hi there guys. i just joined, after reading a bunch of great threads i wanted to be part of the fun!

im a 16 year old kid from california and i have just gotten back into bikes. a few years back i had a freakish mishap with my lil' onehundy. wiped out and somehow got my fingers in the rear sprocket on the way down! (&%$#@!)!!! i dont now exacly how it happened, but i was over motorcycles for a while. thankfully i only lost a bit of fingertip on my middle and ring finger, and scar tissue in my pointer finger. but then out of the blue i have regained interest! the bike has been sitting for like three years now and was a pain in my arse to start. i had to push it up my driveway a few times and roll it down with the spark plug out to blow out the gas in the motor. then i had to do that again to get it to start up finally.

the thing wouldnt idle so i took the carb apart and sure enough, the pilot was plugged. so i blasted all the stuff clean. pilot is clean now, and all the circuits of the carb "should " be clean. the two brass tubes on the filter side of the carb are clean, and lead to the pilot and main, and the pilot screw section is all clean.

ok, now the weird part. iv got a manual that states the float hight as 12.5mm, so i set my float at that hight. before the float was wayyyyyy low, and for some reason gas would come out the lower vent/ drain when the bike would shake/ go over bumps. so i raised the hight, and the gas leaking problem went away, even tho the fuel level is now higher.

the bike runs rich on the lower range but bogs out of i snap the throttle open with the motor at idling speeds. i put the choke half way on and it doesnt bog nearly as much.

i ordered some new jets just to see if it helps. (100 main, 38 pilot, recommended for a stock bike right off the bat)

if that doesnt fix it il put the float back to around 20mm like it was in the beginning.

any insight would be great. the bike ran good (but most likely kinda lean from the factory) when it went into storage. of course i forgot to drain the gas out of the bowl tho

also do you guys know if it is possible to repair the fuel tank valve? mine drips a tiny bit when it is turned off. (about 2 or 3 drops per min.)

The new pilot jet should help. Your fuel petcock is likely dust. They dry out and never seem to get right again.
You may just have to order a new one. Be sure to get the new o-ring where it seats to the tank.

thanks. this bike is hard to tell if it is lean or rich.

im pretty good with tuning small 2 strokes like chainsaws and my hpi baja, but 4 strokes are hard to tell by sound.

at first i thought the pilot seemed rich, but now that i rode it again today, i cant really say. it sorta pops and sputters when i hold it at about 1/8 to 1/4 throttle which on a 2 stroke means rich, but this thing i guess mild popping means lean, which would make sense with it bogging out.

ok guys, a little update. got the new jets in it (38/ 100) and it runs better. the low end is still lean. if i ride down my driveway(pretty steep downhill) and give it a tiny bit of throttle, it sounds lie it goes flat and produces no power. it also still dies if i whip the throttle open from idle. although anything slightly above idle and it will rev right up. if i turn the pilot screw out alot, it will rev up from low speed, but doesnt idle well at all.

also, i tried changing the float hight and replacing the float with a different one i had (non OEM!!!) and the weird problem of fuel coming out of the overflow came back.

at least i can rule out the pilot jet as the problem.

when i took the pilot screw out, the o ring was shot, so this could be my problem. i did put another o ring i had in it, and it did not help, but im still suspicious of it.

so any suggestions or ideas as to what can be wrong????

so no one knows what could be wrong??


heres some pics of the plug if it helps. this is after some fairly hard riding uphill.

so i pulled the carb AGAIN! and raised the float a bit more (its about 11mm) and it runs way better. it will sometimes rev up wothout bogging. the weird thing is that fuel doesnt come out the drain tube when i shake the motorcycle like it does when the float is lower (confused). now when i go down hill with no throttle i actually hear some combustion going on instead of just dead motor noise, almost like the ignition was gone.

so i have narrowed it down quite a bit. replaced jets, so it is not a clogged pilot, cleaned it about 500 times lol, it is totally clean and all the passages are clear, checked manifold and gaskets for leaks and didnt find anything. so basically, it is my pilot screw circuit that is causing this grief.


oh yea, and my idle is very erratic. if i turn the idle up, it will speed up, then slow back down a bunch.

any ideas why my pilot screw circuit is acting so strange??

You most likely have an intake leak. During one of the 500 carburetor cleanings, did you perhaps remove the carb manifold from the block? There is supposed to be a paper gasket between the carb manifold and the head; if it's not there or torn you'll have a serious intake leak. Also check for a good seal at the cap for the throttle cable.

