Help with ?jetting? problems. At my wit's end

I've just about run out of ideas about this so maybe someone can help point me in the right direction.

Here's the short story. My '99 WR has been running great for a long time. I have had Taffy (lean) jetting for several years now and it has a lot of power, plug looks good, and has never overheated. My jetting is 35 pj, 45 pa screw (1/4 turn out), 160 mj, DTN clip 3, 60 sj, 160 maj, 1/2 second ap squirt, and fuel screw 1-1/2 turns out. I went to the DTN needle from the DTM when I put in the GYT-R exhaust insert last year. The bike is YZ timed. I have to adjust the needle clip and/or the mj depending on the time of year (temp. and humidity) but it is usually only +/- one clip/jet size.

Last September I took my bike to the mountains in western NC. During one ride, the bike started revving uncontrollably. Nothing I could do would bring it down to idle. I had to ride 30 or so miles to get back to the campground with it like that. I had to brake going uphill because it was running so high. The cause was a cracked carb slide and I found that out as soon as I got to camp. I loaded the bike on the truck and didn't ride it again until I replaced the carb slider and o-ring. Ever since then I've been having a peculiar problem. The bike runs normally until I get to about 20 miles (1/2 hour) or so into a ride. Then it intermittently will not come down to normal idle without a blip or two of the throttle. Most of the time it's normal but it will high idle maybe about 20% of the time once it starts doing it. This completely throws off my cornering and/or regular slowing down since I am used to using engine braking. It forces me to use the brakes, which isn't what I am used to or what I want it to be doing.

I have done pretty much everything I can think of to solve this problem. I have rejetted/tested every jetting combo I could think of trying to richen up one part of the circuit. My first thought was the needle. I raised the needle but it just ran like crap. I went to a 38 then a 40 pj but it still ran up and otherwise ran like crap-no guts and burbled and popped. I checked the valve clearance. One intake valve was tight. I fixed that and it still does it. I took spacers out of the carb boot clamps to make sure they fit as tight as possible to rule out an air leak. I replaced the hot start hose in case it had a leak. I have sprayed carb cleaner and/or starting fluid all over the carb with the bike running to see if it had an air leak anywhere else. It didn't. I went up incrementally to a 168 mj, but it lost all the top end along the way and it still runs up. I have adjusted the idle setting during these rev-ups but when it comes back to normal, the idle is too low. I lubricated the throttle cables and adjusted them for smooth operation and made sure they aren't binding at different steering angles. Yesterday, just to see what would happen, I opened the pa screw up to the equivalent of a 75 paj (5/8 turn). It was hard to start and ran like garbage, so I didn't even do a road test. I closed it off to 3/8 turn, which should be a 55 equivalent. It was a lot smoother than with the 45 paj (1/4 turn) and ran strong, but after about 20 miles, it started running up again.

The only thing I haven't done is remove the cylinder head to see if anything got goobered up when the piece of the carb slide broke off. As a last resort, I'll probably check that this week. I have at least a thousand miles on the bike since the carb slide cracked so anything that would have been sucked in should have been long gone. And it runs very strong regardless of this problem so I didn't think there was a "mechanical" problem.

Anybody have any ideas? It ran so well for so long with this jetting that I'm beginning to think it has nothing to do with jetting. Thanks for any help.


My first inclination is that something mechanical DID happen.

Maybe your intake valves...??? Have you pulled the head, had a valve job, etc?

Your carb slide works FLAWLESSLY >> i.e. no hangups?

When something like this is consistantly repeating itself, I start thinking heat, expansion, electrical resistance building, etc...

If you have never had a valve job, maybe you want to start there...???

Be happy, at least you can ride your bike... :)

If it wasn't freezing out again here in NH (snow and severe wind) I could at least take out the kids TTR125LE.


I forgot to list that I have checked the carb slide operation a countless number of times. There is no binding in the slide whatsoever.

I'm beginning to think the same thing as you regarding "heat, expansion, electrical resistance building, etc..."

The last valve job was a couple of years ago. However, I haven't considered that since this problem began at the same time the carb slide cracked and, unless something is physically jammed in one of the valves, I don't know how that would be related. But I am finally going to pull the head to rule that out.

maybe a bent/binding valve >> heat, expansion, binding...???

I don't know Rich... :D

BTW, How is sunny Orlando? I received e-mail on ROUND TRIP from local airport to Orlando for $98 round trip. I thought about flying down there for the day, getting torched (sunburned), hit the OBT and head back.

I left Orlando in 1985 (Navy Base) and re-visted w/ family a few years ago.

I was disappointed. With all the "Attractions", traffic and people that go with it, it has become...well not a place I would want to live anymore.

I could live in Gainesville though! :)

Jax was nice too!

Did you take the slide out of the carb before this started? The little plate on the slide that falls off can go on either correct or upside down. If you put it on up side down your bike will run crazy at idle because it is getting way too much air. Up side down looks right but it is not. I did this on my CRF 450 in 2002 and it was hard to figure out. Hopefullyt your problem is that easy.


The carb slide plate is installed correctly. The "flat" end is down. Thanks, though.

Wish it were only that easy.

A very lean condition can cause this.You may be lean when it's cold, then as the bike gets hotter as you ride or a very hot day it leans out more.This can cause pre ignition and your bike will over rev and no come back to idle.Sometimes in worst case senario you will not be able to turn bike off , even by pulling the plug lead.If this happens simply pull the choke and it should drop revs if the ingnition is on or turn off if the ignition is off.Just 1 more simple thing to try,mine had this problem after i changed bars.Check that your hot start isn't pulling when your turning or just running the cable very tight as it will lean the bike out and not drop revs.

hope this helps dave.

