The Saito 100 is a bone-shaker when it comes to vibration. A very light 4-stroke engine with a large displacement creates huge torque-pulses that manifest themselves as vibration.

Vibration (despite the foam rubber) can easily produce fuel-foaming and that results in bubbles in the fuel-line.

The fact that your engine runs okay on the ground but stops in the air is further evidence to support the fuel-foaming theory. This is because when your plane is on the ground, much of the vibration is absorbed by the fact that the undercarriage is touching the ground -- absorbing much of the vibration. Once the plane takes to the air however, it's free to vibrate more wildly and that's when the foaming becomes *real* bad.

However, just to make sure it's not an air-leak, here's what you can do to check:

1. fill your tank.
2. connect your fuel pump to the clunk line (that normally goes to the engine).
3. reverse the pump so as to suck the fuel out of your tank and look for bubbles.

If there are no bubbles then you *don't* have an air leak and the problem must be foaming.

The "suck" created by a fuel pump is many times greater than the suck of the motor so if air is getting into the line it'll be far more apparent while the pump is connected and running.