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

[John Pavlick]

Aha! Interesting. I wasn't aware that there were two different carb's used on this engine. What numbers are on each carb? My engine is most likely later production and it runs fine in the Venus 2 with the tank on the firewall with reasonable prop sizes.
   
  John Pavlick

J N Hiller <jnhiller at earthlink.net> wrote:
        
            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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