>>I have recently looked into using a Weber 55 DCOE side draft carb.
>>Pretty cheap at about $560. I was surprised. Not easy to lean the
>>mixture as it is a
>>very complicated carb. It will work but the idle will be pretty
>> poor as it must be mounted on the beginning of the p-port intake
>> runners. The throat spacing is too close to mount it near the
>> P-port rotor housings. Besides that it would be subjected to the
>> full heat of the exhaust manifold. Power will
>>be somewhat down compared to a slide throttle due to the
>> aerodynamic drag of the butterflies and throttle shafts. I don't
>> know how much. If you take out
>>the throttle shaft and butterflies and use a slide throttle you
>> will lose a lot of functionality like the accelerator pump for
>> one example.
>>
>>It should work fine unmodified at low altitudes and with a modest
>> HP penalty.
>>
>>Ken Welter has been flying a Weber for a few years on his Coot. I
>> think he is tricking the float chamber by using dynamic air
>> pressure to lower the fuel level.
>>I am not sure. I don't fully understand what he is doing. Perhaps
>> that is possible with a needle valve.
>>
>>Paul Lamar ...No rotor no motor.
>
>   If you look at that photo just above the left velocity stack is
> a hole that is a float bowl vent, I have a controllable air ram
> pointed at the velocity stacks that will increase the pressure on
> the Venturi so it will draw less fuel. The rams are about an inch
> away from the velocity stacks. You can also use an air bleed
> system that pulls on the float bowl vent and it will also lean it
> out which is what they do on snowmobiles.
>  Also the Weber carb is very sensitive compared most carbs and
> when the air is thin it draws less fuel so it compensates to some
> degree but needs some additional manual leaning.
>  Ken

To further expand on what Ken is using one could use a low air
pressure regulator mounted on the instrument panel fed by ram air
pressure from the pitot tube system. The carb could be mounted on
the top of the engine. The idle would be a bit crappy but it should
run fine above 3500 engine RPM (1230 prop RPM with a 2.85:1 gear
box.). This is a very workable, low technical risk, moderate cost
system. No modifications to the Weber.

AT sea level the absolute pressure on the float bowl is 15 psi
roughly. Dynamic pressure is .36 psi at 150 MPH at sea level. This
should depress the float level by about 3% max. I am not sure what
that would translate into mixture change. At 200 MPH at sea level in
theory you could depress the float level 6% max which should be a
significant change in mixture. At altitude the affect may be
stronger as Ken points out.

How practical would it be to have a small carb for ground & flight idle
in addition to the above instrument ? This  idea is still current on
various supermoto bikes which use staged carbs , either the same size
or a mixture of slide & Constant Vacum ones. Look up the Yamaha XT
550 / 660 single cylinder bikes.

Fly & Build good -