Help me troubleshoot, please. My RV6A recently started reading about 500 rpm too low pretty consistently. I have mechanical cable from the engine with a transducer and Vans IE VTACH GEN 3500 with transducer. I?ve cleaned and lubed the cable and still reading low. The engine sounds like normal performance, has normal performance numbers, all other engine instruments reading normal. If I knock on the face of the tachometer, it has sometimes jumped up a couple of hundred rpms on the reading. How can I be sure if the issue is a tachometer issue (which seems the likely culprit) or an engine issue? Is there some sort of diagnostic testing I can perform?