I have tried everything to solve the jumpy tach with the msd ignition. I originally had the white tach wire going to the msd box tach output but it was giving too high of a reading and too jumpy/erratic. I then connected the white tach wire back to the white distributor wire which was the original setup and then the tach worked smoothly to 2k, but if I tried to accelerate hard or go above 2k, the tach would nose dive to 0. I then put the tach wire back to the msd tach output but put an inline capacitor of .1uF (1000V rating), which did not work so I took it out. Finally, I put an inline resistor 10k(ohms) 1/2watt rating, and the gauge is smooth and no more nose dives to 0 when rpm climbs over 2k or under hard acceleration. I just wanted to share my solution with whoever was experiencing the same problem.

 

Thanks for taking the time to share that. When I get around to connecting a few wires to get my box hooked up this might come in very handy!