de-sticking manual advance retard on 61SR

[mikeb]

funny things these BSAs, I must be making progress as I'm almost back to where I started.

On a 61 Super rocket the manual advance retard was sticking - repeatedly - in the K2F's cam ring when set to fully advance. *After* I'd done the points and timing I had the great idea to just put some tension on the cable adjuster to stop it rotating so far advanced/anticlockwise - cured! At least it proves its not due to end play under the cap but now the timing is retarded again and its not a stable solution (tho maybe it could be a sneaky way to adjust the timing without removing the timing cover etc . )

What happens (i think) is the bottom bit of the mushroom shaped adapter (see the pic) drops out of the cam ring groove as the cam ring rotates away from the adapters linear path and this jams the cam ring.

But why? the stop peg in the housing is there and limits the rotation of the cam ring - but maybe not enough? Of could the mushroom shaped adapter just sit to low in its housing. All i can think to do it tap a small screw into the end of the adapter to limit how low it sits - but then the adapter would function as the stop for the cam ring. There must be a better way!

anyone faced this? any ideas???
thanks, Mike

[groily]

Hi Mike, If we can assume that the camring isn't chattering in the housing (there's a felt strip in the groove I think I can see? - that will help if there's metal-to metal wear), and that there is no axial float on the ring with the cover on (ie the mushroom still slips out of engagement even if you hold the camring firmly 'in' with your finger with the cover off) then it has to be a question of the parts and/or the geometry of the set up.
The amount of movement allowed by the large notch might be too great as you suggest, if someone has ground some metal off from one end or the other - hard to see from the pic. I wondered if some grinding had been done at the 'retarded' end from the pic, but probably just the light.
The largest undoctored 'clean' notch I can find on my bench, looking at a lot of K2F rings, is 11mm wide (using the short ears of a slide gauge to measure it quickly). Several are just over 10mm. Some have an angled cut, some are close to parallel-ended. If yours has been elongated, then even with a reasonably unworn mushroom and a decent V notch for it, things could slip out of engagement as you say, due to excess movement.
If all seems within limits (and the plunger is sliding smoothly in its bore, with the cable coming off the centre of it, not tending to push/pull it in a deviant way), then the only thing I can think of (apart from trying a different mushroom or camring), is to have a look at the stop or limit peg. The peg should be the 'sharp end' of an eccentric screw hidden under the pimple outside the cb housing. It should have been set in the right position for best internal mag timing when the thing was built/last rebuilt. However, if it is now at the extreme end of its useful movement (to provide maximum advance in this case), then it is just possible that this is what is causing the plunger to travel too far south and come out of the notch at full advance on the cable.
Unfortunately, it is hard to check the very best position for the pin, but as a rule of thumb the points should just be starting to open a few degrees after the magnetic flip points. If they open too early, you'll lose sparks at higher rpm (usually) but there is a bit of leeway, and you could try retarding it a wee bit by turning the eccentric nearer the centre of its movement to see if it makes any difference to the behaviour of the cable. It might, and there might be no obvious deterioration in sparking performance.
Every time we change the cb points, or put in a different camring, there is some small change in the exact firing point as wear patterns will be different. A new set of pattern points compared to a worn Lucas set can show up quite a difference, and I doubt most of us ever start playing with the limit stop eccentric to compensate because it is so hard to measure the difference unless something is seriously amiss.
Best of luck, and maybe someone will come up with a blindingly obvious better idea!
Cheers, Bill

[mikeb]

Bill - thanks for your super detailed post and measurements - sure helps me understand this.

• the cam ring rotation notch is 10.4mm so in spec with your measures (is that what your measuring?) and with no signs of being tampered with.
• the plunger slides evenly in the bore. some wear on the end but not too drastic.
• this cam ring has a v-notch (pic below) for the adapter. a little wear near the inside upper edge but looks ok to me. I would have thought the straight cut notch would be a better design as would not displace the camring outwards, but my name is not lucas 
• you mention felt in the groove of the camring housing - i have none! can you suggest what type is required?

from fiddling around some more i think the problem is simply the cam ring is moving outwards, so much so that it starts to move around the radius of the stop-peg. this allows more camring further rotation, and the mushroom adapter then slips out, drops down and gets wedged at the bottom.

