Roger Long wrote:
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.

The drop in revs you mention would seem to be indicative of the alternator
"kicking in", causing the load on the engine to increase, hence the drop in
revs. If your batteries are "full", this would happen intermittently as the
battery voltage fell.

Probably nothing to worry about, unless someone more familiar with this
electrical set-up contradicts my prognosis!

Dennis.