Carburettor Size?

[Ian]

How is the size of carburettor determined? Does it totally rely on engine cc, is there a magical formula or is it yet another black art that is developed by mechanics over the years? Any information duly welcomed.

[iansoady]

Interesting question, and depends on the type of carb to some extent - eg whether slide type or CV. I'm much more familiar with the former. Essentially the smaller the bore the faster the air / fuel mixture passes through it. So a small bore will give easier starting and probably better running at small to medium throttle openings. Larger bore will be harder to start, be less flexible but give more top end power. And of course we need to consider the rest of the inlet (end indeed exhaust) tract as there's no point in putting a humonguous carb where the inlet tract is narrow and tortuous.

Phil Irving suggested that for racing purposes 1.1/8" - 1.5/16" would be about right for a 500cc single eg Manx Norton. Although this was in the 1950s, those sizes may be considered small now as maximum engine speeds have increased.

I have fitted a 22mm Chinese Mikuni copy to my Grumph (350cc Triumph twin engine) but as yet have not started it. Standard carb is a 20mm Amal monobloc which is pretty tiny.....

[Ian]

Thanks iansoady. The reason I'm asking is that I'm considering swapping the twin carbs on the (400cc) SRX for a single carb setup. I can actually obtain a single carb to twin manifold from Yamaha to fit the 660 "Grizzly" reasonably cheaply.(Same hole centres) This will entail much alterations but hey ho. Steve lake has sent me some details of what he did on his 600 SRX so I know what I'm in for. I was looking at a 34 mm flat slide version but now I'm not so sure 🤔

[Steve H]

A quote from John Robinson $ stroke give gives

(Carb Diameter in mm squared) /40 = Power in bhp
or
(Carb Diameter in mm squared) = (Power in bhp) X 40
or
Carb Diameter = SquareRoot(Power in bhp x 40)

So if we take output to be 40bhp this gives

Square Root(40 x 40) = 40mm

Also says to add 20% for CV carbs and that smaller carbs easier to jet

[iansoady]

Sounds a bit big to me - fine if you want absolute maximum power but not so good at smaller throttle openings - which is certainly where much of my riding is.

[Steve H]

Sounds a bit big to me - fine if you want absolute maximum power but not so good at smaller throttle openings - which is certainly where much of my riding is.

I wouldnt disagree, just quoting the book. I think that is the value of twin carbs, both being smaller the velocity through each is higher and as such better at smaller openings. I have twin CR33's on my SRX which equates to a single 45mm diameter carb. I dont think it would be as nice to ride with the single 45mm

[iansoady]

Yes, sounds sensible. I think there have been some layouts which use one carb up to a certain throttle opening then bringing the other in.