Bought a used 60" Kubota Z421 with 207 hours on it. Mower was mint, people I dealt with were super nice (Hope that wasn't a good cover). I start the mower, let it warm up a minute, but when I pull the PTO the thing shakes and stalls. I will restart and have to throttle way up and it still happens. I changed the blades to another set he gave me, same thing. I have a brand-new set of blades on the way. Once they get turning it seems fine, does not seem to vibrate like something is out of balance, it is just that initial start. Thoughts on what it may be and where to start? Thank you in advance for your time in answering my question.