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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.