When I replaced the stock Mikuni BSR36 carburetor on my Suzuki DR-Z400 with a Keihin FCR39, I found I didn't like the increased throttle rotation. Going from closed to wide-open meant either letting go of the throttle and repositioning my hand, or riding with my wrist contorted in extreme positions. One commonly suggested solution is the use a throttle tube from a Yamaha R6; however, I didn't find it to reduce the throttle pull appreciably. Rather than continuing to buy off-the-shelf throttle tubes in the hope that I'd find what I wanted eventually, I decided to just design my own.
This throttle tube reduces rotation by about 22 degrees verses the stock tube. It's not a drastic difference, but enough that going from closed to WOT no longer requires me to grab two handfuls of throttle.
I had the model printed by Shapeways. The material is Nylon, and so far it has held up to about 6 months of riding, including several rain storms. I believe it will prove reliable in the long term; however, I will be carrying a spare on long/remote rides until I have put some serious miles on it.
For any interesed DRZ owners, this part can be purchased at the following link: http://shpws.me/Guih