Vintage Dirt Bike Q & A

Gave up on getting replacement parts for old Keihin. So I ordered a spec set carb. First, I ordered a vm round slide 34mm, because my bike originally had a 34mm. I got this running pretty good, but was uncomfortable with the physical size differences and afraid I was either going to rip the air filter boot trying to crimp it that small or suck in dirt by not crimping tight enough. So, I ordered another VM 36mm round slide spec set. As many of you know, these fit pretty darned good, but could use a tad shorter inlet, but it still bolts right on and I don't worry about ripping the rubber boot or sucking in dirt.

Just in case anyone else is thinking of doing this, here is the way I set up both carbs to get them pretty close to where I want them. I'm also curious what others are running on their CRs with 36mm mikunis.

36mm VM roundslide:
From dealer What works
pilot 35 40
needle jet 159Q4 159P6
needle 6DH7 6F4
main 290 290

34mm VM roundslide:
pilot 40 45
needle jet 159P6 159P6
needle 6F5 6F4
main 270 290

Now this isn't perfect, but it is a great starting point for adjustment, at least at 800 feet altitude and 80 degrees. I didn't get into needle clip settings because that is the thing I adust the most for temp differences.
What is interesting to me is that the 34 and 36 seem to really need to be set up close to identical. I thought the smaller diameter would have had a higher pressure from incoming air, hence needed smaller settings. Doesn't seem to be the case at all. However, the 36mm will absolutely blow the 34mm away at long straight away wide open running, and actually seems to have more available grunt all but the lowest throttle settings.

Hope this is helpful. Don't criticize me too much, I'm just trying to get stuff out there I couldn't find anywhere over the past two years.

I cut off about 2/3rds of the flange(mouth) that the airbox boot slips onto on my 1975 cr-250 with a 36mm VM. Not only does it fit better but flows better thru the boot.

I've decided I'm going to do that, too, next time I take the carb off.
How is yours jetted?? I'm still playing around a little with dialing my 36 in, especially at one little spot where the needle ends and the main takes over. The only thing I hate about the Mikuni is that you have to be so careful starting to thread the main jet or you push the needle jet up where you can't thread the main.

I also wasted my money on a so called "SPEC" carb. I sold the bike a while back but if my memory hasnt faded i went back to the stock 35 pilot . I think the carb came with a 330 main and ended up using a 250 or a 260 main. This is at a elevation of about 500 ft above sea level. I know it sure hauled ass when i was done. Also used golden spectro synthetic at 40;1 ratio. The piece i cut off the 36 slipped right over my 34mm on my next project eliminating your airboot problem.

How did you cut it?? Do Cr's have the same problem with the fit?

CRs have the exact same boot and manifold, so the fit problem is the same. I'm also curious what method you used to cut it. I've considered dremel or air tool with 4" carbide blade, but would like your thoughts before committing carburetor suicide.

I used a electric band saw. A sawzall will work but use a steady hand. I packed the carb openings with duct tape. I ran a strip of electrical tape around the flange and used the edge as a guide.Try to score a line around first, dont cut straight thru. Use a vacuum to suck out any debri and compressed air. Then file the edges inside and out. Its not that bad, just take your time. Hope this helps

Also watch the bowl nipple on the carb bowl. I tweeked mine cause it was rubbing on the center case.

When i changed mains ,i just rotated the carb sideways. pull on the cable to lift the slide so u wont interfere with threading in the main.You guys are gonna make me go buy another project!

Mike: thanks a bunch.

And you know you NEED another project!! You really have no freewill in the matter, just accept it.

I have a tupperware bowl of jets and parts for the VM carb. I think i also have a tuning manual if u may need.