Definitely do what Ed said in post 2 first.
I was almost sure that someone had a
B7800 on here in the last couple of days with a starting problem. Oh well, cannot find it. I'll try not to be too repetitive: It may well be any of several safety interlocks. If you don't get it going other ways, I would put a meter on the trigger lead to the solenoid. When you turn the key to the start position there should be voltage to that solenoid lead. If
YES then it is probably just not strong enough to kick in the starter. You can run a bypass wire / clip lead straight from the battery to that solenoid trigger wire and see if that kicks in the starter when you turn the key. If
NO voltage at the solenoid trigger lead, then you have a safety interlock somewhere in the chain (PTO switch, seat switch, fwd/rev lever, etc.) preventing the trigger to the solenoid.