Topic: Battery, Starter Motor or Something Else ?

[jaytee-nz]

Been trying to balance my carbs on my 1972 CB500 K1 and of course the bike was dying as I got the idle speed too low.
The first few times, the bike re-started fine but after three or four times the cranking slowed.
The bike has a 6 month old Motobatt AGM battery that was fully charged. When it got to the stage that it would not turn over I put the meter on it and it read 12.2 volts (low) but dropped to below 9 when trying to crank. Charged it up again but went through the same problem once more.
When fully charged, the weird thing is that if I turn the kill switch to off, it cranks the engine fine for some time but it looses voltage as soon as the engine fires.
Voltage drops by about .6v when the key (only) it turned on.
Re-charged the battery overnight and turned the key on with the headlight on - held voltage at 12.5v (dropped from 13.2 with the key off) for at least 10 minutes.
Is my starter motor draining it, is the battery faulty or is there something else going on ?

[robvangulik]

What volts do you read with running engine at 1000 3000 and 5000 rpm?
That shows if the battery iis chargingwhile riding which is far more important!

[TwoTired]

Are your coils and ignition circuit still comprised of stock components?

Stock bike draws about 10 amps whenever the key switch, run switch, and lighting are on.
The alternator can make 12-14 amps, when it is revved over about 2k rpm.
Alternator makes about one third of max power at idle. So, battery drains at idle.
The stock size and type battery will supply enough offset power at idle to run the bike 15-30 minutes.
Electrical mods and component changes can alter idle duration capability.

Cheers,

[jaytee-nz]

Thanks guys.
The charging system is working as I briefly tested it when I was having the issues (was more focused on the carbs to be honest). Part of the issue may be that the output is low as I couldn't get the voltage at the battery up past 13 volts. Not sure what the revs were but likely to be have been around 4000. Output definitely increased with the revs though.
Coils etc are new stock replacements from DSS with stock wiring other than an electronic ignition. Was careful to make sure all earths are good on to bare metal as well.
I've had to take the carbs off so I can't run right now. Will check it further when it's reassembled (and hopefully running on 4 cylinders and not 3.5).
Cheers