What are you jumping white wire to? To the terminal that is wired direct to the battery? Don't know about your tractor, but if so, is your jumper to the starter solenoid just a bypass around your key switch and safety interlock circuit (PTO, neutral switches etc..)?
I used to be able move forward and reverse lever a little and it would start but it won't do that anymore.do you think it might be a safety sensor or something