resistance - should there be some

[bsa-bill]

RGF up to it's usual old trick today, was going to book it in for an MOT but thought it prudent to give a run out just to make sure all was well -HA
it's been starting in the shed no problem, it went the first two miles fantastic, up hill in top with ease and no pinking.
Pulled in two miles out to adjust idle that was a bit fast, it stopped on the lower idle, from then on poor running, difficult starting mostly only possible with bump starts down hills, I ran out of hills and had to be rescued.

There is a spark - sometimes not bad sometimes a bit weak and often coming from around the plugs centre electrode rather than from it ( I did change plugs several times with new plugs also).
Haven't ruled out carb problems yet but here's a thing, I tested for a short on the kill switch, put a meter from ground to the coutout screw on the end of the Maggie, I get 110 to 120 ohms - should this be, this maggie worked fine on my Flash 7.5 pistons, the RGF has the 9.1 pistons back in and the 357 cam (bad move I think)
Any ideas more than welcome
I don't have that much patience these days that I can afford to squander it on this project, saving grace is the old Flash. utterly reliable, starts with a short prod, ferried me to work for about three weeks without a hitch

[Triton Thrasher]

Is the slip ring dirty from a badly-inserted HT brush?

[lawnmowerman]

Hi Bill

How long ago has the mag been checked or rebuilt? I am told that mags die of old age after a while.

If you could find a NOS mag that has been on the shelf for more than about 10 years you would probably have trouble with it.

May be that it worked on the flash at a lower comp ratio but would not work on the RGF as you would need a stronger spark at the higher comp. Also may be breaking down as it gets hotter which would explain why it worked ok in the garage.

I spent ages sorting out a backfire / misfire / starting problem on my SR and I finally bit the bullet and had the mag rebuilt and the bike runs like a clock now - best money I ever spent!

Jim

[bsa-bill]

Hi Triton Thrasher and Jim

Just in so haven't had a chance to look any further at the bike
Good point TT I had just fitted new pickups, will look closely at the slip ring, it is producing a spark though but maybe not in the compressed air of the cylinder.
Jim I have another maggie on my flash, both give the same result on the project, I take your point a bout the higher compression.
The symtoms are as you would expect from a maggie with "warm starting problems", It was reconditioned at FTW around three maybe four years ago ( I have the receipt so could be more precise)
it was also back to them last year as the cam had slipped, something I could have fixed myself but with it giving trouble thought it should be checked out.
However this time it will not start even cold, should at least give a pop now and then even if I've changed the odd setting a tad here and there (don't think so though)
Plan to check the slip ring as TT suggests also the carb before going any deeper.
May swap back to 8.5 pistons but leave the 357 cam in ( has any body run this combination ) if the higher compression is duffing up the maggie then the choice is get it reconditioned again ( somewhere else maybe) or go electronic ( install two slim 12 volt batteries and a switch arrangement  )

[Triton Thrasher]

The reason I thought of the slip ring is because I have misaligned a brush myself (more than once!) and it puts a black ring around the bottom of the slip ring.  The plug sparks in open air, but when under compression, the HT current tracks around the carbon mark on the slip ring and sparks the other plug!

Easy and cheap to diagnose- hope it's so simple, but which of use ever has such luck with magnetos?

[bsa-bill]

Thought that was it TT, there was a bit of carbon which I cleaned off, checked the gap and went to get some known good plugs to try it.
Decided to test the plugs for continuity with a meter, lot of variation one new plug failed completely.
However I got two good plugs and tried the maggie, nice fat blue sparks
Next checked out the carb, nothing wrong in there so put it back on and went to try the bike - nothing, kicked for a good while before taking the plugs out to check for a spark, no spark whatsoever
And the plugs were not wet, tried two other plugs that had given sparks when trying the maggie - nothing.
I hear what you all say about the high compression killing the spark but surely it should produce a spark again when back out of the cylinder, it disappears?
What in dozen or so revolutions can happen to a magneto that is producing a good spark to kill it, no resistor caps or plugs used, I'm more than a little rusty on the black art of windings and high voltage but my mind points me to thinking about the condenser (due to the fact I can't think of anything else).
Think I'll give FTW a ring, they reconditioned it

Also found some info on plugs today that I'll post elswhere

[Triton Thrasher]

Once, the thick wire broke off from the condenser on mine, but I think that's unusual. 

[bsa-bill]

Think this has the makings of something unusual TT 
Couldn't resist one more go, been down the shed gave the slipring another clean ( was ok), took the earth screw out and looked at it uit it back.
Wondering about the end cover, can't see what could go wrong with it although it could have got swapped with the one off my other bike but there both the same I think.
I jury rigged a fuel pipe to the carb and tried it, it fired - once, took the plugs out and tested for spark and they did spark so tried again, it fired - once, still sparking outside the cylinder too so some progress maybe.
As I've had the carb off maybe it needs a tweak, tomorrow

[Triton Thrasher]

You can ru with no end cover if you can stop the cam ring falling out.  Is the points spring keeping clear of the cam ring?

