Welcome to the forum. You will find alot of good info here.
Hi, I have two 90 SP's. I recently rebuilt the carbs on both and have had the same problem. Mine ended up being a slightly stretched throttle cable. I had to keep playing with the cable adjustments at the carb linkage and finally think I got it right. Has been running great the last few times...
I have a second 90 SP that I rebuilt the carb and and cant keep running at all, so I think it is a matter of trial and error with the adjustments. Hope to have this one running soon.
Good luck. If you find out the problem please post. Maybe we can help each other out. Ill do the same
Ray