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Reply to "1915cc carburetor set up"

It would depend on how much room you have between the top of the carb and the inside surface of the air cleaner cover.  You need a minimum of 1-1/2” of space between the underside of the top cover and the top of the velocity stack for the carb to breathe without restriction.  

A velocity stack directs and accelerates the air flow into the carburetor throat so, yes, ANY carburetor will benefit from the addition of velocity stacks, especially a heavy breathing 1915 and larger engine from mid-range toward the top end of rpms.

My belief is that under 3K rpm you might not notice much difference in performance if you add velocity stacks, especially the short ones we can stuff inside our relatively small air cleaners.  BUT!   Stacks will add a bit of flow into your carb throats that might require a size or two larger main jets to match that new flow so you might see a performance boost from 3K to red line from them.

Generally, at higher rpms, the taller the stack the better.  That’s why high reving engines, like in dragsters, have really tall stacks.  Speedsters are limited by the vertical space available between the carb top and the underside of the body.  I can’t remember how tall mine are, but they’re probably under 3-1/2” max. and really only get effective over 3 grand.

BTW, it’s interesting that Thomlinson devotes over a page in the Dellorto book to velocity stacks, but zero words on them in his Weber book.  Don’t know what that means, just noting it.

Last edited by Gordon Nichols
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