Aircoupe Alon A2 with C90 engine with two props, one 48" pitch and the
other 52". It doesn't make the static runup RPM minimums with either
one, for instance the 48" prop should be 2225 to 2325 rpm, I averaged
out to 2162 rpm. Likewise the 52" prop falls short 100 rpm or more.

Three years ago when I checked the tachometer, it read a little
slow--maybe 10-20 rpm at speed, that is it would indicate 2300 and the
electronically measured rpm would be 2310 or so. Assuming that's
still true, what would cause the engine to not make minimum rpm?
Where should I start troubleshooting?