djlyon is right about the 91 FX needle valve. The side of the needle with the o-ring is fed from the tank. The fuel then passes throught the needle valve opening, traveling away from the o-ring end, and then goes to the carb. On the other hand, the OS 61 FX needle valve seems to feed in the opposite direction, with the o-ring exposed to suction from the carb. It seems strange that OS reversed the fuel flow direction on these two engines.

The fuel draw on the 91 at high throttle settings may cause suction all the way through the needle valve and to the tank. This could explain why there appears to be air being drawn into the needle valve around the o-ring.

Another possibility is that there are bubbles coming from the tank. The bubbles would be smaller before coming to the needle valve and would then get bigger after passing through the needle valve where the pressure is lower.

I will try to find the vapor pressure for methanol. Liquid vapor pressures go up with temperature. Vapor pressure is simply the combination of temperature and pressure where the liquid will begin boiling or converting to vapor. For example, water reaches atmospheric vapor pressure (about 15 psi) at 212 degrees F. If you reduce the pressure, for example at an altitude of 7000 feet. water boils at a lower temperature, perhaps around 190 degrees. This might put this idea to rest one way or another.

I emailed Hobbico and they are sending me a free replacement o-ring.