The tach on my Yamaha 2QM20 started dropping back intermittently about 100
RPM. A couple days later, I noticed that this was accompanied by the
briefest flash of the "Charging" light on my minimalist engine panel. The
engine is absolutely smooth so I'm certain there is no actual change in RPM
or cylinder misfire.

Although infrequent, these events have increased slightly and happen most
often shortly after startup. In the past couple of days, there have been a
few large drops of about 500 RPM, equally quick, with brighter flashes of
the charging light. These are about the quickest flicks that the needle of
the instrument can make. After startup last night on the way to the
fireworks, the tach did it several times in succession with the needle going
wildly back and forth and the charging light much brighter, flickering in
sync with the needle movement.

I've checked all wiring connections and tried wiggling the wiring harnesses
to see if I could create the problem. No result. I adjusted the alternator
belt after the first few occurances. Batteries 14 V and engine starts
briskly.

The tach on this engine is the kind that has a sensor that reads the
starting ring gear teeth going by.
Is it time to pull the alternator and have it checked out or are there other
places to look before I disable my boat for the several days that will take?

The alternator is a 35 amp Hitachi, pretty long in the tooth. I have a 55
Amp Hitachi with the wrong kind of internal regulator that I've been
thinking of having modified for an external regulator, perhaps one of the
Balmar three stage units. The engine has plenty of power for the boat so
the extra charging load shouldn'g be an issue with batteries that don't get
drawn down much. Maybe this would be a good time to get that done.

--
Roger Long