Heres an idea.. alittle off the wall but worth a try..
Pinch off the return line.. just use a small pair of needle nose vice grips & pinch it off..
Let it sit the "normal" amount of time & try to start it.. if it starts, you know what the problem is..
If you look back thru our conversations, when we first started this "project".. I said, I've NEVER SEEN a set-up like that..
Where the return had an orifice back into the inlet.. Remember??
I do remember, every GC1705 has one so I don't know what to tell you.. On the left side half way down you will see "fuel return" and you will see the system that's on these GC1705's.. Mine is no different than the other 10,000 that out there..
AGCO Parts Books