Loss of power and trouble idling are classic symptoms of a worn piston/sleeve which means the pinch - which is what seals the combustion chamber - has decreased to the point to compromise proper running. When the pinch is gone there is combustion gas blowby into the crankcase which leads to a loss of power and generally creates all kinds of havoc including poor throttle transition, trouble idling and power loss. When the pinch/compression is gone or decreased where you can notice at room temperatures the problem manifests itself when the engine warms up because the brass sleeve expands more with heat and makes the tight piston/sleeve fit excessive causing combustion blowby.
By looking at them it is likely you wont notice any physical signs of a worn piston/sleeve unless of course the engine has ingested some dirt/debris which has scored the piston. Many engines last for several gallons of fuel but don't be alarmed if a particular engine lasts much less; especially if it has been run overly lean and/or otherwise abused.
You may want to see my post in this other thread which has some engine information:
http://www.rcuniverse.com/forum/m_87...tm.htm#8764466