VM carb conversion and manifold build.


Active Member
I've made a lot of mods to this bike, one of the first was a conversion to VM carbs. The bike came from the factory with BS38s, which are a vacuum type carb. Being 50 years old the carbs weren't in the best condition, needing new diaphragms and what not. However, diapphragms were $130 each, so I decided on a conversion to VMs. The VMs came with manifolds and rubbers, but when fitted I found the throttle cable interfered with the petcocks, so the carb had to be tilted inwards. I didn' ,like that idea, so decided to make a couple of manifolds that would site the carbs further back, thus clearing the petcocks.

I made the manifolds in two pieces: A flanged shaped to fit the inlet and a 40mm tube which was pressed into the flange and the mounting surface machined flat. The flanges were then drilled and barbs pressed into position for syncing. The carbs are sited about 30mm further back, thus clearing the petcocks.


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You got the easy part done. Jetting is 'interesting' getting them as good as modified stock CV carbs, even more interesting
You gotta be kidding, a chimpanzee can jet carbs, hardly interesting at all! And it goes a lot better than it did with BS38s.
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Yeah, right. fitting two stroke carbs to a four stroke is easy.
They have completely different fuel requirements so 'jetting' where you just unscrew one and fit another is easy.
Getting fuel delivery right with the wrong parts, not so much.
I ended up making emulsion tubes and buying new slides for mine, then went back to the BS38's anyway.
You actually rode it with the BS38's before the diaphragms broke?
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