new needle problem

26 replies to this topic
  • yo_its_matt

Posted November 01, 2000 - 02:00 PM


james have you used a 4 gas analyzer to set idle mixture? i was thinking of taking my bike to the shop and getting a base line for comparason before i change to the 48 pilot.

  • James_Dean

Posted November 01, 2000 - 07:50 PM


I have not used a 4 gas analyzer to set the idle mixture. If you get a chance it would be a good point of reference. I would like to know where the limits are (how many turns out) for richening up on a #45 pilot and #48 pilot. This is with the #100 pilot air and a slightly fast idle that starts easier.

The needle jet may start to bleed through on a fast idle which can confuse the situation. This is where the needle straight diameter can be a factor.

Low speed hesitations occur when lean or rich conditions exist in the transition from pilot to main(needle jet) circuits. Adjusting the pilot jet and needle diameter is the basic cure. An earlier taper start is what makes a smooth transition complete. This can throw the jetting off up at higher throttle positions. Multi-taper needles are the ultimate answer. Maybe next year, for the 2002 YZ426? In the mean time Clark and I are making our own needles. :)


[This message has been edited by James Dean (edited 11-01-2000).]

  • Taffy

Posted November 01, 2000 - 11:36 PM


James i have to ask this question.

given that the air jet & pilot jet were set by the factory why did you go a larger diameter 'straight bit' in your needle experiments & not stick to the same dia.?

i.e. i use dxM you had a dtM (i think!) so you went to a dvP? surely a dvM should have been your starting point at the time?

the needle dia. would have been naarrower at any given point on the needle as of when the taper started correct? that's 2.715 for the M & 2.735 dia. for the P.

your low speed circuit would have remained unchanged or at least it might have had to be less dramatically altered.

if you had used a DVM where do you think it would have lead your jetting as regards the beginning of the taper to the tip including the main jet?

yours intrigued!


Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • James_Dean

Posted November 02, 2000 - 10:08 PM


Good question. I did not have a DTM and DVM is identical except for clip. My WR is a 2000 and came with a DRS. The straight diameters run LMNPQRS from rich to lean (2.705-2.765mm). I had the advantage of the "Honorable Mr Clark Mason's" previous experience and in-depth evaluations. Clearly P was working good, and N might too. I wanted to try the E taper too so jumped 3-4 steps richer to EMP and EMN. They both worked right out of the box first try and no hesitations. Then came the surprise that the YZ426 was using E taper EKR. Since EMN might risk fouling, I posted a recommendation for EMP (EKP is same) at 1 clip leaner than stock YZ426. Clark worked very hard to get the EKP working and found EKN (EMN) worked better in his bike with a smoothing at the transition to the taper. It may be EMN is better for the masses. They are both close.

We have had many discussions on the cutaway and you have been questioning me about more options for getting fuel down low. Thus 2 more needles in the bike. EMM and EML(2.715 and 2.705 :D ). They came and here is the scoop... They both add fuel down low and MUST have a lean pilot circuit (you were right). The feel at the wrist says it is a half-breed 2-stroke jetting setup. EML#4 has the faintest sputter at 1/8 throttle. Crack the throttle up to 1/4-3/8 and the front pops up. :) This is walking a tightrope for fouling plugs, risky. The accel pump needs to be backed off a little, just as you said. If a factory wanted to hire me for it, I would sort it out but for now it's on the back burner.

To be clear, my focus has never really been on the straight diameter. My interest has been on the taper possibilities. The batch of needles I got went for varied tapers, D/E/F. The standard D sounded absurd to me, it is ONLY .75 degrees. This is half or less than almost all the 2-stroke dirt bikes, but they don't have accelerator pumps. They have triple tapered needles. Many are worse for it because the designers don't ride dirt bikes and the bike testers don't get a clear picture what is driving the jetting. I graph the profile and ride it. The jetting is predictable before riding and the focus is on throttle position. The results show Keihin is right about the standard "E" taper in the FCR. The standard straight diameter is overly lean for dirt bikes though. The same was true for the 2-strokes relative to "standard". A few steps richer is better and several choices are workable.

