It's *usually* not so severe as that, and I've never seen it happen with a tank
that has a clunk with a cross groove cut in it. (as most do)
Some of the cheapo clunks don't, and can suck tankwall in that case.

The problem with the clunk hanging up usually affects the plane more in a roll, bank,
or an outside maneuver, when below half tank. The clunk gets hung up in the air.
But, not seeing his tank install, and having had it happen to me, it's a valid "check" item.

I agree that it's probably simply "too lean", for whatever reason.
I had this on a Kougar once. It only showed up in high speed loops or pullouts, and I
fixed it by looping extra fuel line in front of the engine, and going around back to the carb.