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Leak Jet on a YZ426?

By 642MX,
in YZ400F/426F/450F/450FX

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Grand Master

642MX

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Anybody put a leak jet equipted bowl on a YZ426?

I ordered one from JD. Put it on yesterday, but I still think the throttle response could be better, so I ordered a 40 lj today. (it came with a 65)

Just wondering if anyone has done this, and which leak jet did you end up with?

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Grand Master

grayracer513

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Unless you increase the output of the accelerator pump by some other means, adding a leak jet to your pump circuit will give you a net decrease in fuel delivered to the pump nozzle.

The leak jet has been somewhat misused as a tool for trimming the pump performance, and that really was not its primary purpose in the first place.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=5634237#post5634237

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Grand Master

642MX

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You are aware that the smaller jets actually make it richer, 'cause it's a fuel return(hence the name "leak") jet, yeah? Just making sure and I don't mean to question your intelligence.?

Yes, I've done some reading on leak jets. My wifes bike had a lean bog and I changed the leak jet to a 40 and cured it.

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Grand Master

642MX

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Unless you increase the output of the accelerator pump by some other means, adding a leak jet to your pump circuit will give you a net decrease in fuel delivered to the pump nozzle.

The leak jet has been somewhat misused as a tool for trimming the pump performance, and that really was not its primary purpose in the first place.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=5634237#post5634237

I thought about sending the carb off for the zip ty treatment, but its cold here and the rain/snow won't stop, so I thought this would be a good time for some "garage time"....lol.

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Grand Master

642MX

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Gray- Do you think adding a LJ is a bad idea? I thought (once I get it right) that it will enhance the throttle response.

Its not like the bike has bad jetting or bad throttle response, I'm just trying to make it better. Since I added the JD bowl, it acts like the 290 did (slight lean bog on the stand) before I swapped the LJ.

I'm thinking that a 40 LJ will make the throttle response "instant".

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Grand Master

grayracer513

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Life in the midwest. ??

But read that post I linked to. Leak jets became popular along with the trend toward wiring the accelerator pump as a way to tone down the really excessive gains in pump output that mod produces. The original problem is the accelerator pump actuator spring on most of the early FCR's is a bit weak, which leads to weak delivery in some models. The truly proper fix for this would be to use a stiffer actuator spring, which is just what Yamaha has done with the '08, but no one sells such springs that I'm aware of.

The next best is the O-ring mod, but O-rings tend not to last long in that role, so people went to wire. Wiring introduces a host of other problems though, because when you wire the pump, you also have to all but remove the diaphragm stop button, which radically extends the pump squirt. Leak jets were then used to blow off some of the excess, even though that's not what they were for to start with.

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Grand Master

642MX

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Ahhhh yes, the good life here in the midwest...... ?

I read that thread, its got a lot of good info. I normally have the 0-ring installed, but it was missing when I removed the carb....

Is there any good threads with pictures on how to properly wire it up?..... I've searched but didn't find much. ?

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Grand Master

642MX

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I personally don't recommend wiring, but some swear by it.

Wiring it looks like more trouble than its worth. I'm going to the hardware store tomorrow and get some 0-rings. ?

Hey Grayracer, do you know whats great about living in Indiana this week?......

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King_Air

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Well, I have the California State Legislature (a wholly owned subsidiary of the Democrat Party). I often think I'd like to trade them for Winter.

Well I would rather deal with winter that subsidiary:eek: At least winter goes away:thumbsup:

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  • 2 weeks later...
Grand Master

642MX

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Today I installed the 40 lj. Started it up, re-adjusted the fuel screw and then grabbed the throttle and opened it as fast as I could. The result.... no bog what so ever, it was on the rev-limiter before I could let go of the throttle.

The 40 lj is better than the 65. I rode it down the street (40 degrees today....burrr) and it has more snap in just about every situation. I almost wheelied over in front of my house..LOL.

IMO, this is a good mod. My bike had no bogging problems before I started this, but I thought it would be fun to try and make the throttle snappier, which it did.

So here is my jetting specs....(if anyone cares)

2002 YZ444F (FMF exhaust, hot cams, 110 octane leaded fuel, Ready filter w/ FFT)

* 0-500 feet above sea level

* 40 degrees today

Pilot jet 42

Main Jet 170

JD red needle (3rd clip)

Leak Jet 40 (JD float bowl)

Fuel Screw 1 5/8 turns out

O-ring mod

Boyesen Quick Shot (original version)

Throttle response is absolutely perfect... ?

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