Why Would Starting the Port Engine, Increase the RPMs on the Starboard Engine?

Question:

I have Farria tachs hooked up to the Cat diesels in my 1994 Silverton. The starboard tach works fine when only the starboard engine is running. When I start the port engine, the RPMs on the starboard engine increases drastically, the RPMs approximately double. My marina thinks that there is an electrical issue causing the problem but have not found the cause. Do you have any idea what could cause this? I would appreciate the help. Thanks in advance. - Ed

Answer:

Sorry about your troubles, unfortunately never encountered this problem before.  Perhaps the tachometer on the starboard side is also sensing the tach sensor from the port? This might cause the tach to double it's RPM. Vary the RPM on the port side and see if it varies the RPM on the starboard tach.  Please share your solution with us!

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