Topic: Charging system question (1974 CB750 K6)

[tentawon23]

Hey all-
I'm new to the board, and just had a quick one for you.  I've been helping a friend fix up his '74 CB750 K6 and we've gotten the motor oil tight and back in the frame, and moved on to the charging system.  From what he tells me, the battery slowly loses charge when the bike is being ridden, but there's no discharge from the battery when it's sitting still, so we've ruled out a bad ground.

OK, so yesterday we tested the voltage regulator and rectifier, and they both checked out OK.  The interesting thing we found is that the resistance on both the field coil and the stator coil are higher than what the Clymer manual lists as spec.  The manual lists 7.2 ohms as the resistance spec for the field coil (we read 7.9-8.1), and 0.2 ohms as the spec for the stator coil (we read 0.8-0.9).  Seems to me that the higher resistance on the coils is probably the reason the bike is not charging effectively.  Does this sound feasible?

I'd be more certain that it was the coils if one of them was grounded or open, but it seems logical that the high resistance could cause the problem he's been having.  What do you think?
--Ethan

[bryanj]

Have you taken into account the resitance in the meter? short the leads and deduct the reading you get from the readings taken for the coils. 99% of bad charging is bad grounds most of the rest is regulators not working right

[Bodi]

Measuring very low resistance is difficult, inexpensive digital meters give an approximate reading. The resistances you gave sound fine to me, if you used a quality analog meter zeroed out properly or a good digital meter and subtracted the lead resistance then they are a bit high but still reasonable.
My experience is that once the coils and rectifier check out OK, you should test the voltage on the regulator battery terminal (black wire). Since the alternator uses an electromagnetic field coil, the voltage here directly affects the output power. On old bikes with weathered wiring harnesses, I have read as low as 9 volts on the regulator with the battery reading 12.5V. After cleaning and tightening all the bullet connectors and spade lug multipin connectors I can get back to 12V or so, only dropping a half to 3/4 volt in the harness and switches. From 9V to 12V is a 36% increase in alternator output!

[tentawon23]

Thanks for the help!  I'll check that out when we get back to work on it this weekend, and I'll keep you all posted.
--Ethan

[mrbreeze]

There are people here that will tell you alot more than I know about the charging system on your bike but I do know one thing....Your K4 is a 74 or your K6 is a 76 unless you have a newer motor in an older frame or visee-versee! Post up your frame and engine #'s to make sure.

[tentawon23]

Yeah, you're right.  It's a '76.