First sunny day in what seems an eternity, and got rental time on the Tiger. Preflight looks good, taxi up to run up area, and then check the left mag. RMP's drop by 200, plane violently shakes, sounds like hitting only 2 (maybe 1?) cylinders. Leaning to burn off carbon has no effect. Just to check, both mags, full throttle - only goes up to 2100 RPM. Called Ground, and taxied back to ramp. No flying today. Yikes - it was rented/flown only a few hours before. Glad it broke on the ground and not with the previous renter 3000' up.. First time I've ever had a problem with this club / flight school. I've sometimes wondered why we do run ups - no one ever really finds anything wrong. I've changed my mind on that.