Chainsaw Repair

Have you tried using the lead (supplied with the tacho) and attaching different points near the high tension lead.
Limited coils often make rev counters play up soon as the coil shorts out (reaches rev limit).
For example,
try clipping it onto the lead 1st, and get the counter away from the saw.
try other places close to the lead, like onto starter housing etc, the lead will pick up the pulse same as pointing the counter at the saw.

I still use an old 2 stroke / 4 stroke REVCO rev counter, works as good today as when I bought it back in the 90's.
I'm not saying they are the best, there are a few different ones on the market and they all work, I guess you get used to reading different saws a different way.