M33 Magneto points gap

[Rog1]

I managed to cock up the points gap after I pulled the contact breaker to bits, cleaned and replaced it. The gap was too big, the bike ran hot, the exhaust became discoloured through heat. I've reset the points, but is the 12 o'clock position where the gap is fully open?

[Jim S]

Hi Rog1
I'm not sure if it is at 12 o'clock but it is easy to figure it out. Just determine where the high spot is on the face cam. That is where the points are fully open.
Have you checked the magneto timing? If this is not set properly you can overheat the engine. For my B33 I use service sheet 604.

[Rog1]

Okay, thanks. I looked at service sheet 604, but in the end I found a Lucas service sheet in the 'M' section and from that it seems easiest to just take the cam out and have a look. I'll check the timing, but I stripped and cleaned the contact breaker in the first place because the bike was getting less easy to start. I found that the points faces were not quite lined up and the cam follower was sticking. I had to dress the points a small amount.

[maddogmargetts@yahoo.co.u]

did the starting get easier after you had cleaned and lined up the points?

[Rog1]

Perhaps, er, dunno, but I had cocked up the gap. After I reset it, I couldn't get the bike to start at all. More investigation required...

[Rog1]

Hi Rog1
I'm not sure if it is at 12 o'clock but it is easy to figure it out. Just determine where the high spot is on the face cam. That is where the points are fully open.
Have you checked the magneto timing? If this is not set properly you can overheat the engine. For my B33 I use service sheet 604.

I haven't yet had a chance to look at the timing and points, but I've found conflicting information, mostly in service sheets and handbooks on the BSAOC website. Service sheet 604 tells me the points gap is .010" to .012", but elsewhere it's .012" to .015. Some sources say the timing should be 7/16" BTDC, (sometimes stating fully advanced) others say 3/8", but don't say if advanced or retarded.

Can anyone give me a definitive answer? It's a late '53 machine.

(Signed)
Confused of Buckinghamshire

[Pete C]

I am sure that the correct values are 12 thou points fully open and 7/16" BTDC fully advanced.

[Jim S]

Hi Rog1
My Service Sheet 604 indicates on the first page that it was revised in 1958, so it should be applicable to your bike. If you have the face cam configuration, you should use the non G.B. method (the first one listed in 604).
One point not mentioned in 604 is when turning the engine back to get the 7/16 btdc, I turn the engine back beyond 7/16 then turn it forward to 7/16. This takes up the backlash in the timing gears.
I use Service Sheet 604 to set the timing on my 1949 B33 and it starts usually first kick with the timing set at about half advance. I ride it fully advanced.
Good luck.

[berniej]

Rog, does your mag have manual advance? If so then fully advnced is cable pulled tight if it enters from the top right but fully slack if it enters from the top left.
7/16 BTDC fully advance and 12 thou sounds fine.

If you have difficulties when set like that it might indicate that the mag timing is out at the pinion.

Cheers,
Bernie

[JulianS]

In 1955 BSA issued a service sheet ammending the B33 and M33 ignition timing slightly to 3/8 inch before TDC fully advanced.

[Rog1]

Thanks to all for your informative and rapid answers. I'm going to go for 7/16th BTDC fully advanced. I think it is the 1955 service sheet that muddied the waters; I wonder if petrol had improved and so by 1955 the timing could be changed. What that means for the vegetable stock that passes for modern petrol I don't know, so 7/16" it is. I will pull off the cam to see where the fully open position lies and give the cam some grease and reinstall it, then check the gap and the timing again.

Jim S, I take your point about taking up the slack in the gear train. I have printed off sheet 604.

berniej, I have manual advance, and the cable enters from the top left.

[Rog1]

Rog, does your mag have manual advance? If so then fully advnced is cable pulled tight if it enters from the top right but fully slack if it enters from the top left.
7/16 BTDC fully advance and 12 thou sounds fine.

If you have difficulties when set like that it might indicate that the mag timing is out at the pinion.

Cheers,
Bernie

Here's an update.
I checked the timing and it was miles out. If anything it was slightly after TDC instead of 7/16" before it. The nut that holds the pinion on the taper was barely nipped up, so I reckon the pinion has slipped round. This explains why the bike was getting harder to start, and in the end was backfiring and on the last trip out I did, the down pipe to silencer part of the exhaust got so hot it changed colour! My cack-handed first attempt at the points gap probably made no difference.

Regarding the points cam, about three quarters of it is the "high spot" and a quarter is the low spot plus ramps between low and high. I measured it with a micrometer and apart from a tiny, tiny bit of wear, the "high spot" is all the same thickness (height).

[berniej]

Good news! Yes, the cam plate is flat with two ramps as you say but that's OK. It's job is just to switch the points between closed and open quickly and cleanly and for the right duration.
Sounds like you have it sorted now 

One difference between coil and magneto ignition which might be of interest is that with a coil you can have the ignition set way off and you will still get a good spark when the points open.
A magneto only reaches maximum charge at one point in its rotation and it's designed so that this is also where the points open. If the timing is off at the pinion then although you can compensate a little with the advance/retard lever you may be getting a weaker spark as the points will be opening either before or after the mag reaches max charge.

Cheers,
Bernie

[Rog1]

I've reset the ignition timing. I used a dial test gauge that I've had for years, and just made a bracket for it. I borrowed a magneto pinion extractor and it didn't take much effort to extract the pinion, so that's confirmation that the timing had slipped. The guy I borrowed the extractor from has had an M33 for many years, and gave me a few tips, so following his advice I checked for a bit of clearance between the magneto pinion and the one that drives it (it was fine), removed the magdyno and checked the pick up brush as it's tucked away when the thing is on the bike (also fine). The earth brush and the dynamo are also good.

His best tip was to turn the crank over via the nut on the R. H. end; much easier and more controllable than using the back wheel in first gear.

I took the bike out for a test ride last night. It started first kick, hot or cold. But after a while it started backfiring again so I checked the timing again. It's less than 8 thou out, so since most people seem to do the job with an old knitting needle or similar, I'll call that spot on. It was a very warm evening, and I noticed when I'd got home the advance/retard lever wasn't quite in the fully advanced position, (perhaps I knocked it) so I will test the bike again tomorrow.

Thanks to all for your advice.

[Rog1]

I did a 10 mile trip to visit family, then about a 30 mile trip home. The bike is fine. Yippee!