Aug 2, 2001
Hi I have a 1987 YZ 490. Can I put a 38 MM flatside carb on it. Will it work. What would be the difference from the original which I believe is a 40 mm. They are both Mikuni.

Please any solid advice would be greatly appreciated.

Thank you,



Jun 25, 1999
You can actually replace the round slide Mikuni VM carb that is now on there with a tmx. I replaced mine on my wr to a Keihin 39.5 pwk and noticed a little more power off the bottom and more in the middle. As long as the length of the carb is similar (so that it fits from the reed cage to the air boot without modification), all that you should have to possibly change is the throttle cable. If you need some base guideline jettings, let me know as a riding partner has a 91 wr with the TMX 38 flat-slide on it. If you already own the carb (tmx) and it fits, than it is a very worthwhile investment. Many of the tuners of those bikes changed out the 40's for the 38 to get better bottom end on the bike. That bike really doesnt need any more on top.
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