Please help

I recently bought a YZ400. excellent bike very revy, great handling. Just spent £££ serviced it and bought motomaster disc/caliper. I took it to the dyno and it was a lil flat at the bottom of the rev range so the bloke said lift the needle up one to '5'. So I turned the carb side ways to get access to the top of it and removed the needle and moved it up one. Put it back together and now it wont start :thumbsup: looking in the manual it shows a sping and a washer inbetween the locking nut and the circlip of the needle( i dont remember seeing them when I took it to bits)

I pulled the spark plug out and it was all black so I used a lighter to take off the excess fuel and then put it back together, still nothing, it sounds like it wants to start but cant. So I put the needle back down to 4. Still wont start so I pulled the hot start and choke out and it worked?? as soon as you take the choke off it dies, it wont idle?

Any ideas? I have the carb off atm and am in the process of cleaning it. The slide seems to get stuck slightly right at the bottom of the run. Is there an explodeable diagram with parts on for the carb with part numbers etc online??

Sounds to me like you may just need to turn the idle up a bit. There should be a spring and washer, but they will be attached to the allen screw that you had to remove to get to the needle. Hope this helps.


There definatley wasnt a spring and screw when I took it off :thumbsup:

I cant see how the idle screw changed by me changing the needle height..

There should have been a keeper screw in the top of the slide that holds the needle in position during use. If you go to the Yamaha parts catalog, it is find number 16 on the carburetor page. As for the idle adjustment getting turned, it doesn't take much of a bump to turn those sometimes. And a half a turn will make it hard to start if it is already on the low side of idle. These bikes like a high idle, especially when starting cold. One thing you may try is giving it one shot of the throttle before you try to start it. Just twist the throttle one full revolution and hold it there for a second and then release, and follow the drill. Hope this helps.


Thanx Josh, Looking at the exploded diagram there doesnt appear to be a tab/spring but in my book it shows 2 other items on top of the circlip/needle.

number 10 on the diagram is ever so slightly sitcky at the bottom point, could this be a problem?

Nothing should be sticky in a carburetor. # 10 is the throttle slide, have you had it out? These can be putin backwards and cause problems. The plate on the side that is removeable can be put in wrong also. Take a close look at the daigram, and be sure it is assembled properly. If you don't have the #16, you need to get one. The drawing is not detailed enough to seet he spring and washer, but they are there. If you have anything sticky in the carb, I would take the bowl off and give it a good cleaning with some spray carb cleaner and shop air. Hope this helps.


I gave the carb a good clean last night with compressed air and carb cleaner and I think I know whats wrong.

If you look at this pix


the idle screw doesnt seem to push the throttle cable housing up


there is a screw in the throttle housing slide which seems to give some movement to the throttle cable housing (to the right of it) but not enough to move it ontop of the idle screw :thumbsup:


There is a screw that locks the throttle slide in place but it doesnt seem to move to the left enough to make the throttle cable housing sit on top of the idle screw. shown in the other pix with the white circles


The only thing I can think of right now is that maybe the throttle shaft has slipped out of the throttle position sensor and that may be what is holding it out of the housing. The TPS is the black thing opposite the throttle pulley. It should have a slot in it that the shaft should fit into and I think that maybe the shaft has slipped out of this slot and turned thus holding it out so that the screw does not engage the stop. I hope this makes sense, if you have a manual, reference the TPS Replacement section and see if you feel comfortable removing it to get it aligned properly again. Or you could just mark the housing of the TPS and the carb so that you can get the alignment as close as possible after you get it lined back up with the shaft again. Hope this helps.


I also see your brass keeper in the picture above. It looks like there may not be any spring and washer on the 99's. That looks very similar to the keeper that is in my 02 WR only it is solid where mine has a spring attached. I think you are good to go as far as the needle keeper goes.

Dood thank you very much for your help :thumbsup:


Should number 1 move left into the centre thus making number 2 sit under/ontop of the throttle cable housing??

Exactly. The black plastic piece should have a couple of wires coming out of it. This is the TPS, and it will have a slot in it that the shaft that goes through #1 should fit into. This I believe will be clocked wrong and is holding the shaft out keeping #1 from being in the center where it should be. Hope this helps,


Is it possible to move the right hand side throttle cable housing back and forward and then maybe it will slide into the tps?? Im reluctant to take the TPS off as I'll never get it working again :thumbsup:

Thanx for being so helpful :thumbsup:


You may be able to get it that way. I have never actually had my TPS off either, but I know several guys have disconnected it and prefer to run without it. Not that this is what you should do, I would try turning the throttle pulley while holding pressure against it and see if you can get it to drop into place. I don't know if it is possible to get the TPS 180 degrees out or not, but you will just have to try it. I am sure we can get you straightened out here though if we get it wrong. Hope this helps.


Ill give it a go tonight and let you know 2moro

fanx again

No problem, just let me know how it turns out.


Got the carb sorted last night.

Thank you very much for your help

Was it the TPS like we suspected?

Yeah, if you take out the middle screw holding the throttle slide in place you can apply pressure to the bar that joins the TPS and throttle cable housing. It went in easy :thumbsup:

Great, glad to hear it. Now, go out and keep it dirty side down. That is unless you are a freestyle rider. :thumbsup:

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