common reasons for compression loss in 1 side after warm up?

[allthumbs]

I am considering buying a bike that has this trouble. can anyone say off hand what I might be getting into? ie, is it more than rings? or other (relatively) simple fix?

thanks much for your opinion

[davomoto]

Valves too tight comes to mind for me. Generally, the rings seat better when warmed up. If there's not enough valve clearence, the valves won't close all the way when warm, and can do some serious damge to piston.

davomoto

[e3steve]

I agree with Davo; my diagnosis would be valve clearance(s) too tight on the errant pot; as the motor warms up, the clearances decrease due to expansion of all components (aside: Concorde, at just over 200ft long, grows in length by almost a foot when travelling at Mach 2!). If it's the exhaust valve, you run the risk of burning the valve seat and/or valve. The inlets get cooled by the incoming charge so don't, as a rule, burn easily.