I had a very similar sounding problem recently with my G20, which like your engine had been working perfectly and was left unused for about four months. The engine would start just like normal and run perfect at a max of high idle all day, but any attempt to increase the throttle would cause it to immediately shut down. I did all the checks you did plus some without any success, and after several days of hair pulling and head scratching it turned out to be a partially clogged felt clunk. Replaced the clunk and all was well again.

Karol