dang good.



How involved is the crank seal replacment?


I personally have put more than 150 hours on the bike in the last year.

Then again there was the year before, when I got it, for another 100 hours or so (I ride around our backyard track pretty often).


I recon it should indeed have new crank seals.  Is that something I can do from each side, or do I have to take the whole engine out and split the cases and all that?




Also check your float height is correct, I had a 250 2 stroke that just wanted leaner pilot jets until I found the float level was too high and after adjusting that it was back to stock pilot jet and running properly. 

Good point.  While reading the manual last night, I read about proper float level height.  I will check that as never have before.

I will get the parts to do a proper leak down test.  If it doesn't hold pressure, might mean needs crank seals.  If it does hold pressure, means it doesn't need crank seals.




Old thread, old bike, old man.


Had the crank seals replaced by the local pro-shop.

New piston.

Cleaned PV.


Still gurggly at low RPMs and smokes and doesnt want to idle.


While reading the manual last night....about float level specifically....they mention a "special tool" to check float height.  It is a bottom of carb plug, with a hose barb on it.  Replace carb plug with this and put a clear hose on it so can physically see float height while holding carb level.  Is that needed?  The manual also shows how to check float height via taking the carb off "holding it just right so the tang rests against the needle with out compressing it, and measuring float height.  Is that the defacto way to check it?  Or should I get the special carb plug?


Makes sense that if my float height is to hi, to much fule will be in the carb and "spill over" and "flood it out" and make is seem ultra rich.

SO.  To finish this, which I revived from some time ago.


Took the carb to TZR as long as I was going to the doctor's office by them.

Not float....it is proper set height, if not a shy "lean".  Never dripped out of overflow.


The shop warned me, it is probably the needle sleeve.  Old carb like that and the needle goes up and down in the tube a million times and wears out.

Sure enough, he took it apart and can see the out of roundness of the needle tube (jet that needle rides in).  Not a replaceable part....it is cast into the carb.

He advises me to contact CPW (CarbPartWarehouse) and ask about a thicker (leaner) needle as it would help.


CPW is pretty good at describing what is going on.  A thicker needle would indeed help, but it is a crap shoot.  How much thiker?  No way to know as it wont be in a round hole.

CPW also advise that if it wont idle, there is another issue.  There is a o-ring in the body and if carb was soaked, that could be suspect.  The fact that the slide has been replaced at some point (he asked if it was chrome.  Yes.  Means it was replaced).  That might be wearing away at the carb body.


CPW was willing to sell me a bunch of parts for it, but of course offered a whole new PKW38.  "It's a great carb and on many bikes.  Yours is just worn out.  They last about 12-15 years...yous is way past that"


The asked me what pipe is on it.  What size piston is in it.  What reeds are on it.  So seems they are paying attention.

$220 later and whole new carb will be here next week with proper jets and extra above/below.


New cracnk seals and piston and now carb....should be factory right.

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