"..the spring atop the carb slide..."
I've never seen a PC50, does it really have a slide valve carburetor? Just in case that doesn't make sense, does the throttle cable go into the top of the carb, and pull a piston up and down?
If that's the case, add a cleaning of the slide valve assembly to silverfox's advice; the gasoline in the carb can make it hard to keep the slide greased or oiled, so you just have to keep it super-clean. Plastic scrubbers work well.
So, grease the throttle mechanism, oil your cables, make sure the return spring in your carb is taught enough, and perhaps clean out the slide/slide tube part of your carburetor.