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1987 Revenge 25' with twin Yamaha 150's
Both tachs are reading very high at idle, over 4000 and go higher when I rev it up a little. Some times they just bounce around, seems strange that both would fail the same way unless it's related. Anyone ever have a similar problem?
the first thing I would try is to make sure the tachs are still functioning properly. Over the years the contacts can get corroded simply from moisture in the air.
There should be a pole selector switch on the back side of each, which fits a blade screwdriver. Rotate it back and forth about five times to clear the contacts, and reset to position #6. See if that fixes the problem.
If not, you need a new rectifier on each engine. How is your voltage when running? It should be 14.2 volts. The rectifier is also part of the charging system and if it is going bad, may show low voltage problems, and lack of adequate battery charging.
Problem solved, you were correct, rotating the selector screw on the back of each tach solved the problem, at least the tachs look like there working correctly now. I returned the screws back to where they were, it looked like the number 12 but it was hard to read. The next number was 6, I was thinking since my engines are 6 cylinders it should go there but I don't want to assume.
