my tachometer reads eratic at times jumping all over and at other times it reads right. does anyone have an idea what might be wrong ?

rebs

If your motor has two voltage rectifiers like my Merc has, wire in the other rectifiers siginal wire.

Many, including myself, were able to cure the wandering tach problem by checking the electrical connections. Mine was loose on the post on the back of the tack. Make sure they are tight, and clean.

Mine did that for a while before the voltage regulator (rectifier) and main coil went out.