no rpm drop during testing of carb heat (found to be the connection rod between engine and lever loose under the hood) (PA18)
-during mag check found engine rpm varying rapidly between 1000 and 1700 rpm on one magneto. (PA18)
I've found those in the PA18 as well, on top of that an engine that would not produce more than 2000rpm in level flight (not really a pre-flight discovery). Mags left on, I once found a PA28 with almost no fuel in one tank- he must have been minutes away from fuel starvation. Water in the fuel is the most common one, often in the tanks it's just condensation but if its in the under engine drain it could mean trouble. General load of rubbish in the cockpit as well.