98 kx250 fouling plugs

[blinkerfluid]

I have a 98 kx250 and i seem to be fouling plugs. I live in iowa so im around sea level i beleive...

I was riding lower rpms over some woops and the plug fouled on me.. every plug ive taken out has been black...

little info on the bike

vforce v3 reeds (no cracks or chips on reeds)
Pro circuit platinum pipe, and factory sound pro circuit pipe (long)
new clean air filter

45 pilot
170 main
55 power jet
needle on 2nd clip
stock NGK BR8 plug gapped inbetween 19 and 23

screw turned out 2 turns

the bike will not idle on its own, it will bog down and die. might have to adjust the black plastic screw up top..


the needle was on the middle clip when it fouled the plug.. its splooging black goo out the pipe..

i put the needle on the 2nd clip and went out today, was about 80F and it didnt foul, but if you held the throttle in any position, it sounded like a harley idling..   sounded like a horrible misfire...

starts up on 1st or 2nd kick every time.. 1st kick with a new plug.

carb is good and cleaned.. but there is one "jet" i cant find ANY info on.. or find a replacement ANYWHERE... parts diagrams dont show it... and it is a #85.. no threads just pushes into a round hole..

the part in question is circled red in this picture.. it has a jetting number on it #85.. but i cant find any info or replacement jetting for it.. nor any info telling what it is.. its not a main, pilot or power jet...

this is NOT my carb in the picture, just the only picture i could find.. mine has a power jet on the top right angled to the outside

[Philthy Phil]

what ratio of two stroke oil to fuel are you using? and what kind of oil is it?
have you recently had anything changed or work done on the bike? did it run fine before this problem?

my 04 did the same and it happened to be the wrong spark plug and air/fuel screw setting from the previous owner...

[blinkerfluid]

32:1 yamalube 2r   I want to run klotz, but the only place that has some was closed.

on the top end it runs strong and can sometimes pull a wheelie just going through full throttle 1st-3rd gears

it did run fine and didnt foul a plug at all with the stock reedcage, but plug was still black around 60F
i changed out to vforce v3 reeds and the temp was 70-80 and it fouled a plug on the low end

im thinking the vforce reeds might require jetting change but im unsure.. i just know the plug is black..


it ran fine in the winter (20F weather) but now that its warming up to 70s i think its too rich.

i checked the needle and its a NOZG on the 2nd clip.

the little brass screw towards airbox is turned out 2 turns and wont idle on its own.

the plug comes out nice and black..

i got it to idle awhile back but i had to turn the screw out about 3 turns and fiddle with the top black plastic screw, and still wanted to die after riding awhile.

i didnt foul a plug when i went out yesterday (when i dropped the needle clip to the 2nd position from where it was at third position) but if you held the throttle still at any position and rpm range, it would sound like a vtwin and wasnt smooth at all and still didnt idle on its own.

im thinking the pilot is fine at a #45, but the mainjet is rich, as the last plugchop i did around 60F came out black.

but then the question arises.. WTF is that long brass tube with the #85 stamped in it???

[dirtjunkie85]

I and a few other of my friends in my area have that same machine. I just recently got mine and I'm at a bit higher sea level than you are but everybody that's in my area runs the klotz super techniplate oil. (the smell is my favorite) I also have the same vforce reed cage. In my opinion I would just say drop your main jet to maybe like a 165 or so. And if that doesn't help you can also change your oil to fuel mixture up here I'm running at about 36:1 but yeah like I said I'm up quite a bit higher than you are. Here's a few links that I use to get both my oil and my carb stuff. Oil: http://www.rockymountainatvmc.com/t/44/85/285/934/2-Stroke-Engine-Oil Carb: http://www.rockymountainatvmc.com/t/44/54/213/930/Carburetor-Jets
Let me know how it goes!

[blinkerfluid]

can ANYONE explain WTF that long round brass tube is? it is some mysterious tube no one knows about, has the part for, info on it.. or wants to even talk about it

[weymouth399]

The long brass tube "in the bowl" is the vent, you don't touch that that's why nobody talks about it.

