Hi!
Most 180 degree 2-stroke twins for snowmobiles have wasted spark CDI magneto ignition systems, consisting of 1 trigger coil, 1 "Black Box" and 1 dual output coil. In some cases the coil and "Black Box" can be in one unit.
The flywheel then has 2 magnets or "lumps" activating the trigger coil, giving 2 sparks per crankshaft revolution.
An XS with points has 1/2 spark on each coil per crankshaft revolution, with the TCI (late model electronic ignition, with flywheel trigger) it has one spark per crankshaft revolution)
Either way, you will get incorrect reading. Also, as CDI primary voltage is way higher than battery/points/coil ignition, it may not read at all....
But it may still be possible to make it work, but that requires more electronics knowledge than what I posess. I once purchased a "Krober" racing tach for my -75 RD 350 with points ignition (1 spark per crankshaft revolution). Later I fitted a "Boyer" ignition to the bike, with "wasted Spark" (2 sparks per revolution). Then I sent the tach to Krober for recalibration. This indicates it can be done, it just requires detail knowledge...
Good luck with this mod, please post your findings, positive or negative!
If this tach does not work out, a UK made "Scitsu" racing tach most likely will, but is at least $250.....