[font=comic sans ms, cursive]MCRIPPPer,[/font]

[font=comic sans ms, cursive]Over a period of years, I've had to work on 3 of these XR/CRF 100s, which were ridden by my wife & daughter. One universal thing that I learned about all 3 of them was this: [/font]

[font=comic sans ms, cursive]"If these bikes sit for more than a couple of weeks with gas in the float bowl, they have a tendency to really gum up those little Keihin carburetors." [/font]


[font=comic sans ms, cursive]All three bikes have done this. Over time I figured out that I have to drain the float bowls after each ride. I had to buy a new petcock for 2 of the 3 XR100s we've owned because they had "slow leaks". Even with the petcock lever in the "OFF" position, they would allow the float bowl I'd just drained to fill back up over time...... [/font]

[font=comic sans ms, cursive]You did very good by buying a new pilot jet. But I wonder if there still may not be some gunk or varnish junk in one of the tiny machined passageways for the idle/pilot circuit in your carb. Have you been able to spray some carb cleaner through those passageways, and then hit it with some compressed air? Sometimes, it takes a few tries of this (spray carb or brake cleaner in the passageways, and then hit it with the air) to get all the crud out.....[/font]

[font=comic sans ms, cursive]Anyway, shut your fuel off & drain your carb after each ride. Todays gasoline, along with something in the metallic makeup of these carbs tend to combine to gum up the works, somehow..... I've found a place near me that sells non-ethanol fuel, and it's waaaay better about not forming crud in the carb. But I still drain it anyway. My wife & daughter go for long periods of time without riding sometimes..... Replace your fuel petcok, if it's needed,, to keep the fuel out after ya drain it. And keep working on the carb cleaning, and be sure to get all the circuits cleaned. Again, it make take a few tries, but you'll get it eventually...... [/font]

[font=comic sans ms, cursive]Jimmie[/font]

thanks guys. im pretty sure all the passages are clean. i blew compressed air through those breather tube things on the intake side of the carb and air came out of the main, and air came out of the pilot and pilot screw spots when i put air through though the other one. i THOROUGHLY inspected the whole carb and all the holes with a little flashlight, and saw that the breather hole in the pilot jet aria looked like it goes to the main, but i didnt see any place where it came out?? maybe they just over drilled a little while manufacturing the carb. could be just a blind hole. does one of those brass tubes feed all the jets????????


i checked for air leaks already. i have never taken the manifold off the motor. iv only taken the carb off. im thinking the only problem that remains is the little oring on the pilot screw. i found another that fit, but it might not be sealing the needle. although i did check for an air leak there too, and tried putting grease around it. maybe my pilot screw is bad.

ok, this thing is really pissin me off.!!

turns out the low speed needle is actually rich, even with it almost fully closed.

so i guess my high speed jet is lean for whatever reason! i made the hole in my old 98 jet bigger and it runs better. i noticed iff i have it revved up with alot of load on it, it actually makes more power at 3/4 throttle then at WOT. this would actually make sense with it bogging out if it is not going fast enough. i tried raising the needle one position and then it wouldnt idle, but the bog issue was slightly better.

so basically i have a rich sputtery low end, great midrange, and super lean top end.

i removed the main jet and pit a light in the hole and could see light in the little brass tube so i know it is clear. why would my carb have a lean high end even with a high float level and a 100 main jet???

i put a drilled out main jet and the thing still bogs. im starting to think its not the carb!

i checked the valve clearance and the intake valve lifter was super tight. i could barely slide it on the pin. so i loosened it ever so slightly so it is about the same as the exhaust side. iv gotta get a gauge to adjust it properly.

so that wasnt the problem.


im starting to think it could be the plug?????? maybe it fouled out when it sat for a while and then started up with a bunch of fuel in the chamber.

it almost seems like it is loosing spark when i give it full throttle.

wanst the plug. found a spare and it didnt help.

