I'd suggest a set of new bearings (Front and Rear)...

While you have the engine down for bearings...Get yourself new gasket and O-Ring set's too...You'll wanna R&R the needle socket o-rings for sure...

Also while she's torn down...Take some compressed air and blow out all those passages in the case, regulator body, needle socket and throttle barrel...

I'm sure you used new gaskets when you re-assembled the engine after replacing the diaphragm...You kinda have to watch the regulator gasket...I have found that sometimes the gasket can partially block one of those passages if your not careful...I almost always wind up enlarging that hole in the regulator gasket a little...

Here is a plumbing diagram for the YS 45 but the 61LS is plumbed the same...I have a PDF copy of the 61LS manual if you wanna PM me your e-mail address I can forward that to you if you like...

The first thing folks go for is the Regulator Adjustment Screw....You gotta learn to just leave it alone...Set it flush and leave it there until you have the engine up and running reliably with the pipe tuned...

I know they can be a pain when they are running like that...I have one torn down that was acting exactly like yours...Waiting on bearings and parts as we speak...

But when they are running right..They are Awesome!!

Good Luck getting her going...

Chuck