Bob W

[blinkerfluid]

oh ok that makes sense then! I got a friends dad that works on dirtbikes all the time and can rejet it in 5 minutes. said my pilot circuit sounds rich...

but a 45 sounds lean enough, especially after putting it on the 2nd clip and it feels rough if you hold the throttle at any position and sounds like a harley..

it didnt sound like that or be as rough if you held it at any throttle with the clip on the 3rd position...

the top end wakes up pretty good, but plug chop says top end is rich..

i think i might drop it off at the friends and have his dad put a 165 main in and see what that does.. its gonna take some tuning time but i dont want to have to push the bike through dirt a couple blocks to where my vehicle is when a plug fouls..

[weymouth399]

Where's the air screw set at?? Turn it out 1/2 turn at a time from stk setting.
1 1/2ish turns out from seated in the carb (not buried), should be a good place to start.

Bob W.

[blinkerfluid]

before i took the carb off, it was at 2 turns out, and wouldnt idle on its own, in gear or nuetral.. had to constantly rev it or it would just shut off...

not sure how many turns out the top plastic screw is, or how many it should be turned out

[JamesyK]

Hi there
My 97 does the same, or did. I've reduced the pilot jet to a 38 if I remember correctly and dropped the needle right down. Most of the time I'm running it at 40 to 1, and even run it with a No7 plug, if you're not running it at race speeds and revs all the time this will be okay. Always make sure you set up the pilot jet correctly first before moving on to the main and needle jet. With a correct pilot jet the bike will sit and idle fine without touching the throttle. Remember though that the standard jetting is for a bike with a brand new top end, as it wears the compression changes and you may need to rejet to suit the condition of the engine. I run at the moment with a 165 main but have went down to a 158 and that was with V-force reeds fitted.
Hope this helps you sort it out.

[blinkerfluid]

I rejetted the bike and took it for some laps around the track.

160 main
40 pilot
NOZG needle in 3rd clip (middle)

also found a gas station that has 92 octane without ethanol.


rode down around the wooded trail section and didnt foul a plug, bike idled on its own once warmed up. still need to dial in the mixture screw.


it doesnt ride as smooth as when it was running real rich.. unless your going WOT.

if you hold the throttle in ANY position, 1/4.. 1/2.. 7/8 it sounds like a cammed v8 and is pretty rough.

I havn't had time to pull the plug and check color.. one thing i did notice recently, is there was a nickel sized droplet under the intake boot of premix (re-used the reed gasket, yes i know shouldve got a new one)

With the old jetting it ran great and was super smooth at any throttle position even if you held it at a position, we pulled the needle out and raised it.. and then it was rough if you held the throttle... so when we rejetted it i raised it back to where it was.

im actually afraid to hit the tabletop now, because i do it in 2nd gear lower rpms (still learning jumps) and its not smooth at all.

[blinkerfluid]

stuck a br9ES plug in and fired it up with the choke on, after about 10 seconds it fouled out choke still on.

the br8es i removed was black on the whole electrode.
so i took the needle out and put it on the 2nd clip to lean it up some

i put the br8es back in and adjusted the mixture screw to 1 1/2 turns out.
took it for a ride near my house for about 10 minutes.

it ran good and sounded good but still was stumbling at any rpm if you held it. rode it back to the house slowly n omore than 1/4 throttle

let it cool down some and pulled the plug, noticed it was lightly tan, faintly white, and a hint of chocholate in some spots.. but for the most part lightly tan

i turned th emixture screw to 1 1/4 turns out and didnt test it yet.

i tried to take a picture with my gopro2 but it didnt like the close up or i dont know how to run it yet 

im thinking im getting real close on the jetting and just need to dial it in a hair more now.

hopefully someone can give me some advice on where i stand.. the only issue im having now is not being smooth when held at any throttle position