"Roger Long" wrote in message
...
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

I would suggest connecting a good quality digital voltmeter to your
alternator output and ground. See if the voltage drops when the tack rpm
drops. If that is the case I would suspect the regulator rather than the
alternator but I guess it could be either one. You didn't say what type
alternator you have. Is this the original Hitachi internal regulator?
By the way, I have an 80 amp Yanmar/Hitachi alternator rebuilt and modified
to use a simple external regulator. Never used since rebuild. The
regulator allows you to increase the alternator output voltage to compensate
for isolator diodes, long wire runs and other voltage robbing factors.
Yours for $200 plus shipping from 98281.