EDITED:
So what to do? some threads blame the end cap (see pic below) and I say should replace it. But any replacement cap may not be better and they are hard to acquire in these parts.  The inner lip of the cap protrudes 1 - 1.5mm which I thought would be enough to hold in the cam ring. But the cap screws up tight on the outside of the housing. when tight the inner protruding lip sits well clear of the cam ring so doesn't hold it in.  One idea: grind 1mm or more off the face (where the red arrow is) so it sits in tighter... or is there n easier way??? like  make a spacer (of gasket paper?) to sit between it and the camring?

Another partial solution could be to rotate the eccentric stop peg so it sits out further in the housing and hopefully stop the cam ring from walking around its radius. are those easily adjustable? (a pic of the base too). i now understand this may adjust the spark point, but when I've ready ground the unevenly worn camring lobes that precise setting may be well lost.

Thanks for any further suggestions

Mike

[Billybream]

To prevent water ingress I used thin section O Ring(1/16" section), this sat in the bottom of the mag end cover and sealed with the mag body, this may well help your situation with the manual A/R

[groily]

Looks as if the problem is the cap isn't bearing down on the camring when it's done up Mike. Suggest you test with a thick bit of something cut to ensure it matches the camring's edge and see if it's better. Billy B's point is good!
For felt, anything will do, I use sheets of the stuff obtained long ago from I don't know where. A bit of old felt hat and a sharp Stanley knife would do! Should be there, with a bit of grease on, to lubricate, and can also have the benefit of reducing any chatter on worn parts.
Your stop pin screw looks a bit chewed up, but as long as it holds firm  . . . the pin that's meant to be up the middle of it acts as an expander to lock it in position. In despair, a screw or bolt with an eccentric machined or ground on the business end can work, using thin washers under the head to get it 'tight' in the right position of the peg. Let's assume the thing is in roughly the right position then - but making sure the peg is far enough in to operate as a proper stop is important. Sometimes they are a bit worn down. You don't want the pin preventing the camring from fully going into the housing though.
You're right that having done this and that to the bits there will be some small influence on the firing points, but probably nothing too serious, there is some leeway. With the mag on the bike it's hard to be able to judge where the flip points are exactly, but if you get good strong sparks at kickstart speeds and it doesn't go funny on you at high revs, it's somewhere close to right. It's only 'fine tuning' after all! A good ruse to see how your low speed sparks look is to take a pair of old plugs and grind off the earth electrodes. This gives you a gap of about 4mm usually, which is a better 'fresh air' test than a typical 20 thou gap and simulates sparking under (some) compression. Should jump the large gap easily. Lucas always used 5.5mm three point test gaps to check mags, on the basis that that was the rough equivalent of (I think) an 8000 volt energy requirement. And they expected a K2F to deliver nearly all sparks from 130 magneto rpm (260 rpm of the crankshaft).
Cheers, Bill

[duTch]

 These manual units are out of my league these days, but my first thought also was an 'O' ring in the cover- but Billy then Groily rounded me up
 

[mikeb]

thanks for the suggestions....

So I’ll try fitting a spacer between the cam ring and the end cap’s inner lip for a test – easiest to fashion one from plastic but anyone know how hot it get inside a K2F?

There’s 5mm of thread on the cap (where the red arrow is in the pic above) so I reckon I could shave off 1 – 1.5mm of it to get the end cap to hold the cam ring in closer…. Anyone keen on that idea?

If I’m reading this right, an o-ring would be a weather-tight seal between the housing and end cap – where currently I have a rubberised seal. tho that’s not where my main problem is with regard to the cam ring moving.

Bill – great info on the K2F spark. I’d assumed with so many threads reporting wet plugs at idle (like mine) that it would be a much weaker spark at low revs. I’ll give it a test.

Cheers

Mike