How many run their YZF without the fuel pump?

Who has a nice running YZF that does not use the acceleration pump at all?

How do you inactivate it?

Thanks

Who has a nice running YZF that does not use the acceleration pump at all?

How do you inactivate it?

Thanks

What would the purpose be for doing this and why?

Why would you want to do that?? The pump is what lets you grab a hand full of throttle from low RPM. If you don't use it you will have to run very rich all the time which will hurt you down low. Down low is what Thumpers are for!!

Two things.

1) It might be usefull to set the jetting properly.

2) With the fuel pump giving a 4 sec squirt, I'd be closer to a BK mod result (1.0 to 0.5 sec) without the pump than with the pump. So, I might try a smaller diameter needle (EJN or EJM instead of the standard EJP on the 426s) and give it a try.

Thanks

The presence of the accelerator pump has no bearing on your jetting, as the jetting is judged at various throttle openings in a steady state, so the pump is not active. Not using an AP would necessitate richer slide cutaways than are needed otherwise to help transition the engine, and that would mean an engine that would not be as sharp as it could be at part throttle. This, of course, is how all four-strokes were at one time, and learning how to "roll on" the throttle was a required skill.

I have and could again live without one, but I see no advantage.

Two things.

1) It might be usefull to set the jetting properly.

2) With the fuel pump giving a 4 sec squirt, I'd be closer to a BK mod result (1.0 to 0.5 sec) without the pump than with the pump. So, I might try a smaller diameter needle (EJN or EJM instead of the standard EJP on the 426s) and give it a try.

Thanks

Look at our jetting setups in the sticky. I have a feeling you are having a frustrating time jetting as I asked a similar question in the midst of a jetting nightmare.

Concern yourself that you might be richly jetting if you have burbly problems. For some reason there is a hysteria around here about jetting WAY rich .

Look at our jetting setups in the sticky. I have a feeling you are having a frustrating time jetting as I asked a similar question in the midst of a jetting nightmare.

Concern yourself that you might be richly jetting if you have burbly problems. For some reason there is a hysteria around here about jetting WAY rich .

Well to come back on what was previously said, my bike runs like an old school 4 stroke at the moment, any fast throttle twist will make him bog.

I'm not frustrated at jetting my bike.. not yet. :applause:

It's that I will be putting stock YZ jetting in my YZ-timed WR426 in a couple of days since as is, my bike can't handle fast throttle variations.

My pump is timed OK and the squirt does no hit the slide.The bike seems to run stronger than what I could handle almost no matter what the jetting is. But still, I have a bog when I open the throttle too fast (almost no matter what the RPM or initial throttle opening is). I've been working around with mj from 165 to 175, DQR needle clip 3 to 5 and pj from 42 to 48 and I can't clear out this bog.

I hope jetting like a YZ will ELIMINATE the bog I have.Diff with the YZ are the smaller paj (WR is #75 instead of the YZ #100) , DQR needle instead of the YZ's EJP and the 162 main. Ah, and the bigger starter jet....

I was thinkin about doing the BK but I can't believe yamaha did such a bad job that I need to drill holes in my carb...

I just tried another engine (polaris) that has absolutely no bog and is plain stock. I'm sure mine can do it too.

I'll try the stock YZ jetting and see what happens.

I'm just a week end warrior that has plenty of power except when he needs it on a urge.

The "bog" you describe sounds lean to me. :applause:

This, of course, is how all four-strokes were at one time, and learning how to "roll on" the throttle was a required skill.

I have and could again live without one, but I see no advantage.

I don't think Husabergs had an A/P until the '04 model when they switched to Keihin carbs. The '00 I rode had great throttle response, you could wick it like a 2 stroke.

Anywho, back in the early days of this site there were a handfull of WR400 owners who disabled their A/P altogether, I think most went back to at least limited A/Ps.

I was then, and am still, solidly in your corner in thinking that turning it off altogether is not the best idea.

OTOH nothing perked up the response on both my 426s like reducing the pump stroke by 2/3. Everyone I know who owned one (5 guys) had me do this to theirs also, and all of them raved about the difference it made.

Having said that I accidentally put mine together wrong once, disabling it totally, and it ran awful.

The "bog" you describe sounds lean to me. :applause:

Put a WR needle in your YZ, wick the throttle open, and then get back to us. :eek:

JK, the WR needle is very lean due to the mild taper. His problems probably begin and end there given that his motor now breathes like a YZ.

But if there is/was ever anything resembling a consensus on this site about anything, it is/was that the stock 426 accelerator pump was on the generous side.

I don't think Husabergs had an A/P until the '04 model when they switched to Keihin carbs. The '00 I rode had great throttle response, you could wick it like a 2 stroke.

Anywho, back in the early days of this site there were a handfull of WR400 owners who disabled their A/P altogether, I think most went back to at least limited A/Ps.

