leak jet

Has any one changed there leak jet on the 03`WR 450 I made all the other modes and still have a small crack in the throtle about 1/8 to 1/4 if I role it slower I do not have the problem, but sometimes it will die and it does not seem to be because of to much fuel. any thoughts on this a friend of mine lowered his on a 03` WR250F he had a problem worce than mine it would die all the time. :):D

No need to change the leak jet. I think the Pro YZ450 riders use a #40 leak jet but I doubt that it will fix your problem. JD jetting kit (available at our TT store) is your best bet at improving your jetting. His needles are the best I have tested. :)

there was a posting on the 250f side about differant yamaha accelerator pump diaphrams. a little searching in that forum may find it. there are a couple sizes available that allow the the amount of gas "squirted" to be dialed in. leak jets can be changed to fine tune the setting.

sounds like a major pain in the butt. just changing the clip position on the j.d. needle made a big differance for me.

one thing you can try for free is to adjust the accelerator pump timing. it's on page 4-19 of the '03 manual. give the screw a half turn both ways to see how your bike is affected. it may help you decide if the accelerator pump is where you want to go to dial in your jetting. you may even find out yours was out of adjustment to start with.

I may be nuts, but the needle shouldn't make a difference either way.

If the problem is too large a squirt, then the leak jet or diaphram are the only way to adjust that.

The thought I had was to delay the squirt and then lean out my pilot. Is that nuts, too? Anyone?

The gap in acceleration is lean condition. The needle will have effect if the needle strait diameter is smaller e.g. more fuel at 1/4 throttle and under. I noticed that after changing my original pipe to open pipe the bike is much more leaner. As you do not know much about jetting I highly recommend to you to get JD kit!!! It will fix your problem and you are not going to screw up things even more. I'm talking of personal experience! After spending 100+ usd on jets and 10+ hours of testing I can tell you that everything in this carburettor is balance between compromises. JD has spent maybe 100+ hours of testing and REALLY knows his stuff. Get his kit and use your spare time to ride!

You guys are missing the point. If you read the signature for WFO_IN_IT, you will see that he ALREADY has a JD kit. Telling him to buy something he already has is non-productive.

It seems that some members of this forum suggest a JD kit for anything that ails them. Got a sore throat, get a JD kit – got a bog, get a JD kit.

For those of you with a kit installed, what suggestions do you have for WFO_IN_IT? I am curious what the next step will be.

Ride fast - take chances


Exactly. I, too, have the JD Kit, which I love. I, too, still have the off-idle (lowest of low RPMs) bog and I THOUGHT it was from too much AP squirt.

I'm not sure if this helps, but somebody posted a Yamaha tech update on the YZ450 forum a while back talking about adjusting leak jets for the accel pump. I didn't pay too much attention, but maybe that's what you're looking for. Try searching over there for "tech".


I think the Pro YZ450 riders use a #40 leak jet but I doubt that it will fix your problem.


I dont know why, but I recall the article on the YZ450's used in pro Arena cross having switched to a #40 leak jet. I am not recommending this but the question was asked and I offered up the tidbit. :)

This is perfect. Now... someone tell me if this is a rich or lean condition causing this problem.

We're all talking about the same thing, right? Crack the throttle at idle and there's a quick bog (maybe even dies) and then it takes off. Mine does it a lot more when it's colder than hot.

Too lean? Too rich?

wow, that went all over the place. the tech sheet seems to express yamaha's view that the off idle bog is from a lean accelerator pump squirt and gives the tuning options available from them (yamaha). the leak jet allows for fine tuning. pump diaphragms are bigger jumps. it's an option, why not try it?

as mentioned before, our carbs give us the ability to set the squirt timing. the manual gives the factory setting. the timing can be advanced our delayed with the turn of a screw driver. if some of us don't seem to be fighting the "bog" and others are (j.d. or not), this might be worth checking out. it's free and might give a hint to the problem.

just my thoughts.

forget i said anything about the needle clip position. it wasn't the original question.

grahmo, your's sounds lean.

I'm not sure if the '03 has it, but the '04 has an adjustment screw for the accel pump. Assuming the '03 has one, have you tried playing with it?

Oops, mtrablue beat me to it.


I do have the JD kit and power now red needle clip in middle MJ 48 PJ have tried several clip positions and jets,It runs real good just not perfect I believe it`s not getting the fuel it needs from 1/8 to 1/4 and I thought the main and needle do not take over till 1/4 throtle, when mine dies I have to wick the throtle or pull out the choke to start it, hot start "even hot" does not start it. and it seems like it only does this when I am ride with my girl friend or working on trails, I am not ride as hard as usuall. and not riding with my girl friend is not an option. :):D

I have no bog or hesitation. I believe fine tuning the Accelerator pump will help all you guys with 03's and 04's with this issue. :)

I have no bog or hesitation. I believe fine tuning the Accelerator pump will help all you guys with 03's and 04's with this issue. :D

I agree the acc pump is what you need to mess with and it got rid of my bog. Don't forget to have a fast idle as the flywheel is larger than the YZ and needs a running start. :)

Well said 5spoke. I run my idle at about 2000 rpm.

The pros run the #40 leak jet to increase the squirt into the carb. The leak jet spills off some of the squirt from the Accelerator pump and therfore a smaller leak jet provides a large volume squirt into the carb throat. Therfore if the duration adjustment is not enough then you can go for a different size leak jet. :)

Right-O. #40 is the smallest leak. I looked at a parts fiche, and I could be nuts, but it looks like the diaphrams are about $22per. Anyone else price these little pieces of brass/aluminum and rubber?

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