I had a similar problem with my 1650. Turned out to be that the float wasn't dropping enough and when the engine burned about 1/2 tank of gas, it would quit. I let it cool a while and it would run again. Replaced the coil... same thing.
I checked the fuel valve for rust/crud, clean as a whistle!!
I finally removed the carb to check for a blocked passage somewhere and I noticed (carb was still hot) that the float wasn't dropping down far enough. So when the carb got hot, the float needle magically closed SHUT!!!
Some OLD WISE man once that "Ve get too soon old and too late schmart". I bent the tang away from the seat a little more and the problem disappeared.
Perhaps someone else can learn from this experience...
FFGino. Happy Cubbing!!