. . .and, as has been said many times on this forum, after checking that you have a good flow of fuel, you need to make sure you have a good, white spark that'll jump a quarter of an inch. I had just such a problem with an 801 a few years ago, and it turned out to be one of the woven distributor wires that was hanging by just one thread. It wasn't letting enough juice through. Choking relieved it at low rpms but not at high.
Good luck, and please let us know how you make out.