Msd ign problem (not)

Just a lesson I learned!! We upgraded ign for my son's mud race truck this winter and had nothing but problems. All new msd dist, 7AL3 box,hvc2 coil and msd wires. Kept having random rpm ign loss. Tested everything, dist, coil, coil wire, dist cap, all checked good. Bought msd ign tester and checked box operation-tested ok. Fought this for 3 weeks!! On a whim and a suggestion from a friend I unplugged tach. Did 3 test hits in field behind shop, presto ran perfect 7000 rpm as fast as you could shift. Any ign problems from know on will unplug tach first!!!!!! Just a heads up. Cory

I think i had that problem with a 6AL box years ago!!

so how do you use a tach with it?how did you solve that?

Hey guys, what we found was the tach was either shorting out or backfeeding ign, tach was autometer with external shift lite. Engine would act as if ign was turned on and off rapidly at different rpm. Thanks cory

Had the same thing happen years back with a Sunpro tach it would go to missing at different times and different rpm I changed all the ignition from dizzy to spark plugs ignition box. My brother told me to unhook my tach and see if the problem went away and it did. Never have ever bought another Sunpro tach I bought a Mallory tach with shift light and rev limiter that was back in the early 90s. I still have it in my 69F100 to this day and have never had a problem out of it.