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I did a full MSD ignition last year. I saw your post in the spark plug thread about tach problems. You can install the MSD ignition and the motor will run fine. The problem is the tach might not register the correct RPM. The problem I had was the tach in the boat was reading 4600 RPM. Actually the motor was spinning 5400 RPM! BIIIIGGGG difference! Your tach might be fine it all depends on the tach manufacturer. It has something to do with the Sine wave or square of the sine or something like that that the ignition box sends the tach. Get your hands on the MSD Wiring Diagrams and Tech Notes book. I got mine for free from the shop where I bought the MSD unit. Everything is explained in this book. If you would like call me at 716-683-0798 and I can tell you over the phone what you might encounter if you know the manufacturere of your tach.
Good Luck, Dave