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Floris

Mr Know-it-all of Dirt Bike's answer about YZF426

22 posts in this topic

Hello, on Dirt Bike's site, someone asked a question to Mr Know it all about the YZF426. The question was: "Q. If you can help me with this problem you'll also be helping about five of my friends. We all have a 2000 YZ 426F and we all have the same problem, if you blip the throttle off idle or low rpm the bike will cut out or die if you don't release the throttle. We live at 1000 ft above sea level in a rather continental climate" and he answered: "I've answered this question already. You and your five friends truly should have known".

As I have the same problem, does someone know what his anwer was ?

Thanks a lot.

Floris.

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Originally posted by Floris:

Hello, on Dirt Bike's site, someone asked a question to Mr Know it all about the YZF426.

As I have the same problem, does someone know what his anwer was ?

Thanks a lot.

Floris.

I subscribe and I've never heard him answer this question. I even asked it myself and never got an answer from Mr. Know-It-All.

But I'll bet he said to go one size richer on the pilot or at the least go richer on the fuel/idle screw, in quarter turn increments (counterclockwise is richer) and maybe raise the needle a notch or two. The hiccup of the YZF is widely attributed to a lean spot off idle, hence the suggestions to richen things up.

When making jetting changes it is better to change in smallest increments and only change one thing at a time (so you can tell what is/isn't working).

Hope this helps!

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if you go over to the WR site i was part of the 45 on the steering head angle & i mention my previous experience.

on another page James Dean & i get going in the "bottom end dead spot" (posted by RodH) in which i change the subject to "slide cutaway".

i can't spend your money for you my friend but when i see everyone going up on PJ size i know that .25mm off your slide height has a very real chance of working.

i get a flywheel weight this week & i will change to YZ timing & the needle to go with it. i will do this mod if i run into this problem. but honestly- how much is a throttle valve, peanuts.

i hope you & your six mates can afford a whip around.

[This message has been edited by Taffy (edited 07-11-2000).]

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Thank you. I will try this.

Floris.

Originally posted by Taffy:

if you go over to the WR site i was part of the 45 on the steering head angle & i mention my previous experience.

on another page James Dean & i get going in the "bottom end dead spot" (posted by RodH) in which i change the subject to "slide cutaway".

i can't spend your money for you my friend but when i see everyone going up on PJ size i know that .25mm off your slide height has a very real chance of working.

i get a flywheel weight this week & i will change to YZ timing & the needle to go with it. i will do this mod if i run into this problem. but honestly- how much is a throttle valve, peanuts.

i hope you & your six mates can afford a whip around.

[This message has been edited by Taffy (edited 07-11-2000).]

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i can't spend your money for you my friend but when i see everyone going up on PJ size i know that .25mm off your slide height has a very real chance of working.

I am very interested in what you are saying but unfortunately I'm not sure I understand you. (Language barrier? Are you English?)

You mention cutaway and slide height adjustments. I am ignorant of how to adjust my slide height and don't understand the cutaway thing either.

You seem to be pretty knowledgeable so I'll ask you this:

What about changing the pilot AIR jet? In the manual it says that when pilot jet is changed the air jet should be changed, but that may be another language barrier thing (service manual is sometimes contradictory and hard to understand).

Plus there is only one PN for this air jet (while there are several for needles, mains etc.)

So please explain cutaway, slide height and give opinion on air jet.

Thanks!

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Taffy,

Does $242. US sound like peanuts for a throttle valve? Carb needles are $12. US.

--- This is why I suggest changing the carb needle, not grinding off the bottom of the carb slide(throttle valve).

--- Taking length off the bottom of the slide is reducing the cutaway of the slide. This richens the low speed circuits over a range of 0-1/4 (maybe 3/8) on a flat slide carb. This same effect can be had with a mix of pilot/needle straight diameter/and clip position. (maybe air jet too) The reason not to lower cutaway is that you have more specific control using the other components and their prices are an order of magnitude cheaper.

James Dean

($242 was from Yamaha , aftermarket slides would be cheaper I'm sure, but same goes for jets and needles)

Hick,

The pilot air seems to have a greater effect at slightly off idle (1/10th) throttle, just before the needle straight diameter. But the keihin manuals also mention going up on both the pilot jet and air jet together, as you said. Not sure why...

James

[This message has been edited by James Dean (edited 07-12-2000).]

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i'm gobsmacked at the cost james. i don't claim to spend time checking prices. i also wouldn't expect to experiment & make a $242 mistake. i'm trying to be objective using my experience which is floored because things change on carbs-but not the rules!

but yours is floored, you've taken ages to reply because you had to read a book first. mine is experience based & it's quite clear you've set yourself up as the jetting guru james.

whilst we all change exhausts, models & year of bike that's fine. but what are you going to say when clark has changed his, airbox, ignition, cams, big bore, big valves, exhaust , valve timing & it's hesitant at 4,000rpm & dies off at 10,000rpm? what are you going to say then?

if, hick, you've read all the old info what is it that has got you in a quandry? i'll answer any specific point as analytically as i can.

"i can't spend your money for you my friend but, when i see everyone going up on Pilot Jet size i know that to take .25mm off the base of your slide; thus reducing the height of the slide cutaway, has, a very real chance of working".

James

to say that you can get the SAME results with other parts is wrong full stop. you have more control over fuel mixture the quicker you can make the air go through the venturi. air goes quicker through a small hole quicker given the "must" of the cylinder.

i'm not a parts list book worm but i can see that the throttle valve & the slide cutaway are separate. if it's just a flat plate have an engineer make one up for goodness sake! can you not buy just the plate?

hick

air is introduced to aireate the mixture through an air valve. i'm off to read a book. it ain't about jetting!!!

[This message has been edited by Taffy (edited 07-13-2000).]

