Sounds like an intake leak or maybe had some fuel leak down to the trans fluid. I had the same issue not too long ago. i replaced all the seals and changed the oil, and it was good as new! My right side flywheel seal was weather rotted and kickstart seal had seen better days. I tested for leaks by spraying some carb clean around the seals and when the idle climbed I would kill it. If it gets too high and you've got good brakes (and some balls!), you can hold on tight, grip the brakes and pop it into gear.