I screwed around with a Speedhut tach for a couple of years. They have a lifetime warranty on the electronics, so I sent it back in twice, but then I got tired of sending it back and gave up. I also engaged in extensive email dialog with the techs at Speedhut.
At first we suspected the problem was low-frequency noise in the power circuit, possibly due to coil "bounce", so I ran dedicated power wires to the battery
(via the fuse of course, and a dedicated switch). This did not help, and the gauge crapped out again in short time.
There were some other problems with the bike, a failing battery
, a dodgy ignition switch, a worn alternator rotor, and a failing coil, all of which I fixed before my third attempt with the tach. I did not go to resistor plugs or leads, but I did use shielded wire for the pickup.
The gauge worked for a little while and then failed again. My conclusion, or theory at this point is that the gauge cannot tolerate voltage fluctuations. The Speedhut unit is made for automobiles, not motorcycles, and the much larger batteries
in automobiles provide a more stable voltage.