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Mikuni TMX answer (for real experts only)

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I have two carbs, 02 and 03 (my friend gone to PWK). They looks same and i thought they have same needles and jets.

But manual said that there are different sets of needles for 02 and 03.

 

2002

7eyp9n-5kc.jpg

 

2003

7eyp9u-gwo.jpg

 

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As you can see, even the needle of the same diameter have different names. How can I compare the needle from 02 and from 03?

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Those are different tapers. Similar but a slightly different taper.

Is there a specific question or were you just wondering what were the differences?

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I tried to jet my 02 carb, i want to play with few 02 and 03 needles, which i have.
As i can to know if i will replace 6BEY30-74 (2.740 diameter) on 6BHY38-74 (2.740 diameter) i will get rich or lean mix?

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I never had an 03 model but the straight portion will be relevant from one to the other (02-03). The straight portion will effect from 1/16 to a little over 1/8 throttle. The taper is the rest of the region until around 3/4 throttle. The clip will effect the starting point of the taper so depending on the clip position, the straight portion can start below 1/8 & start above it. Use the clip to dial in the sweet spot of the needle. Once you find how the bike runs, you can realize what part of the needle needs to be different. If it is good everywhere but goes rich from 1/2 on up, you need a less aggressive needle taper. If it goes lean from 1/2 on up, then you need a more aggressive taper. If it is consistently rich or lean everywhere, then the fuel curve is good bit you need to adjust the clip. What you are after is a needle that gives the proper fuel curve for the engine requirements.

I like the needles from the 2001 model. They have a less aggressive taper (a leaner needle from about 1/2 on up). With that said, I've never tried the 2003 needle options. Let us know if they are richer or leaner. You can measure the needle with a caliper to find out if the needle is thinner (richer) or thicker (leaner) from 1/2 on up.

Edited by KPRacing

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I remember something about the 02 having vertical mounted reeds vs. 03 with horizontal mounted reeds.? wondering if this was the reason for the needle variance for those years, and whether it made a difference with how it ran.. You should compare the 01 mikuni needles as well to see why the 02 is so different..

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The difference from the 02-03 reed cage is the 02 is a two piece unit with 4 reed petals vs. the 03 unit is a one piece with 8 reed petals like the cr500.   In 03 Honda redesigned the reed cage to get better flow & throttle response.  The addition of revised & improved oem cyl porting specs with also a redesigned expansion chamber all contributed to help throttle response.  I think Honda went to a richer profile 03 needle to compensate for all the changes made to the motor.  

After doing some testing with the 01,02 & 03 oem mikuni needles as well as the red & blue JD needles I found the 02 needle to work best on the 02/03 motor for mx use with an S8 nozzle.         

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H4L, if you still have the 2003 needles can you tell us the difference in the taper of the 2003 needles in comparison to the 2002 needles? Are they a more aggressive taper or a less aggressive taper? Where (throttle position) are the different?

Different pipes have different pulse waves at certain rpms. Different port timing as, well as port sizes, intakes, case volumes ect, also change the "signal" at the carburetor. So different engine configurations will require different needles, clip postions, jets, timing requirements, ect.

Knowing how the needles vary will help in regards to making a choice as to what series needle one may want to select for their build configuration. What works for one guy may be a bit off for another.

Edited by KPRacing

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KP -  I did the testing on the needles back in 2005 & sold the 03 CR250r owned a few years back so I no longer have the 03 needle to compare.  I do understand that different engine configurations can require a certain type of needle - clip positions, jets, timing requirements that would best suit that particular engine set up.   That being said for the average weekend warrior or racer the mikuni jetting needs to be leaned out a bit from the oem specs.  I think Honda's R&D team at the time the 02-07 CR250's were being developed must of been out to lunch when it came to the oem jetting specs.

The mikuni carb can work it just needs some patience with jetting to the type of riding conditions desired.          

Edited by H4L
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I’m no expert but I tried researching the mikuni needle codes several years ago when I got the 01’ CR 250. What I found may help somewhat but not entirely. Apparently Mikuni designates the taper angle/angles with a letter. Each letter is worth 0*15’ (zero degrees 15 minutes) so…
A=0*15’
B=0*30’
C=0*45’
D=1*0’
and so on.
If applied to the Honda needle numbers then the…
01’ needle 6BEH1 appears to have 3 tapers (BEH) the needle transitions from the straight section to a B taper (0*30’) then to an E taper (1*15’) and ends with an H taper (2*0’)
02’needle 6BEY30 = 0*30’ to 1*15’ ending with 6*15’
03’ needle 6BHY38 =0*30’ to 2*0’ ending with 6*15’.

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