Vintage Dirt Bike Q & A

As you may or may not know, I went to a Mikuni 36mm carb on my MR250 a couple of years ago. Bought it spec from Sudco, but it ran blubbery rich when I installed it. Played with needles, needle jet and jetting and got it pretty darn good. But no matter what I did, I had one flat spot I could never get over. I could move the spot around, but not get rid of it. I just accepted it and lived with it. But got to tweaking it again this past weekend trying once again to eliminate this spot. I installed the double taper 6DH7 needle on middle clip and, voila!, smooth as silk and runs awesome. But the weird thing is that in the two years of tweaking, I've come full circle and now have the carb exactly as it was shipped from Sudco, except I have a 270 main instead of a 290 main. I don't understand why this setup seemed so blubbery when I first installed it but runs great now (?? bang head against wall three times??). But it does. Current setup is 2.5 slide; 159Q4 needle jet; 6DH7 needle; 35 pilot; 270 main. Plug chop looks perfect; not even a hint of detonation in anything I've tried. The only thing I can think of is that maybe I was in such a hurry when I got the carb, that I never let the bike completely warm up enough when testing it two years ago. Or the more likely truth is that I'm just sometimes an idiot !! My engine is stock except that it is .040 over, I have a FMF muffler (shortened 4" so it doesn't look so out of place) and I have the airbox top removed for the CR tank and seat, so it does flow better than it did stock.

Update: the difference between 68 degree weather and 86 degree weather is one clip setting on the needle. So, as I have read a few times, the Mikuni VMs ARE much more persnickity with settings than the original Keihins were.