I have a 99 summit 600 that I bought last year. It ran great at first and then started getting sluggish on the top end. one plug was fouled so I replaced them both along with new plug wires. Still no fix, so I very carefully cleaned the carbs. Still a dog. It then burned up the PTO side piston. In the process of rebuilding, I found the exhaust valves were stuck. I cleaned every thing and was very careful assembling the engine. It was better, but still kind of sluggish. I have double checked wires and connections and tried new plugs again. When test riding, it would pick up and run strong for a couple seconds and then fall on its face. I am out of ideas. It acts kind of like it isn't getting enough fuel. If any one has any ideas, my wife would appreciate it. (it is her sled and she wants to RIDE!!!)

jets clean ? pluged jet = bad day, carb boots look ok ?

Jets are clean and carb boots look good.

I hate NGK plugs!!! Brand new plugs bad. Put in 2 new BOSCH plugs like I run in my XC ( same set for 4 seasons) and all my problems are solved!

I actually have an answer for you, check the Y-pipe, because if thats leaking then you don't get the back pressure you need, mine was wore out, so it would only do about 75mph, instead of 103mph

Sounds like pre-ignition detonation problem Check main jets, are the correct size for elevation. thing runs good when cold and pipe temp low and loses power when hot . Sounds lean.What did the burnt piston look like ? Hole through the middle or melted on Exh. side?