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Old 09-18-2005, 04:32 AM
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Keihin FCR 41

Hey Guys
A difficult one here,I was wondering if any of you had any idea of how many horsepowers worth of air my carb will flow.
The engine is a 650cc 4 valve single with 37mm inlets and 32mm exhaust valves, the carb has a 42mm venturi a good bellmouth and a 2.75mm needle.
Any help would be much appreciated.

Clive W

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Old 09-22-2005, 03:30 AM
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There is no solid answer to your question. The absolute air flow capacity of a carburetor has no direct correlation to power. Too many factors influence the actual power generated.

However, for a really rough estimate, an old racer's rule-of-thumb is 140 CFM (cubic feet per minute) air flow for every 100 horsepower.

Now, the hard part to answer is the CFM rating of your carburetor. Actual flow measurements incorporate numerous variables. Not the least of which is the carburetor itself. Different carb models with same butterfly size measure vastly different CFM values. As much as 40 CFM in one comparison test I found. http://www.groupk.com/tec-carbs99.htm

In that test, a 42mm Keihin (not the FCR model) measured 195 CFM. Yours may flow more, or less.

I hate to put real numbers on all this guestimating, but to answer your question...

100 HP / 140 CFM = 0.714 CFM per HP.

Therefore, 195 CFM should support about 140 HP.

I make no guarantees on the accuracy of any of this!
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Old 09-22-2005, 04:55 AM
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Re: Keihin FCR 41

Thanks Pantaz
That's very interesting, the FCR is fitted to a Kawasaki KLX650 and it pulls from 2000 and revs to the red line (7500) with ease.I'm looking at 47-48 BHP, presumedly the figure of 140 would be on a bigger engine.
Thanks again for your help.

Clive W.
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Old 09-22-2005, 11:38 PM
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Quote:
Originally Posted by klx650sm2002
I'm looking at 47-48 BHP, presumably the figure of 140 would be on a bigger engine.
The subject was maximum power. The carburetor that provides the best driving results is not always the "right size" (mathematically) for the engine. A 650cc single is a pretty big piston for a bike. Which means it probably pulls a very strong signal on the carb. I'm not surprised it has a "big" carb. A 650 twin or four would likely run terrible with a 42 mm.
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