chronic fail to start, or if it starts, it sputters and dies, then hard to start.

Propane Tank changed
plenty of spark..occasionally starts and revs well then dies
Checked vacuum line to lock off...operates
vacuum to lock off, press button gas flow through pipe to carburetor
Shot some starting ether in carb...no start...THAT's puzzling.
Points, cap wires all look good..cranks good.

Any further pointers? Non electric Lockoff.