Why don't you try going to a baseline setting of everything, and start over? According to the microfiche found HERE, the stock carb should have the #98 main jet and the #35 pilot jet from the factory. If you have the same carb as shown above, it is a fuel screw that is located in front of the float bowl, underneath the manifold. That needs to be turned OUT to richen the mixture, unlike an air screw, which has to be turned IN to richen the mixture.
You don't say whether or not you are using the stock airbox and airfilter, is it thoroughly clean and oiled properly? You don't make mention of a stock or aftermarket pipe, another item that can affect jetting. You don't say whether or not you're at sea level, or 2000-2500' above sea level, another factor in the jetting process.
Using a baseline setting gives you a focal point to return to if you are going the wrong way with your jetting thoughts. I try to use the stock jets to start, put the jet needle clip in the 3rd notch from the top (the middle), and set the fuel screw at 1-1/2 to 1-3/4 turns OUT from seated, to begin. These settings allow you to move up or down with the main and pilot jets (assuming you have a few different sizes of each on hand, to work with), move the clip up or down two notches either way, and also be able to adjust the fuel screw out to 3 turns to richen it, or at little as a 1/2-turn from seated, to lean it.
The pilot jet controls from off-idle to 1/4-throttle, the jet needle controls from 1/4 to 3/4 throttle, and the main controls from 3/4 to WFO, but those ranges overlap somewhat.
Also keep in mind that you cannot expect these little carbs to go from idle to WFO just by the flick of your wrist, they do not have accelerator pumps like the bigger bikes do.
Lastly, jetting is not something you can do effectively within 5 or 10 minutes, like putting new grips on your bike. You have to devote time and patience to do it correctly, and if done so, you will be rewarded with a bike that will run like a top.

i started out with the bike at stock spec. (12.5mm float hight, 98 main, 35 pilot, needle clip in the middle) and the thing would die if i give it full throttle. this problem is not merely a lag in throttle response, it will actually shut off the engine. for example, if i have it in, say, 2nd gear, going down a hill with the engine at aprox. idle speed, and go wfo, the motor will keep turning(because im on a hill in gear) but it feels like the kill buttin is pushed, no combustion at all. as soon as the engine speeds up, it comes to life. if i do this same thing, and give it only half throttle, it will go just fine.

at the moment, i have it very close to stock setting. iv got the float at 12mm, 100 main, and 35 pilot jet. the weird thing is the low speed fuel screw is only about 1/4 turn out, and the low end is way rich. im wondering if 12.5 is the real float hight for this motorcycle. when i first took the carb apart a few years ago, the float was way lower then 12.5. it was more like 20mm. this would explain the rich low end, but not the crazy low rpm bog.


(note that the bog was not always there, so the problem is not that i dont have an accelerator pump. )

oh yea, the bike is stock exept for a few holes in the air box. i live at around 3k ft. i sometimes ride down to about 1000 tho.

Okay, you seem to be somewhat counterproductive with your jetting theories. At 3K feet, you should have to run bigger main and pilot jets from the start. You cannot run the stock #35 pilot, have the fuel screw only a 1/4-turn out from seated, and be rich on the low end. The more you turn the fuel screw IN, the more you are leaning the mixture OUT.
the float level or amount of fuel in the bowl has no direct effect on whether you are running rich or lean, unless you're riding the bike WFO in 1st gear, and run out of fuel; you're using the fuel faster than the tank can supply it to the carb bowl. Under normal circumstances, both jets are submerged in fuel all the time, regardless of whether your float is at 12mm or 20mm. You can take the float setting out of the equation, as long as the casting seam on the side of the float is parallel with the edge of the carb body, that's the standard rule of thumb.
You really should have a few different sized jets to work with, both main and pilot jets. Sometimess you just can't make do with what you have, and arbitrarily drilling out jets doesn't do anything when you have no idea how big the hole in the jet is after drilling. Ideally, having a #38, a #40, a #42 pilot and also a #102 and a #105 main jet gives you the oppurtunity to see how the bike works with accurate jets.

from my understanding, higher elevation would warrant leaner jetting.

MCRIPPPer, on 26 January 2012 - 05:41 PM, said:

My mistake, you are correct. I was thinking of colder air being denser and requiring one to richen up the jetting, and not about the thinner air at altitude requiring one to lean the jetting.

what i dont get is why this thing started doing this after sitting. i cleaned the carb until it was spotless, and it still does it.

in theory, with a 38/100 setup it should not be lean. for some reason the high end is, and the low end is rich...

Sometimes you can not get the internal passags clean very easy. I have spent countless hours cleaning jets, replacing jets and cleaning carbs. Sometimes it worked sometimes it didnt. Recently I was fighting the same exact problem you describe on my sons 2000 XR100. After speding no less than 6 hours cleaning jets and passages and what ever else I could do to it. It ran good when the choke in the middle possition. It popped and ran crappy with the choke all the way off and would run very flat and rough wide open with the choke off. So after all that I happen to find on ebay that you can get new/reman carburetors on ebay for 20 to 30 dollars. At that price I dont think I would bother with cleaning it ever again. Just something to consider. Unfortunately we got traded the bike for a KX100 recently so I dont know if that would have fixed it.