I'm new to this forum, and haven't posted in a forum in about 15 years so I'm likely doing something incorrectly - so apologies in advance for that! I'm going out on a whim that this is the right section to post my issue... but I'm sure someone will know best
I have a '55 G9 and it recently cut out on a short ride, shortly after losing power and smoothness in its running operation. A careful hand over the exhausts confirmed my theory that only one cylinder was firing, as the left hand side pipe (sitting on the bike) was more-or-less cold.
I've been in the garage with both plugs out, cranked the engine with them held against the cylinder (the extent of my mechanical ability) and diagnosed that both plugs and leads are fine after some switch-a-roo-ing (technical term). And that's where I'm at now. I took apart some pieces that were accessible and, whilst not wearing a glove, gave myself a very mild electric shock from the side which doesn't spark. Shouting and screaming aside, it was a good indication there there is some voltage there - just not enough to really spark.
What should I do now? I thought if I did what I could without breaking anything further, and then report it here, you lovely people likely have a much better understanding of my next steps.
Let me know if you need any more info, photos, etc.
I would start by looking at the pick up brush for the naughty side. If it is short, tight in the casing or the spring is weak that may be the problem. Other easy option is to replace the plugs on the theory they can spark happily in the open air but fail under compression.
This is likely where I make everyone face-palm... where would I find/how do I access the brushes?
As for the plugs, I did swap the plugs around - and the leads too - and though they both work fine, only on the right hand side. Out of the naughty left side, visually, there's nothing.
At the end of the leads where they connect to the magneto there are the 2 pick ups held in by the spring clips that swivel. Simply rotate the clip and gently remove the pick ups being mindful that inside is a spring loaded carbon brush. It won't fall out of the holder but you don't want to snap it off. Also unscrew the acorn nuts that hold the leads as over time water can enter there (especially if rubber boots haven't been fitted) and the end of the lead starts to corrode against the metal base in the pickup, this weakens the spark as well.
Hmmm. A good magneto should make a 1/4inch spark from the bared end of a plug lead held away from the cylinder, at kickstart speed. It will be stronger when the magneto is fully advanced on the cable, but should perform almost as well when retarded. A spark at a plug is indicative, but a 20 thou gap is no test of its ability to operate under compression. A mild shock is probably nothing like what's needed - but better to get a light dose than a full one maybe while playing!
Maybe start with the contact breaker . . . suggest you check that the points are clean and that the gap is close to the correct 12 thou on each lobe of the 'cam ring' that they go round inside. There is scope there for a few reasons for failure to proceed, on one cylinder or on both.
You might want to bear in mind that the points fire the gearbox side, rear, pick-up when the fibre heel of the opening point is on the lower lobe at about 4 o'clock as you look at the contact breaker, and the front pick-up fires the other side on the lobe at about 10 o'clock. You can therefore see if there is something funny that relates to the cylinder that isn't firing.
Then, maybe take off the HT pick-ups by undoing the clips that hold them on, carefully take them off with their brushes inside them and look for signs of breakage, complete absence or other peculiarity. You could also clean the 'slipring' that the brushes run on using a bit of rag with cleaning solvent on it, on a non-metallic stick (to avoid more HT shocks!) through one of the pick-up holes, turning the engine slowly with the plugs out. The slipring needs to be clean and not have any wear ridges.
The HT leads need to be well attached by the knurled nuts that hold them on, and they need not to be cracked or damaged in any way. They can leak when they are very old, cracked, dirty, etc. Sometimes, running an engine in a darkened garage reveals all sorts of mini-fireworks hopping off the pick-ups to the nearest big lump of ferrous!
If you want to start getting a bit tekky, you could put a multimeter on a high resistance scale between the brass section of the slipring (it is an arc of about 90° on the thing and it will show up at the pick-up holes easily enough) and the magneto body. It should show about 5000 ohms. That's the resistance of the HT coil. 'Open Line' or Infinity is bad as it suggests a break in the winding, or a damaged connection between it and the aforementioned slipring. A problem there needs professional help in most cases so let's hope it passes that basic test at least.
The condenser is another possible source of trouble, but you can't test that easily in situ unfortunately. Typically, though, that would start to cause grief when the machine was hot, and more than likely not just on one cylinder . . .
But you can test the resistance across the points, - when closed, as close to zero as can be, with the points squeaky clean; when open, about half an ohm. Very low - that's the resistance of the Low Tension coil on the armature.
It is also worth taking out the earth brush. That sits near the flange on the drive end, under a large-headed screw. It should be making good contact with the armature within, and it shouldn't be covered in slime although it is unlikely to be pristine.
Mechanically, any end float or up and down movement on the armature spindle should be 'barely discernible'.
There is a limit to what you can do while the magneto is attached to the machine, and we are fast approaching it.
But maybe something in the above will help.
Or the comments already made might, with which I agree!
I had these off, but didn't compare them - and in the knowledge of what you said, I now see that the side causing problems is stuck fast. This might explain the very weak current I was getting and the no spark. I've attached a photo of the 2 side by side
I some how knew it wouldn't be that simple... I cleaned it up until the spring moves freely, reinstalled it and still no spark. I'll read through what Groily wrote now
The pickup on the right in the pic?? Cracked by the looks of, with a brush that's a bit sticky maybe? You say it was it on the offending side but clean-up made no odds? . . .
Ought to be replaced all the same, even if not the cause . . . 's not a good un!
Spot on chaps! Well done for spotting it. Really chuffed I put the photo up and it showed that, otherwise I could've found myself down an expensive rabbit hole. I retrofitted the working side pick up (the one of the left in the photo) and tried it, and SPARK! So must be a knackered pickup. Any recommendations where to purchase another one? Thank you all so much!
