what is causing this to jump into gear

Hi I have a model # 917.272420 craftsman 18 hp ltx 1000 no matter how i try to adjust the shift linkage it jumps into forward gear with dangerous speed actually picking up the front end if not careful to account for that going to happen any fixes or diagnoses I bought it used a few weeks ago it works alright but it jumps into forward and reverse works sometimes and sometimes it doesnt

I have seen transmissions jump out of gear but never into gear. You must have some kind of linkage problem. That a new one to me. Are you sure you aren't hitting the shift lever some how?

:welcome:

Now you need to clarify things a bit.
Are you saying that you can put the mower into neutral come to a stop put your foot on the brake then when take it off the mower has selected forward or reverse all by itself then tosses a wheelie.

Or are you saying that when YOU put it into a gear then when you release the brake it shoot off like a startled rabbit.