Re: 1969 20' engine choice
The back of the tach should have a small rotary switch witch can be moved with a small standard/flat screwdriver to the appropriately marked number of alternator poles. My Merc, with a teleflex tach uses the 6 pole setting. Of course, you can also try the different settings and see which one seems right. It probably won't hurt the tach, it just won't read right.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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