James Dean

  • Clark_Mason

Posted November 02, 2000 - 05:22 PM



Also do not forget that 2yrs ago when I started jetting the 99 WR the 98-99 YZ's had the DVR needle. The DVR is leaner on the straight diameter than the DVP and starts its taper a little sooner. The remainder of the jets in the YZ carb are close to the WR.

When I switched to YZ timing and went to the YZ stock carb setup it was vary obvious to me that the DVR needle was too lean down low for my riding conditions. I went to the richer DVP and started experimenting and it worked out vary well. The DVP is a consistent performer and in the number 5 clip position falls right in the middle of the available needles across the throttle position possibilites. I still feel its a excellent match for general use.

James and I have been experimenting with "E" and "F" series needles trying to garner more performance, however, my experience so far indicates its a fine balance to get one of these to perform across a very braoad range of riding conditions. Thats why you see comments like it worked great on saturday and the next weekend it ran like crap.

That said I am making progress along with James and his help and feel we will come up with a improved jetting combo for WZ 400's. The main problem for me is TIME it takes a lot of it and work has been very demanding of late.


  • Taffy

Posted November 03, 2000 - 05:47 AM


so you're moving toward;

richer on the straight (even than std)--L.
idle jet MAY come back toward OEM.
starting to back off the APJ at the top.
if you go for -M- it needs machining at the start of the taper.
if you go for -K- it gets too rich further down.

if you go to a two-stage taper you end up shunting the start of BOTH tapers everytime you move the needle a clip!! fun!! looking for trouble!!

scott is sending me that cracked slide. yes, you may question my sanity!! i will modify it & as long as it's up to a short back to back test i will try it.

looking into my crystal ball again; i can't understand why Yam start the APJ late ('00).

my rules (in my head of course) are that the drop in atmospheric pressure & the rush of air to match it should need an immediate squirt. this smacks of another factory fudge-unless you can open my mind. clark recently said 'start the APJ early & finish early' my opinion 100%.

there maybe another reason, this bike is simply overcarbed & there is even the question of a 37 mmm carb. you open tthe throttle & that air should go insane & need a squirt. you put the hesitancy & yams Y2K answer together & you begin to think.

could it be that the very slow air speed that causes the flat spot is the cutaway height & that if we lower the cutaway, get the airspeed up we can then wack some fuel in off the bottom & not 1/8 throttle.

the slide cutaway height is an old focus point of mine & i hope to finally be able to help you both.

now another idea.

variable air screws. i've bought a low speed one which will replace the 100, they may even do a high speed one.once you've got them calibrated (1 turn=100, 1 1/8 is 110 etc)

i've seen some confusion here so lets clear it up (not by you two) the air jets bleed AIR that sucks up FUEL it can therefore be used over a range of revs to richen or lean the engine.

how about using these two air jets to help balance the needle?

if it's important not to have too big a carb we should also remember that the exhaaust header & the ports shouldn't be too big either.

my thoughts aloud.


  • James_Dean

Posted November 03, 2000 - 12:56 PM



If I lead you to think that Yamaha reduced the accel pump in '00, it was incorrect. The pump is the same as '98/'99 from what I can tell. The measured stoke is still .115 inches (3mm). It is easier to adjust with a simple screw. The comments about reducing the pump flow are for low throttle near where the 2.705 mm(EML) runs rich and could foul a plug in tight woods riding.

Lots of former 2-stroke riders can relate to going to smaller pilot jets to clean up low speed sputter. The EML needle is approaching that same situation. The accelerator pump isn't needed at 0-1/4 throttle with jetting like that.

Add a needle with more taper like 1.25 degrees (F) or 1.5 degrees (G) and the accelerator pump may not be needed at all. Some of the stock CR500 Hondas used this same profile - 2.705 diam and 1.5 degrees is a R1370N. I have ridden with a 2.725 diam and 1.25 deg combo (FMN) and it is getting close.

Maybe it needs the next generation P-24 lighting to work.... :)


[This message has been edited by James Dean (edited 11-03-2000).]


Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.