MB5 VM20 Tuning Woes

Rupus Crunchbucket /


Writing this mostly for catharsis but any tips would be appreciated.

This MB5 came with a hack job VM22 and manifold, 4 petal reeds that I cannot properly identify, but they fit and seat fine, and someone's pipe which I cannot identify either. It has the "import" 70CC cylinder and head. As delivered the bike's riding characteristics were a severe, almost boggy low end and then some decent performance above 10K, but that just didn't seem right.

I checked the jets and it had a 30 pilot and a 100 main. From everything I read, this just plain seemed off. What I read seemed to center around 17.5 pilots and somewhere in the 170 range for mains.

I tried all sorts of jetting combos and needle settings and decided that perhaps this carb was just screwed and that the best thing forward was to get a fresh VM20 with a new (not hacked) manifold.

After installing that I again pissed off all my neighbors with various all day tuning sessions and still could not get solid low, mid, and top end performance. One factor here was I was limited to a 140 and 180 main jet options. So perhaps these swings were way to much, but all in all I felt I would be getting close one way of the other. Didn't happen.

Decided to replace the stock reeds and stock exhaust in order to try to start over with just the carb. I was running 17.5 pilot and 180 main. This is the best combo I have had so far, but still there are "holes" in the tuning. It just has zero power at some throttle positions. It usually has good power over say 9K regardless. It also seems to respond to being loaded up, like if I ride up my steep driveway it has more power at lower RPMS then when I am just riding down the street at the same RPM / throttle.

Bike seems to get worse as it gets warmer. Like less responsive and more bogging in the mid range.

I just ordered the Treats 160-200 jets set, so I will be able to move the main around more granularly but the problems seem to be more in the low to mid range. I have tried every setting on the 17.5 and 20 pilots and just cant find a setting there that is right.

At a bit of a loss.

It is worth saying that I have my other MB5 with a PM20 tuned just fine. I have an AR50 with a Delorto 18MM carb on it that I had no problem tuning either. Both responded to the tweaks and changes I made and I got the right feed back from the tunes to get it set correctly.

The mystery of the bike coming with a VM22 with a 30 pilot and 100 main still haunts me. Bike ran OK with that setup.

Right now it is set up with a 20 Pilot, needle in pos 2, 180 Main. It just seems way off just about everywhere, but I am sort of stuck on which direction to go on either.

Any thoughts? Do I abandon the Mikunis? Could something be wrong with the engine that would cause it to have low power until 9K regardless of the carb tune?

Thanks in advance.


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