2, Contact breaker when the piston is at TDC. The point gap is 0.003 in this picture. This has been set in accordance with be information given by BSA for this bike.
Hi Billington,
I wonder if you misunderstand the ".003" at TDC at full retard" message from BSA.
I think you have been doing this: You set piston to TDC, set the lever to full retard, then adjust the points to 0.003". Correct?
But this is not what BSA meant. Not at all.
Let's separate this into two parts: 1. Setting the points gap, and 2. Timing the magneto to the engine.
1. Setting the points gap. Imagine the magneto is out of the engine and on the bench. For it to give a nice spark when rotated, the points have to be adjusted to a gap of 0.012" (12 thou) when the points are fully open. Looking at your photo of the points, the points rotate clockwise (the fibre block that runs on the cam ALWAYS trails), so from where they are in the photo at about 2 o'clock until they close at about 10 o'clock the points gap should be about 12 thou. So, set the gap to 12 thou, but it doesn't matter too much if it wobbles a few thou as you rotate fro 2 to 10 o'clock.
OK. This is what the magneto man does when he renovates your magneto, at his place, when it's not attached to your bike. The points gap should not be touched again until wear causes the gap to change.
Now the magneto man spins the mag and there is (hopefully) a huge spark at full advance (the tight wire rotates the cam towards the rubbing block so that the points open earlier), and a slightly less intense spark when the magneto is retarded (loose wire, cam rotates away from the rubbing block, so spark is later).
All done. The mag is working fine. Don't touch the points. Next we have to time the magneto to the engine. Because the spark is best at full advance (tight wire), the plan is to have the best spark where the engine spends most of its time - at a few thousand rpm when the spark needs to occur at something like 35 degrees before top dead centre, which corresponds to the piston about 9.6mm before TDC.
Now there are TWO WAYS to set the magneto timing that will give the same result: the first is to set the points to open 35 degrees/10mm BTC with the magneto at full advance, OR the BSA method to set the points to be 0.003" at TDC with the magneto at full retard. BSA are saying the the points are just starting to open - by 3 thou on their way to 12 thou - as the rubbing block is sliding up the ramp on the cam. When the magneto was new, these two methods should give the same result, so if the first method is used, and the magneto is fully retarded and the engine is rotated to TDC the points with have opened 0.003", and vice-versa.
The first method is better in 2022 because stuff has worn, been replaced or fiddled-with in the 90 years since the bike was made, and the important thing is that the spark occurs 35 degrees/10mm BTDC at full advance. The BSA method is easier but indirect, and in 2022 could easily give the wrong ignition timing.
So...
2. Setting the magneto timing. If the mag was fresh back from a rebuild (with its points set to 12 thou), the drive gear would not be attached to its taper. You would fit the mag to the bike, set the engine to 35 degrees/10mm BTDC on the firing stroke (both valves closed) (always approach by rotating the engine forward because of backlash in the timing gears), set the mag to full advance, rotate the magneto until the points are just beginning to open, push the gear onto its taper, and do up the nut.
(The BSA version would be set the engine to TDC on the firing stroke (both valves closed) (always approach by rotating the engine forward because of backlash in the timing gears), set the mag to full retard, rotate the magneto until the points are just opening by 0.003", push the gear onto its taper, and do up the nut.)
Now here is my suggestion for how you set your spark from where you are now:
1. Adjust the points gap to about 12 thou, on average for the time the points are open.
2. With the head off, go to TDC on the firing stroke (both valves closed as you go either side). Measure the position of the top of the piston relative to the barrel. Rotate the engine BACKWARDS by half a turn or so, then bring it slowly forwards until the piston is 10mm lower that it was at TDC.
3. Take off your timing cover, and have a look at what you've got there. Between the inlet cam gear and the magneto gear, I think there's a larger idler gear? I think this can be slid out? If so, take it out.
4. Set the magneto lever to full advance. Rotate the magneto so that the points are JUST OPENING - i.e. the rubbing block is just impacting on the cam ramp and causing the points to crack open. Hold the points there and see if the idler gear will drop back in. It will probably be out by some fraction of a tooth (hopefully not exactly half a tooth!) so drop it in on the nearest tooth. You should be able to get the points to open somewhere around 10mm BTC - say 1mm either way. If there are 40 teeth on the magneto gear, one tooth will change things by 9 degrees.
5. Button it up. Done.
6. Ask if you don't understand.
Leon
