Hello,
After a full day of jet swapping and test riding I think I found a good setup for the Mikuni TM40 on an XR650R.

First of all, I'm below 1000ft. altitude(Florida, USA). The motor is stock, headers stock. The pipe is a White Bros. E-series w/ 12 discs. The stock intake manifold and airbox has been fully unrestricted and a UNI aifilter (no holes in side covers, yet).

I tried pilot jets from 22.5 through 27.5 and test rode each. I ended up sticking with the 25 pilot (pilot screw 2.25 turns out). The bike starts first/second kick with no choke, response is crisp and strong. Popping/overrun when closing the throttle is at a minimum but is still there (22.5 pilot had almost no popping, but didn't feel as stong as the 25).

The 9DJY4 needle worked great in the middle postion so I didn't mess with it. No surging while cruising (like stock carb), and accelerates clean and strong from 1/4-1/2 throttle on up.

I was unable to get full throttle very much, so the main jet is still questionable. 3banana recommended that a 140 was a tad lean, while a 150 was perfect (dyno'd with aftermarket headers). However, I tried a 142.5 and 145 and left the 145 in. With stock headers, I figure definitely leaner than 3banana's 150 main jet. So a 145 main 'seems' about right. Acceleration was clean and strong during short periods of full throttle. Extended full throttle might show something different though.

I'm quite pleased with the carb to say the least. It really wakes the 650R up. The whole powerband is strong and smooth with no noticeable holes or hiccups (butt dyno ). I still plan to experiment when the accel pump kicks in, right now it comes on at ~1/8-1/4 throttle on through 3/4 throttle.

Just my findings. Still open to other suggestions... Thanks.

Hey Snaggle

just checked my jets

pilot 25
tube Y6
Main 150
needle 9DJY4-98
clip middle
brass idle 2 turns out
white air 5 3/4 turns out = 1400rpm

your findings back this up really well


have fun

3banana,
Thanks a million for your time and efforts, you were a great help. The properly tuned Mikuni really maximizes power output for the stock motor. I can also see how it would make a bigger difference with a modified motor. Thanks again, ride on!!

nobody seems to mention the air jet, located in the throat of the carb, controls the emulsion ability of the needle.
when having mine dyno'd made a HUGE difference to the midrange fuelling by messing about with the air jet.
just a thought loike.
www.mikunioz.com/hs40__tm36-68.htm
download the tuning manual, become a carb expert!!

just curious as to what your accel pump is set at? thanks, jeff

it is set at standard which is in the first quarter of throttle opening

on tuned XR's with open exhausts it helps stop bogging on snapped open throttle from idle

X81 our resident set up guru said leave it alone it's fine where it is