I can't remember if the 50 has the points at the engine flywheel or running off the cam....i'm just going off my SL70 from 30 years ago
Check timing with a timing light.
Could have sticky or loose counter weight/springs causing strange idle problems.
Also try removing the air cleaner and with your hand,slowly block off the back of the carby....if the revs rise,you have a lean problem with either manifold leaks,carb jetting/adjustment or burnt out valve/s
You can also try with the choke on and see if it runs better or worse.
Just a few things for you to check.....cheers.