The Hog Air line of engines are running a Harley carb, but I don't know the
size. I personally would pull that prop and see what rpm I could get with just
the engine being careflu not to redline the motor ( the same as revving it in
neutral ). If you can get close to redline with say half to 3/4 throttle, I
would think that the carb is sufficient. The math formula for calculating CFM
size of carbs is: street carb = rpm x displacement (in CID ) divided by 3456 x
0.85 = cfm size carb. Convert square inches to square centimeters CC x
0.0610237 = CID
My engine: 1915 cc x 0.0610237 = 116.86 CID Now take CID 116.86 x 3400rpm =
397,324 / 3456 = 114.96643 x 0.85 = 97.721465 or simply a carb rated for
approximately 98cfm. That is measuring the engine output at a standard auto
engine efficiency of 85%, not 100%. At 100% you would use the 115cfm figure
before the last multiplying. Unless you are using a racing engine it is always
better to go at least one size smaller carb due to the fact that the engine
cannot reach the rpm necessary to use the air available at wide open throttle,
and therefore will SLOW DOWN at wide open throttle not speed up. I have proven
this on the drag strip changing from a dual plane intake to a single plane
intake which effectively went from cutting the carb in half, to giving every
cylinder access to the whole carb. The engine would actually nose over and
start slowing at wide open throttle compared to running faster at 3/4 throttle.
Also not having some engine heat applied to the mix is only good for wide open
throttle, and not acceptable for part throttle operations. You will fight a
rich/lean mixture problem if the incoming charge cools the air/fuel mix enough
for the gas to begin condensing on the intake walls and then get sucked in by
the next cylinder.
Things that effect volumetric efficiency: air filter, restictions in intake
system, exhaust restrictions, valve train restrictions, cam design, and whether
the carb can fully open at max throttle position. The manufacturer should be
able to give a guidline as to what CID/CC size engine the carb is designed to
support, and what the cfm rating is for the carb.
It is also possible that your prop is pitching out to beyond 48 too early if
your engine will not rev up. That is what was happening with mine and I had to
go down to a 44 to get 47 to 48 inflight at cruise.
FLY SAFE!
Colin & Beverly Rainey
Rainey Mortgage Professionals, LLC
[email protected]
407-323-6960
