my leak jet is blocked off

[dwnlowx]

i just took my carb apart i was gonna take the leak jet out and i noticed that the whole is bolcked off what is the problem here

i have had the ap wired but thats it and the bike has been running great ,i just hear every one is changing there leak jets and i notice mine is blocked off so any input is great

[grayracer513]

There is no leak jet on the '03-'04 YZ450's. To add one, the float bowl would need to be drilled and tapped, or replaced.

[Gerry B]

I have a 2000 YZ 426 and mine was blocked off too. I took a float bowl off of a 2005 CRF 450 and mounted it up and it works great. You can get a float bowl from JD Jetting for around $65.00, I believe for your bike. Or, order a float bowl from the dealer from a newer bike. The only reason I used a float bowl from a CRF is because I got it for free.

[dwnlowx]

so thats why nobody knows what im talking about

well is it needed or what ,the bike runs great, will it make it better

[grayracer513]

Here's my deal: AP mods and leak jets and safety wire are all the rage right now, but they don't apply to every bike made.

I have an '03, which is nearly the same thing as your '04, and the way I ride it, the only way the throttle could be more responsive is for the bike to read my thoughts as they happen. I can force it to gag and stumble if I whack the throttle hard enough at an idle, and I can make the bike bog if I leave it in third coming out of a slow second gear corner and give it enough gas. But who rides like that?

I learned long ago to roll a throttle smoothly because back in the day, there was no such thing as an accelerator pump on a big thumper. The '03 is snappy, responsive and runs clean at all throttle openings. I see no reason whatever to spend more than a very few dollars trying to make it any better than it is. The Zip-Ty mod is probably worthwhile, IF it produces the same kind of power increase in the '03-'04 as it does in the '06, but there's a lot more to it than an AP tweak or a leak jet. IMO, spending a lot of time and money messing with the AP is a waste on the early 450.

My two cents.

[Baron Von Beard]

Gray, I'm disappointed. They added the AP and leak jet on the newer bikes for a reason. That's like saying that there's no point in jetting your bike properly because if you just learn to ride it differently it'll pull you around.

The AP is actually very easy to tune but a leak jet is a very important part of that tuning. Not only will a properly tuned AP circuit aid in throttle response but it will clean up the bottom end jetting considerably resulting in better outta the corner pull.

As far as the original question goes, as long as you are happy with the bike, I say leave it alone, but there is a lot to gain by adding a float bowl with the correct leak jet in it.

[grayracer513]

The true function of a leak jet is not to tune the AP squirt. What they are intended to do is actually to eliminate the discharge from the AP nozzle under circumstances where the throttle is opened only slowly or partially. This is more important on bikes that spend a good deal of time at part throttle than on a race bike, or on smaller, more highly strung engines, which is why the 250F has had one since its introduction, and the 450 didn't get one 'til '05. But it does make for a more sophisticated carb.

Let's understand what an accelerator pump does (I know a lot of you already do, so just hang on here). Say we have an engine either idling or running at low speed with the throttle mostly closed. It's running almost entirely, if not entirely, on the pilot circuit, and no fuel is flowing through the main jet. There's about 15-18 inches of vacuum on the engine side of the carb, and that's what's bringing fuel out of the pilot jet and into the air stream. Now we rapidly open the throttle; what happens? The size of the hole in the carb suddenly increased twentyfold, and because the engine isn't turning that fast, the vacuum over the jets falls to something less than one inch, and fuel stops flowing into the air altogether. The engine stumbles. If fuel doesn't start flowing from the main circuit quickly enough, it will stop running completely.

If we assume that we have to be able to open the throttle that fast, something needs to be done. The solution for at least half a century has been to put a pump on the system that squirts one temporary shot of gas into the carb to run the engine while the flow in the main circuit gets started. It works, of course, but that's its only function.

The AP circuit is little more than a pipe running up from the bottom of the float bowl. Gas has to be raised to the top by the pump. The leak jet is literally a hole in this pipe near the bottom of it, and the effect is that if you want to get gas to come out the top, you have to push it hard enough to make up for the leak at the bottom, otherwise, it just runs out into the bowl. This cleans up part throttle operation and carburetion in general by eliminating or reducing the AP discharge when the throttle isn't opened far enough or fast enough to need the squirt. In street bikes, this is important to clean operation. A bike like the 250F, that has a bigger carb (relative to its engine size), and more aggressive cams, will usually also have a need for a more forceful AP squirt. But that causes a lot of gas drooling in through the nozzle while working the throttle through a big loose corner (for example), thus, the need for the leak jet.

Since the leak jet is there, however, it does make a convenient way to dial in the strength of the AP discharge, and it's actually easier to do it that way than by the intended adjustment methods (AP cams, diaphragm button length, spring tension, and nozzle size.)

The AP on an FCR is intended to be mechanically forced through its stroke by a cam on the linkage. Like most such pumps, it has a buffer spring built into it to limit the amount of pressure applied to the pump. If pushed too fast, the spring gives, allowing the pump plunger to travel at a lower speed than the linkage did. This spring is what is being wired out of function by the wiring mod, thus forcing the pump plunger to travel at the full speed of the linkage. (this is also why an O-ring is a preferable approach IMO; it's more flexible and forgiving, as a stronger spring would be) That, along with shortening the diaphragm button to increase the pump stroke, results in a stronger pump discharge, and probably also a shorter one. The reason for the larger leak jet is that this works a little too well, and some more of it needs to be dumped off than did with the factory setup.

