110 cc engine in a 1969 CT 70


Well-Known Member
I dunno...something is really amiss with your VM22 setup. The $64 question is "what?". Your top speed is about what I'd expect, which points toward a viable air:fuel ratio at WOT...sorta - it could be on the lean side. What's got me baffled is how you keep having to stick with the jet needle at the leanest setting, regardless of the main jet. That makes no :censored: sense, unless something is wrong with the carb. Ordinarily, I'd suggest dropping-down to #150 and lower the jet needle C-clip 4 positions; that'd lean-out WOT mixture by about .4:1 while richening mid-throttle mixture by at least twice that amount. Normally, this cleans-up the mid-range lean spot. However, we seem to keep going around in circles. Thus, I think the VM20 swap makes sense...it should be a lot more forgiving.

As I recall the main jets are the same, so I'd add the next two smaller sizes to your present collection. Then I'd start with the #160 and go from there, finding the rich limit and working back in the leaner direction one step at a time. In terms of top speed, I'm not so sure the impact will be significant. True enough, you're downsizing by ~20%, but consider the fact that the VM20 is still ~20% larger than the 18mm Keihin through which these engines inhaled in factory-stock trim. I've seen VM22s fitted to 70mph+ bikes...and yes, horsepower = airflow regardless of displacement. By the (rough) numbers if a 22mm carb were just big enough to support 70mph power, then a 20mm carb should be enough to support ~65mph. Your motor makes enough power to pull 65mph under ideal conditions only, a.k.a. imaginary top speed. Usable power runs out around 60mph. This could be a win-win...


New Member
OMG, runs amazing now! Installed the VM 20 carb out of the box, used base settings from the VM 22 (Main #150, needle 2nd richest, pilot 22.5).
Has better bottom end and midrange, and even top speed increased I think. Had it up to 55 mph in no time and was still accelerating a bit more.

I haven't even tried other main jets or needle settings, maybe I'll get a bit more out of it but so far so good! I wish I knew what is going on with that other carb??