Following advice on this thread I did a thorough check of the Limbach. No corrosion seen but the two left cylinders had a splendid 100 psi compression, while the two right cylinders had less than 10 psi. After difficult start and gradual warm up to working temp I did a couple of fast taxy runs (and hops) on the long runway. Returned to the hangar, changed the oil which was somewhat dirty, but no metal in the filter. The next compression check however made my day, as three cylinders registered 100 psi and one 85 psi. I guess there was some crud around the valves which burned off. Anyway with new oil, new plugs, and healthy compression time to take it into the air.