Originally Posted by 9D280
Somewhere on the back of the tacho will be a switch with (at a guess) 2-4-6. fiddle with that until it reads sensibly.
Background: the tacho counts the pulses of voltage that the alternator spits out in a second. Alternators come in different guises, with more or less poles in them. Net result of this is that for a given RPM an altrernator with, 4 poles will throw out 2/3 of the pulses that a 6 pole will. If your switch is set to 4 pole and you have a 6 pole alternator, the tacho will think the engine is going 50% faster than it actually is because it is seeing 6 pulses per rev when it is expecting to see 4. (& vice versa)
There is indeed a switch on the back which, supposedly, sets it for 2, 3, 4, 5, 6 or 8 cyclinder engines. Mines a 2-cylinder but, having checked, the switch is currently set at 8-cylinder. Moving it to the 2-cylinder position shows a tick over rpm of over 6,000 !!!