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.
--
Paul Lamar ...No rotor no motor.

   Goggle Dial-a-jet and you will find a kit that is made for
snowmobiles that leans with a pressure differential system, it lowers
the pressure in the float bowl with vacuum to lean as less fuel will
flow through the jets, my air ram increases the [static] pressure on the
Venturi to slow the flow through the jets, the Dial-a-jet system will
work much better than my system and I plan on  going to this system.
  Ken

I mis understood what you were doing. I stand corrected.
Do I have it right know? Lower pressure in the float bowl leans the mixture?
Higher static pressure in the venturi also leans the mixture?

I'll have to rethink this scheme. Perhaps we need an external low pressure to
supply a lower controllable pressure for the float bowl.

I think what they are doing with the Dial A Jet is set up the carb with
a full lean jet to begin with and add air to the fuel and pressure to the float
bowl with the Dial A Jet as required.


Paul Lamar ...No rotor no motor.

Paul,
What is really happening with is not the change in float level. I can change
the float level by a large increment and not effect mixture. What is
happening is changing the pressure on the fuel in relation to the metering
signal. Decreasing the atmospheric pressure to a lower value leans the
mixture. Adding dynamic pressure would have the effect of making the mixture
richer. You need to lean the mixture by putting a partial vacuum on the
float bowl. The float level will remain the same as the float is driven by
gravity (it's specific gravity is less than the fuel).
Larry

Thanks Larry for straightening me out on that.