[bsa-bill]

Is the points spring keeping clear of the cam ring?

Ah now I spotted that and thought I was onto something, it is very close and might indeed touch but only on the part of the cam where the points are closed in which case all the parts are earthed together anyway or have I got that wrong and they should not be earthed to the frame but just to each other.

back to the continuity thing, there is no continuity from the central pin (that holds the point plate in) to the maggie case, but there is some (as said before)from the kill button on the end over to the maggie case - going to swap them over today, also check meter reading on old flash for same thing.

[Triton Thrasher]

The centre screw should have continuity to earth with little resistance. Shouldn't the test current go from that screw, to the condenser and thence through the thick LT winding to the armature yoke, which is earthed to the casing by one or more earth brushes?

Something like this:


Sadly, most magnetos which act as if cursed or haunted turn out to have an intermittent/deteriorating HT winding fracture don't they?

[bsa-bill]

thanks TT that's really helpful, my 12 month apprenticeship in a Electrician come radio/tv shop some fifty years ago seems of little help to me these days .

Do you know which point ( fixed or moving is from the primary windingand therefore the capacitor) I can do comparison with other maggie.

Have tested the end cover from the Flash's maggie, it has a brush that is twice the length of the RGF one, it also shows around double the resistance, looks like what I'm measuring is the resistance of the brush in the end.

I do have a spark now (don't ask how or for how long) but the carb is not right, bike fires once after flooding then stops, not enough fuel I think, it's coming out the rear of the carb.

You know I can't remember braking any mirrors but..................

[groily]

Bill, the fixed point (ie the one we adjust) is the live one. It's insulated by a mica washer and other little bits and bobs that go round the small screw that holds it to the cb backplate. The sprung, moving, point is earthed always through the screw on its tail, to the cb backplate and then through the earth brush(es) etc. It doesn't matter electrically speaking if the spring blade touches anything else that's earthed - but it mustn't touch the brass bit that holds the live point obviously or you'll have no sparks as the points will have been bypassed.  Could have been your prob? unless the cut-out / kill button wasn't right - ie permanently shorted to earth and thereby same result? (If the camring hits though, anything could happen as the points might never even operate properly!)
The live point is connected (at the drive end of the mag) to the live side of the capacitor and the live end of the LT winding. It's done by a link wire that goes from the bottom of the cb centre-screw through a channel cut in the laminates of the armature  - the coil end, the link wire and the capacitor live are soldered up with norralot of room around them. The other end of the LT coil is earthed along with the capacitor itself (and the HT winding for that matter) also at the drive end.
The kill button brush or contact blade (or whatever) to earth should show an Open Line except when the button is pressed - when it should be as near to 0 ohms as possible  . . . If there's permananet continuity, then no sparks natch.
As TT says, there'll be continuity to earth through the centre screw (or live cb point, same thing), that's the resistance of the LT coil only - about half an ohm. Which a cheap meter seldom measures with any precision. Earthed point to mag casing or frame should be zero ohms - but same problem with cheap meters and there's usually a bit of unwanted resistance in any brush and spring arrangement. If readings are obviously high but seem to be correct, then the earth brush is a likely prob as suggested.
Best of luck!

[bsa-bill]

Thanks Goily and TT - plenty to digest and test there, I think the resistance I measured could be down to the carbon brush on the end cover screw ( to kill switch)

I have an old AVO meter that I started to clean, was my dads, I think I'll get it together and test the maggie with the info you've both provided.

Now some good news, Les (friend ) had heard of my problems and came around to investigate, makes a difference when you can share the kicking.
We has little success so decided to go through all the things that usually go wrong other than the maggie, carb, valves, timed to correct cylinder, discussed the events of Tuesday starting with how well it went at first, Concluded something must have broken or moved.
Checked the ignition timing, heel just at the ramp when cylinder at the 3/8th btdc, it was --- thats when the lamp lit, it should be there when fully advanced not at retarded.
Possibly moved when I  bump started it or just not been tight enough, at best timing has slipped at worst tooth/teeth of the fibre pinion. will find out tomorrow.

This still does not explain the disappearing spark

 

[bsa-bill]

OK I got around to making sure all was well re timing both valve and ignition, and had the carb to bits once more.

Put new plugs in and tried it - zilch, the plugs spark outside the cylinder and light up a plug tester when in the engine but it wont start.
We were looking at the plugs and thinking they were dry but thinking logicaly when Les swped the plug leads over we got an almighty crack, I tried the same thing and got two loud reports and on a second kick another bang that sounded like it was in the crankcase, so there must be fuel mixture in there (or maybe this is how the universe came to be, matter from nothing )
The other point to this is that when you swap the leads you then have the spark formed in the cylinder that is not under compression.
This is leading me to believe the Maggie is just not upto the task, both my Maggies perform the same in this engine perhaps I should accept that neither of them is up to sparking in compression over 7.5 .
Now to meter readings, my meters lowest resistnce range is 0 - 200, with the points open readings between centre screw and earth or either points and earth is around 5.6, with points closed the same readings give around 9.
tell me if I'm doing this wrongs folks