One Into Two Manifolds

chrisshropshire said:

sorry I meant to ask if I had a 36 mm carb should I get a bigger map manifold ? 38 or 40 mm and make up an adapter ring
as lots of folks say that the single carb works better with the larger manifold

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That's what I would do if the carb spigot is too small for the large manifold adapter. Jim

Somebody once claimed that a BSA Gold Star operated better with a step in the inlet tract. I don't have any steps in my inlet tracts because I believe that is happening involves sound waves. Typically what happens with a sound wave when you have a step, you get a bulge in the boundary layer - restricts flow.
The main advantage of the single carb is convenience when tuning. A well-tuned single carb can often give much better performance than a pair of poorly tuned carbs. With the single carb, you only have one of everything. So if the needle jets or needles are worn in a pair, you only have one to play with in a single carb. I use Mikuni needles in my 34mm MK2 Amals. The range of available tapers is greater than with Amal and they are hard anodised. With petrol half a thou of an inch wear in the needle jet is the difference between flying and sluggish. With methanol, it is a thou.

It was the BSA factory that proved by accident on their Dyno that the Goldstar did have more power with a sudden step down in size of 1/16 of and inch at the joint between the carb and the manifold. They built more than one version of the Goldstar with this step. The ones with the 1 1/2" GP carburetor had this step, but so did the earlier ones with the 1 3/8" carbs. I think it may be that the GP carburetors were a bit too smooth inside since they didn't even have the needle in the center of the bore like almost all other carburetors do.

From memory, the needle in the GPs have no taper. The adjustment in fuel supply is achieved by uncovering holes in a tube which is in a chamber off to the side of the bore. They might be more affected by turbulence created by the step. GP carbs are obviously British invention - like cricket, they require a certain mindset. One thing I do know is that in the 1950s, the Brits got their Manxes going faster than we Australians could on methanol. Our guys were good in Europe because they were used to the speeds.

pete.v said:

chrisshropshire said:

so if I had a 36 mm carb which manifold should i use 38? plus adapter ring

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This,
http://www.rockypointcycle.com/p-norton ... -2535.html
And this,
http://www.rockypointcycle.com/p-norton ... -2565.html

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This is the manifold and adapter I use for the TM40. Perfect match up and transition.
The Mikuni's are standardize in this regard (for the most part). Its all about the flow. Right Jim?
Go to the mikuni site and check the dimensions. You will see how many different carbs will compare at the OD for the adapter.