Hi Roger,the magneto is independent from the bike wiring and does not need a separate earth wire.The small brass slotted screw is indeed an earth,this has a carbon brush behind it to complete the circuit between the mag body and the armature.have you checked the points are clean of protective grease,and are gapped to (just remembered you sold the M20) the recomended .Check the HT lead is fitted correctly into the pickup housing,and the HT cap is also tight.
hope this helps Cheers Rick
email (option): richardholt@rocketmail.com
thanks everybody I put an earth wire in mag sparking lovely roger 
email (option): roger.beck@node6.com
What I think you are referring to is actually a ground for the magdyno assembly and not specifically the magneto portion. It is to assure an effective earth connection from the body of the generator section of the magdyno through the engine and frame. On my machine the original brass grounding lug with grubscrew is missing. The wire leads off the magdyno body run into the wiring harness and join the battery ground terminal with the other leg grounding in the headlamp shell if I recall correctly.
email (option): teladelujo@ msn.com
The earth brush doesn't seem to be critical for function, although it may well be essential for long term reliability, as I recently noticed mine had gone AWOL and I could still ride the bike home with no apparant problem.
Cheers
Pete
Hi Pete, if I remember correctly the earth brush is there to prevent that current is running through the bearings which would damage them on the long term. So you would better replace it
Regards, Michiel
email (option): m.wijbenga [ at ] hotmail.com
Already done it although I had to make one up 
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1280126&cmd=show
Cheers
Pete
email (option): petercomley@web.de
I've just noticed the re-routed crank case breather pipe on the photo above. Where does it go? back to the oil tank or onto the chain? I'm intrigued!
Kimble
Prior to a rebuild, the engine pictured was passing a considerable amount of oil through the breather. When stopping after a long run, the combined drippings from the primary case and the transmission output bearing along with the engine were becoming a bit excessive to say the least. The lost oil would tend to collect, trapped on the steel engine guard, and which would then run off, en masse, when the bike was placed on the side stand....
A temporary hose off the engine breather valve was routed up hill, along the top of the chain guard and then from the pannier frame to the lower rear fender stay where a short section of copper tube terminated downward just off to the side of the rear fender tip. As least the oil was distributed in several small places under the bike, as opposed to a single largish puddle.
To date, the transmission and primary oil leaks have now been resolved, and the engine gone through, so not as much need for the breather modification, although I still see emulsified oil dripping off the tip of the breather hose when stopping from a good run....seems there is no way around a bit of that.
There has been some mention of routing the breather into the primary, but I have mixed feelings about breather air being pumped into the primary chain case carrying as it does, a witches brew, mixed of acidic combustion gasses and atmospheric moisture. I would be interested in reading some discussion on this idea.
email (option): teladelujo@ msn.com
Hi John,the breather pipe should be clipped with a small like a half round clip
with one hole to the back of the chain case and drain to the road.This from previous posts on the subject.However on a bike (WM20) i had years ago it was routed to empty on the gearbox sprocket,so giving the odd drip of oil now and then.Only trouble with this is it tends to splatter off all over the place after time.There is no provision from BSA to route the pipe into the chaincase,prolly
because of the reasons you highlighted in your post.
cheers Rick
email (option): richardholt@rocketmail.com
Hi John...From the 1970 model year onwards Triumph twins had the camshaft driven breather assembly deleted and from that date onwards 'breathed' into the primary chain case as standard without compromising reliability...Debate does continues to this day regarding the wisdom of the move though...Note however, that as well as breathing into the primary via the crankcase wall there was also an 'exit' pipe leading from the back of the primary case to the rear of the bike, in a similar fashion to the one rigged up on your M20.
It could be argued this was more necessary on the Triumph as the primary case was intrinsically more oil and gas tight than the earlier BSA one...
If you managed to completely cure the leaks from the primary case on the M20 and then chose to route the breather pipe into the case I guess eventually you would end up with too much oil in there......
In my experience some corrosion tends to occur anyway within the BSA primary cases...Shock absorber components, clutch cover and even the chain all have a tendency to rust...
I guess a little contamination from engine oil wouldn't make too much difference in practice....Ian
email (option): ian@wright52.plus.com
It could be argued this was more necessary on the Triumph as the primary case was intrinsically more oil and gas tight than the earlier BSA one...
If you managed to completely cure the leaks from the primary case on the M20 and then chose to route the breather pipe into the case I guess eventually you would end up with too much oil in there......
In my experience some corrosion tends to occur anyway within the BSA primary cases...Shock absorber components, clutch cover and even the chain all have a tendency to rust...
