X-Mozilla-Keys:                                                                                 
JMLBlues@aol.com wrote:

One thought on carb jetting: Carburetors are very sensitive to the nature of
the air flow through the venturis. A nice, steady flow will be obtained with
a big plenum on the manifold. This is the easiest setup to develop. A rotary
with multiple carbs and runners with little plenum volume will expose the
venturis to air flow pulsations. These pulses will vary both temporally and
in magnitude depending on engine speed, throttle setting, and to a lesser
extent, air temperature and (I guess) altitude. Imagine  what air flow
pulsations do in the carburetor -- the inrush picks up fuel. The outrush
picks up fuel. The next inrush picks up fuel again. So in an
oversimplification, the pulsating flow has 3x the opportunity to draw fuel
from the venturi. The higher inertia of the fuel will change the amount of
extra fuel drawn by the pulses in a non-linear fashion depending on the
strength of the pulses.

All this is a long way to say that I "guess" that jetting one carb at a time
on a multiple carb setup is a good way to get running, but considerable fine
tuning may be required when all carbs are running together.

Finally, my feeling is that the cost of new floats and jets is worthwhile
rather than attempting to repair / modify parts that may be on-hand. The
manufacturer's production methods are hopefully more consistent than what can
be accomplished in the field. Also, it is faster to have an assortment of
jets available to swap out, rather than soldering and drilling each time an
adjustment needs to be made.

Jonathan Lauter
Rotary Power Marine Corporation

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