Okay, at the risk of revealing my ignorance through dumb questions,
    here
    goes:

      1) The Tillitson manual says the carb has its own integral fuel
    pump.  It
    seems very unlikely that a pump this size that is driven through
    "pulsations
    of the engine" (i.e. not driven mechanically with a hard link) could
    actually deliver sufficient fuel for WOT operation.  What does this
    pump do
    (starting?  acceleration pump?).

      2) The manual says "Atmospheric pressure on the vented side pushes the
    diaphragm toward the inlet control lever, opening the inlet needle
    to allow
    fuel to enter the metering chamber . . . "  Will this function work when
    ambient static air pressure is significantly reduced (at altitude)?

      3) Dual carbs:  Just so I understand, what are the
    advantages/disadvantages of a dedicated carb per P-port vs both carbs
    mounted on a common rail.  The independent carb set up would have the
    preferable failure mode (one carb failed would still allow one rotor
    to run
    well and get you safely down.)  Are there ANY advantages to going
    with a
    common rail instead?

    Again, sorry for the basic questions.

    Mark Waldron

    They are very good questions.

    (1) Tillotson makes two types. I will copy the one without the built
    in pump and use an external ordinary low pressure fuel pump.
    A low pressure regulator can also be used to ease the load
    on the Tillotson.

    (2) It will if we manually reduce the spring pressure by moving the
    spring
    seat away from the diaphragm.

    (3) Well you have a very good point there. Two carbs would allow the
    engine
    to run on one rotor. The rotary runs real rough on one rotor as most
    people
    report but it could extend your glide. However there are a zillion
    aircraft
    engines out there with only one carb. Carbs I think tend to be very
    reliable.

    Paul Lamar ...No rotor no motor.

The dual Tillotson type could be mounted here.
Advantages are smoother air flow through the venturis at
low throttle opening and a slightly better BSFC due to better
mixing of fuel and air. Saves a bit of money because the required
bell mouths are incorporated into the carb body. The carb will
run cooler as it is further way from the exhaust. Tested on the dyno
with the Weber and Marks injectors are located in this area.
Also better tuning as the Weber style venturis would act as a discontinuity
if located down near the slide throttle.

On this Tillotson web site:
http://www.aerocorsair.com/id27.htm

we find this statement:

*_Here is how the carburetor works:_*

*The membrane of the Fuel Pump is activated by the vacuum pulses from
the crankcase. *The fuel is pumped and enters through the Fuel Inlet. As
it passes the Check Valve and a filter, the fuel is directed to the
Needle Valve which is opened by the atmospheric pressure acting on the
Metering Diaphragm. The pop-off pressure is set by the spring (Metering
Lever Spring)
At Low RPM the fuel mainly enters the Venturi through three Idle Holes
which are situated one after the other. This ensures a smoother
acceleration and prevents the mixture of running too lean when the
Throttle Valve is being opened.
The amount of fuel is regulated by two screws: HI and LO (for high  and low
RPM)


I just wonder if that carb with the pumping membrane is working only on
reciprocating engines?

On the rotary, we don't have a 'crankcase' to take-off the pulses. Or do
I see that incorrectly?
Just wondering.

Rolf Pfeiffer

Your right Rolf. The pump version won't work on a rotary.
It is for two cycle engines with crankcase compression.