SR1 cam and points

Helpful information and requests for assitance and advice
User avatar
G3L1946
Member
Posts: 542
Joined: Tue Feb 25, 2020 1:57 pm
Location: North Yorkshire UK

SR1 cam and points

Post by G3L1946 »

Hi all, just need a bit of advice on SR1 magneto points and cam.
It seems that the lobes, on the cam, are not very pronounced ( there are two, right?). To my oldish eyes this is giving me bit of a problem. It appears, and this is where I would like clarification, that they are at the same point as the slots in the cam? See green circle on photo.
I’ve set the points as required and all seems correct. But I’ve thought things have been correct before! I’m probably appearing to be a numpty but this electrical stuff gets a bit confusing!
Regards,

Steve

1955 G3LS, fitting new breaker points.
You do not have the required permissions to view the files attached to this post.
User avatar
1608
Member
Posts: 2493
Joined: Mon Oct 05, 2009 1:00 am
Location: ESSEX UK

Re: SR1 cam and points

Post by 1608 »

Only one lobe I'm afraid, Steve. Your bike is only a single cylinder after all!! I think you are mistaking the start and the end of the lobe as two points.
User avatar
G3L1946
Member
Posts: 542
Joined: Tue Feb 25, 2020 1:57 pm
Location: North Yorkshire UK

Re: SR1 cam and points

Post by G3L1946 »

Hi, that was the baffling bit! Logic told me just one lobe but the points ‘appear’ to be opening twice in one rotation at a point 180 degrees from each other. That leads me to think I am misinterpreting the top of the lobe. If someone could indicate, if possible, where the top of lobe should be found on the photo, would really assist.
Mmmmm!

Steve
Last edited by G3L1946 on Fri Jan 28, 2022 3:05 pm, edited 1 time in total.
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: SR1 cam and points

Post by Groily »

Yup, one lobe Steve, as 1608 says. As you look at the mag cam-end on it goes CLOCKwise.
The magneto is described as ANTI-Clockwise, because it is - from the Drive End.

The points should just open JUST after you feel the 'flip' of the rotor, turning it A/C from the Drive end. (There are two flips per revolution - as with all mags - so there'll be another one when the points are shut. That gets used where there ARE two lobes for a multi-cyl engine, but not on a single.)The points stay open until the heel comes off the ramp of the cam, then they close so the coil can recharge ready for the next time they open to make the spark.
If you look at it one way (from the points closed period), there are 'two opening points' - but not if you look at it the other way - they open, then they shut, which is right. Points only need to be shut for about a quarter of a rev to do their job.

The slots are in a screw that goes up the spindle, the spindle being a tight fit on a taper, with no keyway. It's coincidence where the slots end up!
This makes the cam position adjustable - so if it is in the right place, don't move it unless you have to! It probably is! There's a load of info on setting the cam in the SR1 Lucas booklet which is widely available, including here: http://www.brightsparkmagnetos.com/libr ... %20SR4.pdf but you shouldn't need to fiddle with it, in 99% of cases anyway.
The reason the cam isn't keyed is so that it can be fitted in a different spot to suit the opposite rotation of the magneto if need be.

While you have the end off, worth putting a simple multi-meter across from the brass HT tag on the coil to the mag body just to check there isn't a break in the winding. That looks like an original one . . . You should see somewhere around 5500-6500 ohms probably. If you see Open Line or Infinity, there's a break. Might still work for a while as the spark jumps a break to begin with, until it burns away and becomes too large - then zilch. Fingers crossed that one is still OK . . . .
User avatar
G3L1946
Member
Posts: 542
Joined: Tue Feb 25, 2020 1:57 pm
Location: North Yorkshire UK

Re: SR1 cam and points

Post by G3L1946 »

Hi Groily,

Thanks , as always! I think I’ve been misunderstanding where the lobe actually is! Does it matter, when setting up the points, if the auto A/R is advanced fully or closed? As I look at the points I’m turning the engine over via the kick start . Would this better be done in gear and rotating back wheel? I seem to just get the shoe onto the correct part of the cam when it flips round.

Steve
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: SR1 cam and points

Post by Groily »

Ha! When setting the points gap, it doesn't matter, no. You can do that on the bench or on the moon.
But when setting the timing ref the piston position, the auto-advance should be wedged fully open - advanced that is - against the springs. A wooden wedge of old clothes peg or something is handy for that. Then, get yer piston where it should be before Top Dead Centre (not after, sooo easily done!!) and on the compression stroke (not the exhaust one, also sooo easily done!!), and yes I'd do that in gear by turning the wheel as it's easier usually. Be sure that the piston is on the UP stroke as you get it to the right spot, as if it's on the down one, there'll be drive train slack that will affect it a bit. Then get the points to 'just starting to open' on the cam (turning the mag the right way!!), make sure the wedge is in there and that the engine hasn't budged, and do the auto advance centre bolt up. Might take a couple of goes, but shouldn't be too horrible.
That said, I believe many people with singles actually prefer to do up the ATD end first and take the sprocket on the camshaft off, as it is easier to avoid unwanted movement just at the last minute doing that one up - but I don't have any singles. Need to get the mag chain slack on the right run too, if possible, so that everything is as accurate as you can get it.
User avatar
G3L1946
Member
Posts: 542
Joined: Tue Feb 25, 2020 1:57 pm
Location: North Yorkshire UK

