Tach question

[Dysfunctionalone2k1]

I have a 68 mt with a mercruiser 165 hp(chevy 250 6 cylinder) stern drive. I finally got the tach working but it is erratic. I have had this problem before with chevy trucks and have put a potentiometer in line to adjust this out. My question is can this be done on this motor? It is points ignition instead of electronic ignition like all of the trucks I have had. Any help will be greatly appreciated. <br />Thank you<br />Richard