CB72 running 20v at battery with new parts

[RustyGold7]

Hi all. Working on a CB72 build by unknown person with apparantly commonly modified ignition system. Problem even with new stator its running at 20v @3000rpm.
Background - was overcharging at 30v dc at batt and carbs leaking. Sortded carbs, got running, tested voltage and got 30 ish v. Old stator appeared to be the oem one from early 60s and needed full rewind - 7 weeks @ £430.00 GBP!
Bike already had a little square 4 pin solid state rectifier as advertised by classic bike parts shops to replace an old selenium one. Made the assumption this was fried by overcharking so replaced with anew on but could not even get a reading on the multimeter - was vibrating at 70v!!!!!.
So.... fitted a Wassell type modern style single phase 4 wire (2 from the stator) reg/rect fired it up and....20v @ 3k rpm!!!!.
With key at position 2, and the little switch on headlight turned to right, with lights on, the bike starts and idles at 12 ish v until reved than gets steady 20v. The rotor is still heavilly magnetised.
With engine running and all 4 stator wires disconnected from loom, stator is pushing a correct 30v ac.
Can anyone shed any light on this? - totally stumped at this point folks.
Sorry but cannot insert images here.
https://www.ebay.co.uk/itm/143048969430 ... bb5535d0d5

https://www.ebay.co.uk/itm/254721494457 ... c55b1ff795

[Tim Allman]

A couple of things come to mind.

Is there any chance that the BSA rectifier is positive ground? Unfortunately I'm not quite sure what the effect would be but I suspect it would either burn out the rectifier or let some smoke out of the wires.

Could the stator have more windings after its rewinding than before? If you increase the number of turns in the coils the voltage will go up in proportion. I.e. double the turns, double the voltage. This is the only way I can imagine getting 70V out of the generator.

There are six coils in the stator but only four should be connected when the headlamp is off. The yellow wire is used by the lighting switch to connect the remaining two coils to help with the extra load. Is this happening correctly?

There is always a voltage drop across a rectifier. The original selenium rectifier will lose about 3V but the new one will be a silicon rectifier and will lose about 1.5V. The whole system is set up to provide about 15 to 17 volts above about 3000 rpm with the selenium rectifier. Therefore, with a modern rectifier you should see an increase in voltage of about 1.5V. With a little hand waving, this might account for the current 20V which is too high for good battery and light bulb health.

A lot of people are using inexpensive combination rectifier/regulator units on these old bikes. Anything for a 12V system should work well as long as it has three wires plus earth which may be on a wire or through the mounting screws or both. I wouldn't proceed with this until I knew that the rest of the system was working well.

You may find this (https://drive.google.com/file/d/1lx0Thw ... drive_link) useful. There's a section on the charging system in the back.

I hope you find this helpful.

[RustyGold7]

Thanks for reply. The stator was build to the factory oem standard (company who did is very old...). Reg/rect is new 12v Jap style with 4 wires inc - earth. As stated output from stator (disconnect from loom whilst engine running) ac is good at ac30v now, issue still is dc20v @ 3000rpm yet 12.5v @ tickover.

[Tim Allman]

tickover?

[Michael Stoic]

I am not an expert on bike electrics, but in general diagnostics terms, if your old stator and the rebuilt stator both produce wrong output, then a second look at other components (including the multimeter and testing methodology) is prudent.

[Michael Stoic]

'tickover' is idle, IIRC.