[NSRCA-discussion] OS 91FX or 120 AX for Focus sport

[J N Hiller]

Bob, I am getting back into this discussion late. I went to the bearing
store and when I got home it was miller time.
I have two AX'es purchased at different times and their carburetors are
different (see photo). I am currently using the OS muffler and the first
motor carb pictured on the left has a serious fuel draw problem and the
other one works fine with the tank against the firewall. I was already into
a 1.20 size airplane when the OS 1.20 AX appeared on the market and I got
one right away, which was likely early production (left carb). The idle
needle didn't withdraw fully from the fuel delivery tube at full throttle
and the barrel has an air bleed hole through the bottom side, which opens to
the intake around half throttle. I shortened the needle, which helped some
but ultimately I added an exhaust restrictor (.350 ID) to increase
backpressure before I got an acceptable mixture setting and transition.
I inquired about the difference but understandably got no reply. The other
carb from the second motor works much better.
I suspect running a 16-10 is a little too much load and I need to advance
the throttle before going vertical to get the RPM's up to produce sufficient
back pressure / heat or it will go lean. The OS muffler is likely sized to
operate with more exhaust volume using a smaller prop at higher RPM's.
I plan to try a double quiet Mac pipe, probably with a pump in the next
airplane.
Sorry about the photo size. I had to make it quite small to fit the file
size restriction.
Jim Hiller
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