Model year 2000, Mercury 50 Hp, fourstroke, "Big Foot". Carbureted engine.

My troubles with these engine never seems to cease.

Finally got the motors running half way decent, but now the tachometer for
one engine has stopped working. I swapped wires at the helm station between
port and starboard engines and the problem followed the engine, the tach
works fine.

I have an after market manual on the engine that provides a wiring diagram
but I can't figure out what the tach output is. There isn't a whole lot to
the electronics (except big and probably expensive modules). The engine
runs fine, but I suspect that the charging circuit is a little weak. With
the engine rev'ed up fairly high I was getting 13.0 volts at the battery
(pretty close to fully charged, it was at 12.7 before I started the engine)
so I am getting at least some charge. I am not too concerned about the rate
of charge but I suspect that the tachometer output might be related to the
charging circuit.

Can anyone give me a guess as to what I need to change to make this thing
work? I really need the tach. I can't really hear the engines from the
helm station (houseboat) and without the tach I can't tell if the engines
are anywhere close to running the same.

Rod