I've read about needing the vent the diaphragm into the fuselage or cowling on some setups, but I left that for last, being that it ran great for a few tanks of fuel on the first outings, then started acting up.

I just pulled the diaphragm off and checked the little lever, fork, whatever it's called. At rest, the back of it is level with the carb body, but it has to move a bit before it starts to lift the valve open. I'm not sure which way to check it, at rest, or when it starts to pull the valve open.

Either way I'll check the local shops for a kit, and order one if nobody stocks them. If that doesn't do it, at least can rule that out...