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Motorcycle Electrical Issues
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mendip_discovery:
So as a glutton for punishment, I own a 1961 Matchless G3LS. I broke it a few years ago so it stayed unloved in the shed for a while. During COVID I got around to rebuilding it. Put it back together and then I ran into some issues, the wiring loom was cracked and shorting out all the time. So I have to change a few things.

I removed the old Lucas rectifier and fitted a modern full bridge rectifier a KPBC3510 which I bought at an autojumble. I also swapped out the battery for a Cyclon Battery.

I'm getting running issues that make me think it is the electrics. The bike has a feature that allows the bike to be started using Emergency Start which starts the bike without connecting to the battery etc but once started if you give the bike any amount of throttle it starts to pop and bang a lot. The issue I have currently is the bike starts (sometimes) then it will tick over happily but as soon as I go to ride off it stutters and pops and bangs just like it is in emergency mode and after a few attempts to go somewhere it really becomes hard to start.

The bike is just 6V electrics which are really really basic. What I am wondering is whether the rectifier is failing as it got hot a few times, not being helped by a battery that charges via a constant voltage type system. There is no regulator for 6V just a magnet spinning around a coil (stator) to generate the single-phase power. I have checked the timing, replaced the condenser and the coil etc. so covered a lot of the basics.

Questions for the masses,
Can I test the FBR to see if that is the issue?
Should I bolt the FBR to the frame to reduce heat as it is currently in the open under the seat?
Should I go back to an old-style lead acid battery?
Any other thoughts on the cause?

I have just bought another battery (gel) and a replacement regulator KBPC5010 & GBPC5010 as one is better designed towards heat dissipation. I will see if I can find a bike shop nearby that can get me a battery as annoyingly the only ones online come without acid[1].







[1] Partly because the postal service doesn't like posting it, the other is because some adults can't be trusted not to use the acid to melt people's faces.
jmelson:
Well, having a fair bit of experience with lawn mowers, if the engine sputters when the throttle is opened, that could be spark blowout, ie. the spark voltage is not high enough to bridge the gap in the spark plug when cylinder pressure is higher.  Is this emergency mode using magneto spark, but normal mode uses battery spark?  That sounds pretty wierd, it would require mostly duplicate spark systems.

Popping sounds like lean mixture, maybe the main jet is clogged, but the idle jet is fine.

Another issue I have seen is a mower that runs fine for several minutes, then gets weak and finally won't run at all.  Then, after cooling off, the whole process repeats.  This was fixed by replacing the magneto unit.
Jon
floobydust:
Can you post the original wiring diagram? Is it a single phase or 3-phase stator?
The original Lucas 49072 introduced 1962 silicon replaced older selenium rectifiers like square plate 47132.
It looks like the stator has multiple windings to permit different output i.e. powering radios: https://commons.princeton.edu/60-tiger-cub/wp-content/uploads/sites/139/2019/08/LucasServicemanual_noSB519_part3.pdf
edit: is this the (attached) schematic?

The rectifier should get warm but not hot. Its case is isolated so it can be bolted to anything. Is it wired in properly, to cause the heating.
Conversion to a silicon bridge rectifier, it will have much more output than a selenium beast I would say too much so the battery will see well over 7V and overcharge which gel batteries cannot withstand at all. You'd have to add the Lucas zener diode system to add some regulation.

The ignition system issues, I don't know the bike but I did once have really weak point spring and they would bounce at any speed much above idle and cause misfiring. It would start and idle fine but it acted like a rev limter. Otherwise, your condenser is suspect or the coil/plug (resistor), wiring.
Circlotron:
If you can reach it, while the engine is idling, remove the plug wire and see how far the spark will jump while idling. Using that as a gauge, slowly increase the revs and see if the spark fizzes out or begins to fire erratically.
Gregg:
Do you have the coil polarity correct?  Most of those old Lucas systems use positive frame ground. The wiring on the stator could be suspect, the insulation they used back then is far from ideal especially considering it lives in a hot oil bath.
I have had a BSA Goldstar, Triumph Cub and currently own a MGA; I feel your pain! I'm glad to be rid of the motorcycles and the MGA has had all of the Lucas stuff except the starter and distributor replaced. 
Also, Amal carburetors have their own quirks.  The mounting bolts distort the body causing vacuum leaks and sometimes causing the slide bore to become out of round. 
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