I was then, and am still, solidly in your corner in thinking that turning it off altogether is not the best idea.

OTOH nothing perked up the response on both my 426s like reducing the pump stroke by 2/3. Everyone I know who owned one (5 guys) had me do this to theirs also, and all of them raved about the difference it made.

Having said that I accidentally put mine together wrong once, disabling it totally, and it ran awful.

Are we all talkin' yz426's here? or WR/whatever. Given how awesome my yz 426 has been in this department, I get a little confused when I hear guys talking about yz jetting problems. A lot of the posts I've seen don't mention altitude, temp, or whether they're dealing with a known starting point (from stock) or not. I haven't had the slightest tendency to change my yz426 jetting from sea level to 4000 ft. Winter to summer. :applause:

R&D P-38 Lightning, & have never had a problem.

Thanks to you all for active discussion.

I did not mention the altitude and so on since I did not want to start a jetting thread in itself. I've been discussing about jetting the YZ timed WR426 with Hick, Taffy, Indy, Hamish, just to name a few TTers who helped me by PM.

I was just curious about the fuel pump thing and if anyone had tried to jet the bike without the pump.

If anybody is still interested here are the specs:

0-1000' / 30-90 degs

YZ timed WR426 with FMF powercore 4 + quiet core insert, air box wide open under the seat, stock header.

I think the best thing to do from now on is to try the YZ jetting set ups. Then I'll go from there. I ordered the parts yesterday. I'll be all set in 2 weeks.

Thanks to all again.

Are we all talkin' yz426's here? or WR/whatever.

WR/whatever? The WR has much different jetting due to the closed airbox and tiny exhaust insert.

Given how awesome my yz 426 has been in this department, I get a little confused when I hear guys talking about yz jetting problems.

This post is about the A/P more than jetting, really. For the 426 Yamaha changed the A/P diaphragm, improving the response, every year. Most '02 owners were happy w/ the stock A/P. Most '00 and '01 owners who had a chance to change the pump stroke liked it better than stock.

By all means, if you have an '00, you should swap the '01/'02 needle in there. Best $12 you can spend.

A lot of the posts I've seen don't mention altitude, temp, or whether they're dealing with a known starting point (from stock) or not. I haven't had the slightest tendency to change my yz426 jetting from sea level to 4000 ft. Winter to summer. :applause:

Well I guess you are more tolerant than I, at 4k ft. I hated the way mine ran stock, but my '01 wasn't as bad as my '00, and I rarely if ever go below 4,000 ft. But I share your pet peeve about jetting posts that leave out environment etc.

Only a handful of TTalkers that I'm aware of who tried the BK mod on their A/P went back to stock, the vast majority thought it was a substantial improvement. Have you tried tuning your pump at all?

Good to know. Mines an '01. It's been great. Thanks.

This post is about the A/P more than jetting, really. For the 426 Yamaha changed the A/P diaphragm, improving the response, every year. Most '02 owners were happy w/ the stock A/P. Most '00 and '01 owners who had a chance to change the pump stroke liked it better than stock.

Here are the stock parts number for the piece 31 of the carb assembly for 2001 and 2002 WR and YZ 426 f. Looks like the stock diaphragm is the same for 2001 and 2002 WR426f.

As for the YZs, the 01 had a different one and the 02 as well.

Have a look:

01 and 02 WR426: 5JG-14940-18-00

01YZ426: 5JG-14940-07-00

02YZ426: 5JG-14940-17-00

I assume I'd better use the 01YZ426 diaphragm now that I'm buying all the YZ jets for my WR carb.

What do you think Hick?

the standard jetting is usually rich as hell. to sort your jetting dramas, in particular the bog off idle have your bike jetted on a dyno with an air fuel trace so that the jetting can be set up for partial throttle and for when you want/need to grab a big fistful of throttle

the standard jetting is usually rich as hell. to sort your jetting dramas, in particular the bog off idle have your bike jetted on a dyno with an air fuel trace so that the jetting can be set up for partial throttle and for when you want/need to grab a big fistful of throttle

If my WR still has this kind of bog once jjetted to YZ specs I'll try it. :applause:

I assume I'd better use the 01YZ426 diaphragm now that I'm buying all the YZ jets for my WR carb.

What do you think Hick?

If it were me I would get whichever one was in stock and then do the BK mod. As I recall the only differences in diaphragms are the center button depth, which limits the stroke, which matters not once you install the pump adjustment screw (if you do this you can experiment w/ running it totally off, totally on, and everything in between if you like).

If you aren't planning on doing "the mod" then I still really don't have an opinion. Since, based on my completely unscientific opinion, it seems to me that fewer '02 owners bitched about the AP then perhaps you should get the '02? OTOH FZ up there has no complaits about the AP in his '01....

You could get both and try each one.......

:applause:

Thanks for your advises. :applause::eek::)

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