Re: SR1 cam and points

Post by G3L1946 »

Hi Groily,
Thanks. I’m assuming to turn the mag the ‘correct way’ is turn rear wheel in direct if bike was moving forward?
Just tested winding. No reading!!! It was running prior to changing points with a good spark ( only decided to change points as, on inspection, they looked quite pitted ).
So, even more to look at and pay for unless my ohm meter is the problem!
Cheers,

Steve
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: SR1 cam and points

Post by Groily »

That's the way to turn the ENGINE the right way - but the mag won't be connected (until it is, as it were)! You need to turn that by hand- to get the points starting to open ready for things to be locked into position - ANTI-clockwise from the ATD / drive side.

On the winding, check again from the brass HT tab to the wire off the coil that goes to the points / condenser. That should show the same reading as 'tab to earth'. If it doesn't, there is a problem almost for sure. On those coils there is only one 'wire off', which is the live low tension one. The earth is internal, being direct to the laminated bar that the coil is wound round, and the connection to the mag body is then 'made' by the fact that said bar sits atop laminated iron bits in the body and is screwed down hard onto them by the two screws you'll see on top of the magneto. Undoing them allows the coil to slide out (sometimes with difficulty!)

I have a nasty feeling that, being original, that coil may be past its sell-by, which was why I suggested measuring to be sure - but it's hard to tell by looking. An Open Line is conclusive enough though, if that is truly the situation, and there is no point proceeding without a rewind sadly. Replacements - often from decent sources - are out there on e-bay; and all the 'usual suspects' will be able to rewind yours if need be. But, as ever, not a freebie or cheapie unfortunately.
The other (well, one other!) thing with coils is what they are like hot . . . the insulation may have started to ooze out, and it might be soft as putty when hot, which is bad. It needs to be pretty solid to the pinch at anything up to about 60°C. Lucas test temp was 50° for performance measurements, but they can get hotter than that on a bike, even in front of the cylinder. If in doubt, unless you really like taking things apart regularly looking for gremlins (??!!) it is going to be worth having that one checked out properly, is my penny's worth.
User avatar
G3L1946
Member
Posts: 542
Joined: Tue Feb 25, 2020 1:57 pm
Location: North Yorkshire UK

Re: SR1 cam and points

Post by G3L1946 »

Thanks Groily. I’ve had a quick look and found a new winding for £64 and is British made.
I’ll make a few more tests and see what needs to done then just get on with it. If I can replace it myself I will go that way. If it looks out of my zone then I’ll need to get it done professionally.
Let’s hope I’m in the zone!

Regards,

Steve

Regards,

Steve
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: SR1 cam and points

Post by Groily »

Drop of penetrating oil on the two screws on top if they're tight. Steel in alloy, always a pain.The coil shoud then be removable if you undo the one wire off that goes to the points. The magnetic rotor will hold the coil in place in some positions - twiddle it to find a spot where the coil lifts off its seating easily with a small lever, then wiggle it out if you can. You'll lose a bit of magnetism for the lack of a keeper while the coil is 'out' but not enough to stop it working. The laminated bar that goes through the coil acts as a permanent 'keeper' when it's assembled in that it maintains the magnetic circuit across the laminated bits on each side, the bits that are profiled to let the rotor turn between them, but it won't be the end of the world if the circuit is broken (or not usually anyway). You might be able to mess about with a short iron bar to bridge the two sides as you get the coil out if you want to - and leave the bridge in while it IS out - but it can be a fiddle-faddle. If doing a 'full job', I usually withdraw the rotor into a tight-fitting thick-walled iron tube to act as a keeper, but that means taking out the points side mounting plate etc etc. Only essential if one or other bearing is fubarred or there is end float to eradicate. End float is bad on these bearings, but a thou or three would be OK.

New coil will probably come with a wire soldered on for the points / condenser link, so just needs sliding in and screwing down from on top. Clean the bits the coil sits on to get a nice clean connection for the earth. Worst case, if supplied without said wire, you'd have to solder the old wire in same position as on old coil - not hard, and won't melt anything in the doing thereof. A cheapo soldering iron and a steady hand will sort that, no problem.

Some coils have a solder bobble where you have a brass tab, but the outer cover has a brass bit that can be bent to make contact. It's about 50:50 what the HT take-off looks like.

Shouldn't have to take the whole thing to bits, so you should be able to get into 'the zone' I reckon. If you do, you'll be happy and in demand among your mates. SRs are the best ones to play with frankly as the system is so like an old car distributor, with a single lobe and no 'cap' of course! They truly don't need attention that often, and even then it's 'just' bearings and a dose of extra magnetism up the rotor in some cases if the coil and condenser are good. Loads of folk have replaced the coils OK in small sheds, pretty sure you'll be able to if you can get the old one out OK. (Don't chuck it away if the deal you do isn't an 'exchange' - it can always be rewound later on and resold! In fact, don't chuck anything away, ever . . . even old points can be recovered if they really have to be.)

I see you have a ceramic EasyCap condenser in there - that should stay where it is, and with your points all shiny new, you should be up up and away quite quickly. Here's hoping.
Locked