I have a 2003 Prairie 650. I bought the machine around 2010 and it ran great for a number of years. Around 2017-2021 it sat idle for a variety of reasons and got little to no use. About a year ago I was able to start riding/using again. After sitting for a few years it wouldn’t start so I pulled the carbs and rebuilt and cleaned them. It fired right up and ran good up until about 3 months ago when it began to lose top end power and eventually got to where it had little to no power.

I pulled it into the shop and cracked the crank case while at idle and got a face full of gasoline mist, indicating it had blow by and the piston rings were likely worn. So I bought a top end rebuild kit for and rebuilt both cylinders. New cylinder base gasket, piston, all piston rings, and new head gasket.

I put it all back together and went to fire it up to notice a loud knock coming from the front cylinder. I have pulled the valve covers and adjusted them to their respective clearances about half a dozen times and the problem persist. I backed the the font cylinder exhaust valves off completely and the knock goes away. This leads me me to believe I have a stuck valve.

Does anyone have any other theories before I go pulling the head apart? I’m usually pretty good with these machines but I’m about at my wits end with it.

 

I would to a compression check first off.

They have compression release so the numbers will be low anyway, like 55 psi. But if it isn't at least that then you know you are headed the right direction with a valve sticking open or something.

 

Maybe the inside of the engine is worn too much, and some parts need to be replaced