Normally air does get into the lines when the engine is pointed down. However there is usually enough fuel remaining in the tubing to keep the motor somewhat spinning.

As long as the bubble is not too big, it will instantly be dissipated when it hits the carb, and the engine is none the wise with nary a hiccup. This is due to the incompressiblity of liquids. Air literally blows through the nipple while fuel takes a while.

A second clunk and T can also be a problem as air can still be drawn through the exposed line.

However all of this is speculation.

Try the test first... it will help tell you what is going on.


It may also be that this could be related to the rigid plastic tube too.

Normally some bending is advisable, as this lets the clunk move towards the available fuel.

Your clunk may be staying out of the lowered fuel level when the engine is pointed down.


I've also had a situation where I did not leave enough of a gap between the clunk and the back of the tank.

I didn't know this at the time, but my engine tended to cut out on accelerations but only in the air.

It turned out that the tubing was stretching, letting the clunk's opening contact the back of the tank helping to somewhat seal it off from the fuel... duh.... was I red faced on that one!