About the only thing I haven't read(and I could have missed it) is the actual inlet to the carb, as well as the needle/seat ass'ly being disassembled and blown out/inspected. Obviously these lines/paths should be clear of all/any micro debri and such.... and yes.. again, you got good info on this engine not needing anything in the way of a pump/regulator.. nothing other then pressure from the muff.

I would go as far as I could with disassembly(as your skills and tools allow of course).. to possibly take off/out that needle-seat assembly off that carb... have a look at what goes on in there if you can..

Also.. if this engine will start and seemingly run out the prime.. what happens when you open the main needle.. in whatever increments.. basically.. can you get and keep this engine running with the needle way out(further then we all might think anyway)...?

I've ran mine with the OS type-F plug from day one... with an 18x10 prop(Menz)...Bisson pitts-style muff.... Morgan FAI fuel for the most part(It was not finicky about fuel. as long as it didn't have nitro).. med. fuel line, using a 24oz tank, w/3-line set-up. Its one of the best(if not the best) engine I own... I sure am curious whats wrong with your engine... hmm.. (scracthes head...)..

Please post back with any revalations.. thanks.