I have a 1970 53 MY with DD 8V71Ns. My question is about instrumentation, specifically tachometers and whether or not the RPM reading can be adjusted. My port tach on the salon console clearly registrers 100-300 RPMs low compared to the one on the flybridge, dependant on engine speed. At idle, the difference is 100 RPMs. At WOT, the difference is 300 RPMs. The starboard engine tachs agree almost exactly throughout the RPM range.

Can the tachs be adjusted or must I replace the port engine main console tach?