KDXRIDER.NET

I need carb data from engines that have a perfect chocolate brown color to the spark plug. If your jetting is a bit off then the color will be lighter or darker and your data is of no use to me. Also the data needs to be from engines that use the full rev range not from riders that just do low speed trails.
I have a theory that if verified should be of use to everyone who wants perfect jetting.
The theory is that at a certain percentage of slide opening the area around the needle at its jet is equal to the main jet area.
On one of my engines that runs perfectly that percentage is 60% but I don't know how common that is.
Getting that relationship right means you get the needle taper right. Selecting the right needle is the hardest part of carb tuning.
I think the KDX is a perfect engine to study and I know many of you have it dialed in to perfection.
Those that participate will receive the following Excel spreadsheets:
Oil Ratio Calculator - shows you the correct fuel/oil ratio to use depending on which oil you use, type riding, and max RPM.
Carb Flow Calculator - shows you the max flow velocity and where that falls within the range designed for enduro to MX bikes.
Carb Needle Calculator - shows you if your current needle has the ideal taper (once I get it finished)
Just message me after you've posted your data.
Needed: Carb brand, throat size, idle jet size, main jet size, needle ID, needle clip position, fuel/oil ratio, type gas used (does it have ethanol?)
thanks guys
ps- I know some people think that this is just legs on a snake but let me tell you my story. I bought a bike and changed the idle and main jet so it ran really good and gave a perfect colored plug and ran really crisp all thru the rev range. But it had an odd lack of effect past 3/4 throttle. I finally figured out that the needle taper was too steep which fully engaged the main jet too soon. If I would of had some guide that dictates how the taper should be then I would of known it was off. So I want to figure out a standard to help others out. My bike is a commuter bike and its very possible this is a trick the factories use to limit power on the street. So this will help many people who are trying to get the best power out of commuter bikes. Also it will help people who are putting a different carb on their ride. Carbs are sold with a certain set of jets and needle. But on different engines then the main jet would be resized and so should the needle so the main comes on at the right % of throttle opening. Look at this graphic showing the amount of effect of the main and the needle and how their effects are equal around 60% slide opening:

Hi. Cool project. Good luck with it.

I don't need luck, I need participants. ;-)

OK now I did a flow analysis and found out the most stable gas/air ratio will be had when the needle jet open area at 80% throttle open is equal to the main jet hole area. This means the needle and main jet need to maintain the same relationship so ideally if you change one then you should change the other. The two should be considered a set.

Here's my page on this subject: http://www.dragonfly75.com/moto/selectNed.html

Would love to give you some numbers, but I am more of a low speed guy that lugs in 2 and 3 gears. Never really reach that middle to upper power band much. Good luck with the study mate!

Can I ask where you got the info for the emulsion tube inner diameter. I've been working on my own project of making replacement emulsion tubes and I don't measure them at 2.9mm

According to Keihin it's 2.9 which is also the needle head OD at the top of the needle above the clip slots. More info HERE

I see the dimension for needle head diameter but I don't see anything for emulsion tube diameter. Could you point out where it is please

I don't remember the site I got this info from:
Keihin needle jet inner diameters
PWK33-39 2.9mm
PJ 2.9mm
FCR 2.9mm
PE22 and PE24 3.4mm

It's the site info which is important for those numbers, I have a worn out ovaled FCR emulsion tube in front of me and the head of the needle will not go in the emulsion tube, I have seen the number 2.9mm posted many times but never from an official source. Hence the reason I question that number

Well the FCR is a slightly smaller needle than the PWK I believe, like in the 2.85mm area I think. Any machine shop should be able to dimension the hole size fairly quickly.

I got those from Sudco but I don't fully trust them.
2.9mm looks right for these carbs for a normal jet to needle clearance: PE34-38, PE36, PWK33-39, PWM38, FCR 28-41

By looking at the range of needle diameters (fat part at top of taper) I think the needles jet is probably around 2.73mm for PE20-28, PWK26-28
That would give the normal Keihin clearance.
That clearance is important because fuel comes up that area even when the throttle is completely let off. When that jet wallows out then you have to really lean out the idle jet to correct at idle but opening the throttle the extra clearance means more fuel all through the rev range and you can change the main for it to run good at top rpm but at low throttle settings it will run like shiet (too rich).

Other things I have learned:
for simplicities sake (thinking we are all 3rd graders) we were told by Mikuni and others that the needle has its influence from 1/4 throttle to 3/4 throttle. Actually on most carbs the needle starts kicking in a bit before 1/8 throttle and has maximum influence at half throttle but then declines in influence till at full throttle it has about 18% influence.
It's the carb flow velocity that determines the right jetting. (the engine size/ max RPM/ carb size determine the velocity)
So if someone gives me a few details I can tell him what jetting change he should make for a good standardized jetting (which then may have to be fine tuned) I've figured out the formulas for these calculations. I don't know if being so smart is a blessing or a curse. hmm

SUDCO double checked and says they measured that the needle jet hole is 2.6mm for PE20-28, PWK26-28

As I thought those original measurements you gave are completely wrong and I wouldn't trust any other measurement sudco give on needle jets either. As you said charts list the straight diameter of the needle being effective at 1/4 throttle but also that is not accurate, it's so important even at idle . I put in my replacement needle jet last week but haven't had a chance to test it, I made mine 2.8mm

a .1mm clearance is probably a tad too close. Some Mikunis are that way and it causes a non-linear fuel/air ratio change as the throttle is being opened. But if you don't putt-putt much then it probably don't matter. The industry standard clearance is .2mm.
And I think I said that that clearance affects everything starting at closed throttle. That's what I know to be true any way.