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| Carburetor Air Velocity |
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Author: Archives Views: 8793 Votes: 2 |
Carburetor Air VelocityMaintenance of carburetor air velocity is everything to drivability. It is the air velocity across the venturi of the carb that creates the vacuum that sucks the fuel into the air stream. An inordinate drop in air velocity drops the vacuum and less fuel gets into the air. This causes a big flat spot or bog.Those who can remember the old days before the CV carburetor we had carburetors whose slides opened with the throttle. Snap the throttle open at low speed, and the motor would almost quit. You quickly learned to modulate the throttle. A CV carb does this for us. The CV carbs are carefully balanced by the manufacturer to fit the loading and air velocity requirements of the motor with stock airbox restrictions. Open up the intake and the now unrestricted air does not seem to generate the velocity required to suck the correct amount of fuel into the air stream. I think that it lifts the carb slide too soon. Some time ago a guy in Arizona named "Tugs" was producing a hop-up kit for Ventures which consisted of essentially a V-Max-like intake manifold. As part of Tugs kit, he recommended stretching the stock carb diaphragm spring from its usual height of about 6.75" to almost 8". What this does is to force the slide to remain lowered until a higher vacuum was generated at higher motor speeds. He coupled this with a much richer slide needle so that when the slide did lift, there was additional fuel available. By this means, he operated the carb largely on the pilot jet circuit for cruise, but had much more power available for acceleration. I have a Max intake and heads, cams and all on my 1300 Venture, using Tugs suggestions and a low restriction intake. I can tell you that there are no flat spots whatsoever, even running with an open air box. I am not suggesting that this is a simple modification to engineer or tune correctly. It is one thing to just stretch a slide spring, but getting the needle jet correct is like dancing with the devil. Rey Kirkman
Last update: 11:57 AM Sunday, September 26, 2004 |
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