Tyler- as Cap'n Kirk noted, the coil-battery ignition system generates a substantial amount of harmonic content in both the primary AND secondary sides of the ignition coil. Using a resistor-type coil wire or resistor-type plug, as well as making sure your condenser is top-notch, will cut down the RF hash noise considerably.
BTW... I'm sure you read the directions, but for anyone else that ends up with a small-engine tachometer- there's two types- types that hook directly to the coil (-) or 'magneto kill' wire, and types that wrap around one plug wire. Doesn't hurt the former to hook it directly to the coil, but hooking the latter will cause the tach to fry.
The latter type that just wraps around the plug wire... that's called an 'inductive pickup'. I believe the Tiny Tach uses inductive pickup exclusively.
Consider an 'inductive pickup' to be nothing more than a radio antenna, and it's listening for the magnetic field generated by a sharp pulse of ignition energy. If you'd like to 'hear' what this sounds like, hold an AM radio next to the ignition system while the engine is running.
As an aside note, when someone challenges me with a poor-running boat, the first thing I do, is turn on an AM radio, and listen to see how the electrical 'health' is. Alternator whine and ignition noise tell me an awful lot about what's happening under the hatches... especially on multi-engine boats. Can do the same for small engines!