First of all, taking breaks is a good thing - it keeps us from hurting ourselves, and from going on a destructive rampage with a B-F-H !
Regarding the main jet, if it were too lean, partial or complete choking should over-come that... not that you'd want to keep running things that way.
You could certainly try turning it out ( counter-clockwise ) 1/2 turn at a time, and see if things improve.
The phenomenon of spark disappearing at higher engine speeds, is usually caused by one or all of the following: oxidized points, points gapped too close, failing condenser.
When the engine is stopped, the mag can build-up enough charge for one fat spark off the impulse, which will fire the engine, spin it fast, and then the mag can't keep-up with the rpms, and the spark dies.
I'll bet every guy on the forum with a Gravely engined-L has stories about a tractor that would "pop once, then die ! "
Not knowing the history of your particular machine, but having resurrected quite a few neglected Gravelys in my time, a couple of big culprits in the fuel system are: dirt, insects ( especially mud-dauber wasps ! ), and rust. Any or all of these can block-up or restrict fuel passages and cause a lot of head-scratching.
If you get a chance, snap some pics when you visit it, and let us see what your're working on...