Not really sure what happened: pump seemed to be working well but I was really nose heavy. I moved the engine back on the mount and had to relocate the pump, and 5 subsequent flights resulted in 4 dead sticks and a ripped out undercarriage.

I tried re-routing the return line from directly into the tank stopper to a 'T' just before the pump, as in the diagram. I seemed to be having air issues, could not get the engine very rich at WOT, and again had a boggy transition.

Removed the 'T' system and tried leaning out the mixture valve. Now idle mix is very lean, high end is pretty leaned out, and although transition is good the engine loads up with excess fuel if you have the stick anywhere in the middle of the throttle range (aka flying the plane). However no dead sticks.

So, my questions now are:

What does the drain hole on the pump do, and what is the result if the pump is unable to drain (i.e. hole not facing down)?

Does a Cline system really work better, and does anyone here actually have experience with the Cline on a larger (120) size engine?

Thanks. Sorry to keep dragging this one on, but this intermittent operation business is driving me nuts and I just want to fly my plane with the tank on the CG (without having to buy another YS this season). Don't really think I'm asking too much...