Start with the basic compression test dry/wet, then examine fuel system for any gel or steel gas can trash blocking the fuel flow, proceed to check the oiling system for flow - what happens here is a new owner may not be aware of the previous oil used and poured in something that is 2stroke but will not mix with previous one and the gel blocks the nozzles - this happens all the time and ruins many motors, next is the red wire to test for overcharging electrical system - this overloading messes up the overall power.
There are so many things that produce this symptom, but diligence will get you to the end and then you will have a summer of fun, a neglected machine will just breakdown over and over making the 4sale sign attractive, once you go thru one of these your friends will be asking you to help them, and it is addicting to play on the water at 60MPH.