I guess a little contamination from engine oil wouldn't make too much difference in practice....Ian
Rick and Ian,
Thank you for the response on the breather. Perhaps my concerns about contaminating the primary case oil with crankcase effluvia are overblown.
What would be ideal, and what I was hoping to achieve with the uphill hose run from the breather would be in effect, an oil/air separator that would trap the oil mist carried from the breather valve and drain the oil back into the engine.
I have had several bikes in years past (1968 RE Interceptor, 1972 Ducati twin) that used large breather hoses that ran almost vertically out of the engine and seemed to rely on the large hose and crankcase opening to keep gas velocities down while allowing residual oil to find its way back into the engine.
Part of my breather "design" problem is the small internal diameter of the breather and hose; the gas velocity inside the hose remains too high I think, and the collected oil is being blown up and out, unable to be drawn back into the crankcase.
I suspect that at certain engine speeds, the breather pressure becomes negative. My thought was that at that point, the separated oil would be drawn back into the engine.
email (option): teladelujo@ msn.com
Hi John..Effluvia...Now that's a word you don't hear every day... ....
Regarding sucking oil back into the crankcase you won't, or shouldn't, do that on a BSA if the crankcase breather valve is functioning correctly...
Gas is blown out through the breather due to the pressure generated in the crankcases by the downward motion of the piston, which lifts the flap valve from its seat...
On the 'upstroke' the flap valve is sucked back onto its seat, shutting off the valve and generating a partial vacuum in the crankcase.
The thinking behind this was that after initial start up power lost through pumping losses would be reduced...
Once the valve is closed due to the vacuum created by the rising piston there would be no vacuum available to suck any residual oil back into the cases....
My RE Constellation also had a flap valve fitted, so I would guess in that case if residual oil was returning to the cases via the valve while the engine was stationary (accepting it can't when the engine is running) then that method would work equally well on the M20 I would have thought, as long as the vent pipe ran upwards from the valve..A small valve and exit fitting size wouldn't be a problem as long as the diameter of the rest of the pipe was increased and it was long enough...
I understood the principle on the Bonneville was that the combined volume of the chaincase and the long breather pipe (which ran to the rear of the bike) exceeded the crankcase volume...
So, when operating, emissions from the breather pipe were reduced as what was generated was a backward and forward movement of an 'air column' within the breather system and little pressure to move oil through to the 'exit' end of the pipe......
If that is indeed the way it works then it would be best to feed the M20 breather into the primary case and fit an 'exhaust' arrangement from the back of the case similar to that on the Triumph....Ian
email (option): ian@wright52.plus.com
Ian,
It looks like this thread has been thoroughly hijacked....!
Many Thanks for your analysis and comments. And in spite of ancient rumors of crankcase gas explosions sparked in the primary case, routing the breather into the primary would seem to be the way to go.
Routing into the case seems pretty straight-forward, but as you point out, the volume of air passing into the primary also requires a passage for an equal volume to exit.
I should mention that my WM is sporting a M21 crankshaft, so another observation; there was no change in the breathing arrangement between the two displacements, yet there has be be a real difference in the demands on the breather system.
The existing primary case vent would seem to be too small for the application and I would like to avoid too much additional plumbing and hole drilling, so, any additional suggestions or observations on how best to accomplish?
Would the existing primary case vent slot suffice?
JDE
email (option): teladelujo@ msn.com
Hi John...There is a reasonable difference in size between the flap valve disc and the hole in the crankcase fitting...I think you could enlarge the latter to give that part of the breather a bit more capacity (by reducing the 'seat' for the disc)..The difference may not look very much but you have to do your sums to determine the actual increase...remember cross sectional area is not directly proportional to an increase in diameter ...
Also there are three versions of the flap valve housing (the piece that screws in and holds the flap valve in place...One of these is considerably more 'cut away' to increase breathing capacity...
I have to say though I have never worked out when each type was fitted and whether they were model specific...It's all got mixed up over the years and the books aren't clear on the subject...
Another alternative might be to leave out the flap valve in the crankcase fitting altogether and open it up as far as possible...
Then increase the copper pipe diameter leading to the back of the case and make a larger diameter flap valve assembly to fit into the back of the case...or look for some other one way valve that could be discreetly fitted to do the job...Ian
email (option): ian@wright52.plus.com
Ian,
It all sounds like a good winter project! I will post on the Forum if I work out a satisfactory modification to the breather system.
Thanks again for the ideas and insights.
Cheers,
JDE
email (option): teladelujo@ msn.com