Try adding a quart of ATF when you change your oil, the ATF will help to clean-up the carbon deposit that may be on the rings and around the valves. I like to do this on engines that have been sitting for long periods of time and for engines that I have no history on. My dad used to drain his oil and then fill the crankcase with diesel, run the engine for a few minutes, drain, then add clean oil. He belived this would clean all the deposits and crap from the motor, he never had any problems with bad rings, low oil pressure, or lack of compression.