Are you sure the flat end goes down? On mine the logical way was the wrong way and your problem sounds exactly the same. The pointy end goes down. Just trying to help. Take a look at it, it can go either way.


Have you tried going back to the DTM needle. You went to the leaner DTN with the baffle. Temps in Florida haven't reached summer highs and your jetting should be richer in cool temps. Maybee DTM in cooler temps and DTN in the summer. Sounds like you have tried everything else. :)

Are you sure the flat end goes down? On mine the logical way was the wrong way and your problem sounds exactly the same. The pointy end goes down. Just trying to help. Take a look at it, it can go either way.


The flat end goes down on this carb. I have the service manual and it clearly shows how it goes. Also, it won't go together with the pointy end down because the slide rollers that connect to the throttle shaft do not clear the slide if it's put in the other way around.

Have you tried going back to the DTM needle. You went to the leaner DTN with the baffle. Temps in Florida haven't reached summer highs and your jetting should be richer in cool temps. Maybee DTM in cooler temps and DTN in the summer. Sounds like you have tried everything else. :)

I spent several more hours this evening messing with the jetting. I went back up to a 38 pj and adjusted the pa screw accordingly. It took about 12 miles before it started but it started high revving again. I went up to a 40 pj, adjusted the pa screw to a 75 and it would barely start. I had to use both the hot start and choke (?!) to get it started but once it got running, it would revv up too high.

I did put in the DTM needle on clip 1 (leanest) and it ran much stronger- I think I'll keep that one in for a while, but it still revved up. I put it on clip 3 and it bucked and gurgled (too rich) but it still ran up.

So, it seems like there's just no other solution--or is there? I closely inspected the pilot screw and noticed that the little o-ring is looking a bit distorted and "expanded". I'm not sure if it is sealing properly anymore. If it's not metering the fuel properly it may explain the erratic idling. I'm going to get a new fuel screw and see if that fixes it. I don't think I can get just the o-ring by itself.

Humm... I am far from being an expert in 4 stroke bikes, but the few times (4 times I guess) that I had problems with erratic idle there was air leaking in. Either through some o-ring, or by a misaligned carb, or by a seal (don't know if this is the right name)... Just my 2 cents. Good luck. Rodrigo

I don't think I can get just the o-ring by itself.

You can get one fron Sudo for 39 cents. They have a $3.00 charge for small orders but it beats the $24.00 for a new screw.

What I don't understand is why it does not give the erratic, high-idle from the get-go?

Why does it take a good 20 minutes before it displays this characteristic?

I wish I could add a solution, but I can't come up with one.


Brandon, that's the $64,000 question. Just about the time during my test rides that I'm thinking "Maybe it's fixed this time", it starts doing it again. Once it starts, it does it for the rest of the day.

An additional thing that makes me think it's the pilot screw is that while it was high revving, I pulled over and turned the pilot screw. It didn't have an effect until I turned it one little bit farther, then the revs suddenly fell back down to idle. Then I blipped the throttle and got it to rev high again, then the fuel screw adjustment did the same thing. It's like it's moving in increments instead of a smooth transition. I think the o-ring may be hanging up in the pilot screw recess. If that's the case, once the carb warms up, the fuel screw hole may be heat expanded enough that the o-ring isn't sealing properly and it starts misbehaving. (I'm just guessing here.)

I think you are gessing it right! When there is little air leaking in you usually get erratic idle either when the bike is hot or when it's cold. If it's erratic all the time it's because it's a MAJOR leak.

I don't think I can get just the o-ring by itself.

You can get one fron Sudo for 39 cents. They have a $3.00 charge for small orders but it beats the $24.00 for a new screw.

Good advice! I called Sudco last night and ordered 10 o-rings, 5 springs, 5 washers, 1 pilot screw and an o-ring for the air filter joint connection for $25. That's about what it would cost for just the pilot screw set alone from Yamaha.

...and it has nothing to do with the carburetor. Hadn't received the Sudco order yet but decided to go riding yesterday and just live with it. About 30 miles into the ride, it started acting up bad. It would idle real high and not come down without a lot of throttle blipping. I happened to look down while stopped and noticed that I could see a light colored vapor coming from the crankcase breather tube. Very unusual and couldn't be good. I rode for a few more minutes and decided to get a second opinion. My mechanic friend had me rev it while he felt/smelled the breather tube effluent. It was full of gasoline vapors! He theorized that the rings were letting a huge charge of the fuel/air mixture blowby into the crankcase. Now it's starting to make sense. The fuel would pass into the crankcase unburned. Then the mixture would be too lean and would idle/rev erratically. I got back to the truck and called it a day so I could get home and tear into it.

Here's what I found inside:

<ul type="square"> [*]The cylinder head looks good. [*]The piston is smooth and glazed (no hone marks are evident anymore).

[*]The ring end gap on both upper rings were way out of spec too big.

[*]The gap between the piston ring slots and the rings themselves and they were within specs[*]Timing chain looks good[*]Valves look good

My mechanic friend has some micrometers that we will use to measure the piston diameter and cylinder bore to see if they are still servicable. If they are, all I'll need is rings and a new wrist pin. Then we can hone the cylinder and get it all back togther. I'm guessing that it will also need a new piston but won't know until we measure. I hope it doesn't need a new cylinder because that's another $300. It's my understanding that the Nikasil coating on these cylinders is very hard and usually outlasts several pistons before they go bad.

I'm seeing the light at the end of the tunnel. I don't feel bad because the piston and rings have about 15k hard miles on them and that's a lot of miles on a dirt bike.

You should replace the piston while you're in there. It's close to 6 years old already. Better safe than sorry ( and broke) :)

I decided last night to replace the piston and cylinder after a closer inspection. The piston and cylinder were replaced in 2001 so they are 3 years old right now.

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