1998 MAXUM 3700 SCR - engine synchronization issue

[RSBeck]

Hello again! We are cruising the Chesapeake Bay in our new 1998 3700 SCR and learning as we go. Our old boat was a single screw 27 ft. Uniflite so the l.o.a. at 39.7 ft., twin MERC 7.4 MPI with velvet V drive is a totally new experience, not to mention having a complete head and galley with A/C is beyond what my wife and I had imagined when looking for a new boat. We had the boat surveyed in the fall of 2020 when we were purchasing it so some of the small items to repair are not a surprise. Lots of scrubbing, new VHF antenna, fix horn wire leads, replace wipers arms with new blades, etc, but a couple of things have popped up. The bow windlass switches have failed, the ice maker needs replumbed since the feed tube sprung a leak and the most confusing is the Port engine will occasionally run way over sync from the Starboard side. With the 2 running at 3600 rpm, the Port will take off to 4000 and the last trip upwards of 5000 rpm. I reduce the throttle to the Port engine and the boat changes plane so I believe it is just a tachometer issue but where to start, sending unit OR gauge on the dash? Any thoughts or experiences?

[donald]

On the back of each Tach is a switch to change the #of cylinders it works on, move that switch back and forth a couple of times and return it to original position.
Hope this works it worked just fine for me on my 2400.
Don

[mmwjr]

I would start with Don suggests as well as cleaning wire connects on the back of the tach. The signal comes from the ignition module so unless the ignition starts breaking up when this happens the issue is with the tach.

[RSBeck]

Thanks! I'll give it a try next boating day this week.