YZF250/WR250F Carb-3rd Jet

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#1
WRROOSTER:
all right, I'm gonna start something here. I told a buddy of mine about the grey wire mod and the YZF timing, and he comes back to me w/a wild story about his friend of his that says he has a YZF250F that has a 3rd jet in the bowl that he has soldered shut and the bike now RIPS and he's wondering if the WR250 has the same set-up. I don't know as I've not looked in mine and I'm wondering if you've heard of this, or is it one of those internet scam stories or what ? Yeah, I know, a 3rd jet? Right, but it looks like you've been in this YZF/WR thread a while, and I'm thinking, if you've heard it, it must be something. Whadda ya think ?
 
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#2
40+,

never heard of the "3rd jet". furthermore, the keihen FCR carb is a complicated piece of engineering. the apparent simplicity of a carb as described in academic "fundamentals of internal combustion engines" books and as implemented on your lawn mower fools you into believing that it really is simple. your lawnmower operates at a single speed under a relatively constant load, that's why the carb design is simple. but a motorcycle application that requires smooth idle, good midrange acceleration, and top end performance, all in concert, is a real trick. thus the need for a design that is actually a few carbs in one, with lots of needles and valves and so on to cover the entire operating range and to provide smooth handoff between operating regions. screwing this whole thing up is that we operate the bikes at differing altitudes and temps and with varying qualities of gas.

where is this 3rd jet in the following link?
http://www.onlinemicrofiche.com/OEM_Images/yam_db_images/01YZ250FNB4.GIF

ps
personally i believe that canondale, while not having the "complete package" in terms of a race ready bike, is bringing some fresh air to the commercially produced moto game with technology like fuel injection. yes, there are some drawbacks, e.g. the need for DC power for startup and so on come to mind, but at least they are pushing the envelope a bit. but as we've seen in the automotive industry, these problems can be solved and then the ECU "black box" can deal with issues like varying temps and fuel quality and so on to give maximum performance under all conditions. i mean, how long did it take to bring ABS as used on commercial aircraft to general use on cars, and finally to sport bikes? another example, my car has stepless variable timing for both camshafts, bmw calls it double-VANOS, and it too is controlled by the ECU. the result is excellent fuel economy and good low and hign rpm performance. give them a couple of years and yama or honda will figure out how to do the same for a midsized and up 4 stroke.

the wrooster
 
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#3
Yup EFI is on Sport bikes now and C dales 4 wheelers. Have you seen those hydro electric valves? no more cam at all! Subtract about 10lbs from the motor, all the reciprocating mass besides the pistion and changing cam timing on the fly via an ECU. Oh what an exciting time to be riding. Cool 4 strokes are about the biggist inovation this sport has seen in years since the mono shock! Hold on to your hand grips boys, sub 200 lb 60 + HP bikes are a commin.:eek:
 
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#4
a 3rd jet in the bowl that he has soldered shut and the bike now RIPS
Sounds pretty fishy to me:think

I would bet that if he was that knowledgable about the whole thing he would be able to give you more specifics instead of just a "third jet" Surely he would AT LEAST know the name of the jet!

My guess is he is referring to the Leak Jet, which has been one of the suspected culprits of the notorious bog. If I recall correctly, WRooster you may have to help me here, the YZ250F comes stock with a #105 Leak jet(which is very lean) and the WR comes with a #60(which is rich) In order to get rid of the bog in their YZ's, many have been going to a #95 Leak jet. While this is not the only thing that can cause the dreaded bog, it is one example.

Tell him that you need more specific info and see what he tells you.

Coop
 
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#6
Pilot air jet???? Nope!

Hick,
This is straight from the manual, chapter7-pg7. "A leak jet is a setting part that adjusts the flow of fuel discharged by the accelerator pump. Since the accelerator pump operates only when throttle is open, the leak jet is used to adjust a fuel mixture ratio for quick throttle opening and is therefore different from other setting parts that adjusts a fuel mixture for each throttle opening(each engine speed)"
Hope this helps:)
Coop
 
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#7
It's good to hear some actual technical info about those carbs! All the mags (i.e. DB, MXA) ever say is "don't twist the throttle like a two-stroke". I mentioned the R&D Performance P-38 accelerator pump on the Thumper forum, but got no response. Anybody try it?
 
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#8
Natester,

What year YZ/WR do you ride? The recent models have been really good as far as accelerator pump settings from the factory go. If you have a 98 or 99, I hear the P38 does wonders. Also, if you are mechanically inclined, several of the guys on the WR side of ThumperTalk have rigged their own version of an adjustable accelerator pump. Research it, and you will find excellent instructions.

BTW: my YZ250F was jetted perfectly from the factory...only needed a small adjustment on the pilot screw.
 
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#9
YZF

Hey AngryCandy,
Thanks for the advice. I was riding a '98 400F, but I sold it (I'm waiting for a 2002 426F). I'm really interested in the carburation issue, because the whole time I had the '98 I had problems. It always ran awesome at sea level, but as soon as I got up to the dez (4000 ft and up) it would cut out on choppy, high speed stuff. I talked to just about everybody I could think of, including Ty Davis at the National Hare and Hound, but I never did get it worked out before I sold it (I'm a senior in college, so I sold it for financial reasons, not carburation). Anyway, I'm really pumped (no pun intended) that Yamaha finally figured it out! I'd get an '03, but I don't think I can wait that long!:p Gotta get back into it before I'm a bonafide vet.
 
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#10
APJ??

Originally posted by BCooper
"A leak jet is a setting part that adjusts the flow of fuel discharged by the accelerator pump.”
That sounds like the accelerator pump jet.

On the 39 mm I don’t think you can adjust the APJ, I wish you could...

If 250f owners can I think it would be worth experimenting with. The dealers may be in the dark, I call Sudco when I want weird stuff like that.
 

Boit

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#11
Natester; I installed the P-38 on my '00 YZ426 after experiencing that unacceptable bogging/hesitating problem. While I can't claim definitively that the P-38 solved the problem, I feel pretty strongly that it was ONE factor that helped solve it. Switching to C-12 and sharpening my jetting ALONG with the P-38 was the ticket. I'm a very happy camper now.
 

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#13
YZ250F/WR Carb

WRROOSTER: Thanks for the informative reply, and I appreciate the response w/out a "funny' as I felt a little foolish reporting it. Your explanation and the following "leak jet" post were most likely on target, but I am going to get more info. BTW, care to forecast a time for "across the board" use of fuel injection a la Cannondale, in the world of dirt ? No more jettin' sounds like a dream come true, but it'll happen. Did you notice how quickly the outboard motor industry picked it up after the first guy (Mercury, I think)went to it? It wasn't any long drawn out affair like I forsee in the MX industry, but I pray I'm wrong. It's funny you mentioned the Dbl. Vanos system in the BMW's. I was lucky enough to talk to the engineers at the South Carolina plant when I took my M5 there to do the driver's school. they had the new BMW scoot there and were trying to brain storm a similar set-up on the scoot on their own. They are pumped about the applications in motors, and I'm sure that the Teutonic mind will come thru. I'll post a response to the carb issue if I get anywhere w/it.