Please understand that I'm this won't work for an '06, but that there are better, if more complicated ways to approach the problem. Also, the '06 as delivered clearly stumbles much harder than an '03 or '04 did. (I also still think that a lot of it can be cleared up with jetting. We haven't done this mod to ours, and my son never complains about how the bike runs) All I'm saying is that it's only going to cure a problem where that problem exists, and that simply because it works for one kind of bike does not mean it will work on anything else. In fact, since the problem with most 250F's is bogging from excessive pump squirt and richness (cured by a larger leak jet and timing adjustments), this mod would be a move in the opposite direction from the need.

Before trying this on an '04, you should understand what it does, and how it will affect your engine, and whether there's even a need for it in your own case. On my '03, there's simply no need. I couldn't use any more throttle responsiveness if I had it, and on that bike, I could get more out of the two hours I'd spend on this mod by taking a nap. Nevermind the money.

[TexN343]

First let me say I have had a few situations where I turned the throttle fast on a slippery turn after braking hard and the bike would just die. From what I understand this mod would prevent this from happening? If so this would drastically help with those first turns out of the hole. I thought if might of been a jetting issue but now I find out I have to spend 65$ but if it's a permanent fix why not. Would this mod happen to change the mpg. I'm starting to race cross-x this weekend and don't have an oversized tank. I would hate to fix the carb and have to refuel more often.

[grayracer513]

I can't say whether any AP work would help you, because, for one thing, I don't know what year your bike is, and for another, it frankly sounds like you're brake stalling it, in which case the engine is dead already. You'd need to determine if such stalling is really the result of an inadequate pump shot or not.

As far as the MPG question, that gets back to misunderstanding the function of the accelerator pump. No fuel flows through the pump unless the throttle is being opened. In fact, including a leak jet may even help MPG marginally by reducing the amount of fuel discharged during minor changes in throttle position.

[Baron Von Beard]

Good grief. I'm too lazy to read that whole thing. You win! No one needs an accelerator pump and messing with things like jetting is ridiculous, no one needs to ever do any of it,along with turning their suspension clickers or changing their undergarments.

[grayracer513]

But if you were to read it, you'd see that I didn't say what you say I said.

[Baron Von Beard]

But if you were to read it, you'd see that I didn't say what you say I said.

It's too long. I believe whatever you say and you are right.

[flintlock28]

Beard, You need to read it.....it's one of the best, well explained, post's I've ever seen, on what's going on with the Accelerator pump function.

I agree with gray, that if you have an 06' or 07' than fooling with the leak jet, may reward you, if you have a stumble or hiccup.

I have an 04', and the bike never hiccups/stumbles, unless you have it at idle, and whack the throttle as fast as possible. Taking the time/effort to drill/tap the carb for the leak jet, would probably be a waste of time, and also a possibility of causing damage to the carb (OOPs!, got over zealous with the drill!)

[dwnlowx]

Well I Have Decided To Just Hold Up On This Mod Im Glad That We Finally Got This Out On The Table

There Is No Leakjets In The Older Yzf And You Can Switch Fuel Bowls For A Simple Solution Or Just Re Tap Your Fuel Bowl If You Do Want A Leak Jet

Maybe Some One Should Do A Ap Mod E Book Like The Honda People Have, It Could Come In Handy To Put A End To These Types Of Forums

[grayracer513]

People will always discuss things, and they should.

The PDF you're talking about is completely applicable to the YZF, except, of course for the instructions about how to remove the carb from a Honda. It's linked in the Common Threads sticky.

[flintlock28]

Hey gray,

I just thought of something that could make you/me a bunch of money......

How about fabricating some type of variable leak jet...i.e. an external screw/rod/linkage, whatever, that allows one to vary the size of the leak jet opening?? That way, you don't have to open the carb, you just try a setting, and if it doesn't work, you simply move the adjustment dial/lever, whatever??

I'm writing this tongue-in-cheek, but is there any pragmatic way of doing this??

p.s. I'll accept any Royalties......

[Baron Von Beard]

Hey gray,

I just thought of something that could make you/me a bunch of money......

How about fabricating some type of variable leak jet...i.e. an external screw/rod/linkage, whatever, that allows one to vary the size of the leak jet opening?? That way, you don't have to open the carb, you just try a setting, and if it doesn't work, you simply move the adjustment dial/lever, whatever??

I'm writing this tongue-in-cheek, but is there any pragmatic way of doing this??

p.s. I'll accept any Royalties......

Not plausible without completely reconfiguring everything in the carb.

[grayracer513]

I agree. You would need to drill in a adjusting needle that intersected the leak passage. Then, once you got it dialed, how often would you want to change it?

[flintlock28]

Guess I gotta keep playing the Lotto.....

[dwnlowx]

well is zip-ty doing this with there carb mod