My wife's bike has developed an interesting quirk.  The idle was messed up by a damaged needle (the tension spring was sitting thingy-eyed over the needle and caused to wear one one side).  It was idling extremely rich.

I replaced the needle and the starting and idling problems have been resolved.

When I took it out on a test ride, it just didn't seem right.  It would accelerate, but not as briskly as it used to.  To make matters worse, it can no longer hit the top speed that it used to. Before the problems, it would easily cruise at 65mph and top out at 70.  Now, it struggles to get up to 60mph.
Up hills, it has the same problem.  Hills that I used to be able to ride at 40mph (flat out), I can barely get to 30mph.

I changed the sparkplug and the air filter, then pulled the carb and checked the jets.  The jets were good and clear I could see no obstruction in either jet.  The needle valve was clear as well.

I put it back together and started it up.  Like beofre, it started fine and idled fine.  On the center stand, it would wind out to full throttle (hitting the rev-limiter).  On the ground, it just does not have the power it should.

Any ideas?

Oh, BTW, it is a Vespa scooter that I'm trying to work on.  I asked over at the Vespa forum but really didn't get any help from anyone.

No, its a 4t LX150.  I put a 190cc cylinder kit on it last year.  It was running fine until lat Jan, early Feb, when it started idling eratically.

I'm inclined to think its the drive belt or the variator, but that would be unique since she has less than 3000 miles on it since the last maintenance.

So it's wonderfully warm out now,  take it for a ride