OK I've got one here that's just got me stumped. Maybe some
old-timey Honda mechanic might be able to help

1980 Honda 750K.

I put a new battery in it start of the season, and I ride it most
every day. The problem is that after about a week of riding,
the battey has very little charge in it - just enough to start the
engine if the engine starts in about 3 revolutions. If the engine
doesen't the starter gets slower and slower and stops. Then
the bike will not start.

If I pull the battery and put a fresh one in, the bike starts
immediately. If I leave the fresh battery in, the bike works
fine for a week then the battery starts getting lower and lower
charge in it and the same thing happens.

I've traced the problem down to the bike isn't charging the
battery unless the engine is 2500rpm or more. I see the
battery terminals jump about a half-volt when the bike is
started, to about 12.5 volts, but I don't see the voltage hit
13.8 at the battery terminals unless the engine is revved up.

Ted Mittelstaedt wrote:

[snip story of failure to charge battery]

Follow the fault finding chart at electrexusa.com. Do all the
checks in order, don't skip any, and you will find the problem.

Did you measure the AC voltage open-circuit or with the stator
connected to the rectifier? You should disconnect the stator
from the rectifier for the AC voltage test.

"Ted Mittelstaedt" toybox.placo.com> wrote in message
news:newscache$3bs62k$l21$1@news.ipinc.net...

Mark Olson wrote:

I like that chart, somewhat similar to the diagnosis instructions at

http://home.earthlink.net/~trinomial/DOHCcharge.html

On Jun 9, 12:17 am, "Ted Mittelstaedt" toybox.placo.com> wrote:

I fixed a drop in charging by replacing a set of brushes
even though the old ones had only about 50%% wear.
Not sure if the brush was beginning to hang up or if
it was just the increased pressure but at $9 for new
ones, it was well worth swapping them as an experiment.

On Jun 9, 7:49 am, "ian field" ntlworld.com> wrote:

Sounds like an energized rotor energized by a regulator,
probably a three wire stator, and diodes to rectify the
A.C. output. What other info did you have in mind ?

ian field wrote:

You couldn't figure out that it is an excited field type from the phrase
"rotor brushes"? I happen to know the type well, having had a CB900C with
the same alternator. I also had a brushless excited field alternator
in my 1977 KZ650B1 which used a clever fixed field coil to deliver the
magnetic field to rotating pole pieces. It had a mechanical voltage
regulator and never gave me any trouble. The CB900C needed the rotor
brushes replaced but was otherwise faultless.

I am an old Honda mechanic and my best guess is that the rotor is going
bad. A quick check is to hold a long (8 inch or more) feeler guage so
that the lower end of it is about 1 inch away from the alternator cover.
Turn on the key and the feeler guage should move over to the cover. My
guess is that it will only move a little. Check the resistance of the
rotor. If it is about 4 to 5 ohms it is OK, if it is below 2 it is bad,
I have probably changed 50 of these "back in the day". If you are gong
to buy a used one be sure to check it out.
Bruce

Ted Mittelstaedt wrote:

"Rob Kleinschmidt" aol.com> wrote in message
news:364d1772-745f-4241-a6c8-cf03a830be36@s33g2000pri.googlegroups.com...

Some alternators have a brushless energised rotor - which would obviously
rule out worn brushes.

On Jun 9, 2:05 pm, "ian field" ntlworld.com> wrote:

The OP had mentioned brush length. Which bikes use
brushless energised alternators ?