[This message has been edited by Taffy (edited 07-13-2000).]

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James - Do us the favor of continuing to share your insights. Your info has saved folks here and at DRN considerable time and money. Don't let the random naysayer mutts of the world discourage you. There's one in every crowd.

Rich Rohrich

Applied Fluid Dynamics

rrohrich@interaccess.com

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Rich, thankyou for the encouragement.

----

Taffy, my silence and delay was to dig through the stack of DB mags to answer Floris' question. Not a high priority.

----

Floris, Dirt Bike Magazine,June 2000,pg14:

Mr.Know-It-All"Delayed Reaction",Quote:When working properly, a 426's ability to accept throttle is nothing less than astounding....you'd do well to check the operation of the accelerator pump. I have seen maladjusted pump mechanisms make 426's stumble when they would normally race toward peak revs. Also, look for obstructions in the fuel passage that fills the chamber to load the pump....

There you have it Floris, not exactly the answer you would expect to fix you and your 5 friends bikes.

James Dean

--------------------------------

Taffy,

Your experiences are valuable and I consider the approach of reducing the throttle valve height a sensible one. After reviewing the cost of the plate alone it appears it would cost $106 not the $242 for the slide with cutaway.(sorry for the misquote) I choose to respectfully reserve my judgement on the outcome at this point.

-

Somehow you seem to be under the impression that I only read books for my answers. Most of my responses are from my own methods developed over years of tuning. During the past 25 years there have been a string of gradual jetting enhancements that allow much better fine tuning. You will not read this anywhere in a manual. The approach is simply determining the fuel passage area versus throttle position. Any jetting change is based on a specific throttle position. The determining components to change are then both based on the fuel orifice area and the engine speed, thus vacuum. The carb slide cutaway is very critical and is given a great deal of attention. In this area on my own bike there will be additional rounding at the taper start. I've not yet gotten to that level on the FCR2 carb. There are bigger factors that come first. Earlier you suggested that the factories share their tuning charts with me. That is unnecessary. I have already charted the carb needle profiles of most Honda CR's 1985-1999, RM's 1989-1999, KX's 1985-1996, and few random YZ's and KTM's. FCR needle codes read like a book now. There are no secrets to hide.

I also have experience with altering slide cutaway on bikes that needed it the most. Several YZ490's, '89RM250's and '91-'95WR250's. Their problems were solved with limited success. The limiting factor was also needle selection and needle jet availability.

The air jet tip was based not on text, but on testing experience and statements from Sudco 's tech support on which I agreed.

--

When answers are short they are almost always incomplete, thus I take more time than most to answer. One thing that is important is that riding and testing surpasses all charts and figures. Only you can jet a bike for your conditions and ability. If it needs richening or leaning at a given throttle position I can certainly help you do it.

James Dean

[This message has been edited by James Dean (edited 07-14-2000).]

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well said jim. you have a far more interesting history than i would have given you credit for. more respect.

i write far tooo crudely all the time & always from the heart. so like i've said in other posts keep it going.

oh & incase you think i've gone completely soft.

rich, don't wash your linen in public. it's only small minded tits like you who have to drop their business name. cheap

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Taffy,

I still want to get back to the fork valving topic over on the WR side/progressive springs. Can share lots on shim stacking, thickness, multi-stage valving, and valve mods. Sometime soon.

James

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My YZF is doing the same thing, and the last MZA had a good carb article in it, it's in the July issue... Anyway, almost word for word, here it is: The fuel misture screw is the most important adjuster on the YZF carb. Located on the bottom of the carb, you'll need a special screwdriver to get at it to adjust it while the carb is on the bike. The stock setting is 1-3/8 turns out, but MXA recommends 2 turns out. This will get rid of the hesitation the carb has right off the bottom... Hope this helps, I haven't tried it yet but intend to...

Mike

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Reply to MikeOK:

Mike,

The stock airscrew setting I believe is 1 3/4 turns out, not 1 3/8. Your recommendation to go to 2 turns out doesn't work, at least it didn't for me. I live in Denver at 5280 ft. and changing the pilot didn't work either. I'm still searching for a remedy.

Dave

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PLAN B: If the increased pilot doesn't help, increased pilot screw doesn't help, the fuel pump is working good, ....

- Try richening the needle clip one slot.

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In my YZ400 manual it states to set the fuel mixture screw at 1 3/8 and 1 3/4. I remember that the manual addressed the fuel mixture screw in a couple of locations and gave contridictory advice. I tried the 1 3/8 and just didn't work so I settled on the 1 3/4. I might now try the 2 turns out. Careful with that screw because it is very fragile. I've already bought a new one after mangling the original.

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Me again- I checked my manual and it says stock is 1 3/8. My bike isn't bogging very bad (yet) but if you guys would, reply and let me know your results. Mine has about 20 hours on it and a couple races and after it is warmed up well it doesn't bog at all. I spend most of my free time just keeping the important maintenance done but if it gets worse I'll tear into the carb. My email- mdws@usa.net... Thanks.

MikeOK

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MikeOK,

Thanks for the clarification. I unfortunately can't check my manual since my bike is getting some work done to it and the mechanic has it. If you would, let us know exactly what your settings are and at what altitude you are. What's your main, pilot, needle setting, and air screw. Thanks Mike.

With my 426, it didn't matter whether it was warm or cold, it would still be finicky right off the bottom.

Daveyg

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Ok here is some more stuff... most riding done between 500 and 900', jet needle right in the middle, I think it's groove 4, pilot screw- 1 1/2 (or so) main jet 162, and I looked again in my manual and it says pilot stock is 1 3/4, I swear it said 1 3/8 the other day but I was wrong it seems....

Mike

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it is perfectly normal because when you blip the throttle it the mixtures off because of the fuel pump which makes it cut off

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