Contact breaker adjustment

[Billington]

I’ve been servicing my 1932 BSA, which has a cam ring contact breaker. In one of the BSA books I own the indicated gap size is .003” when the piston is at TDC, with the ignition is fully retarded. When I measured the gap it was .006”. So I was thinking about adjusting the gap until I read in “Book of BSA” (Year and model appropriate), “with the contacts full open, should be considerably more or less than will just hold a 12 thou’ blade of a feeler gauge(0.012 in.) they should be adjusted. ”

The bike seems to run just fine and the spark plug is a light brown colour. So should I adjust the gap to .003” or just leave it alone?

[Rex]

If the bike runs fine then why bother? That said, it's no hardship to adjust a set of points,  and if you were to do it you might find the bike runs even finer.

[Billington]

Thanks Rex. I think I may simply run the bike as it is for a little while then adjust the gap, so that I can make a judgement.

I’ve not adjusted the contact breaker before, is it simply a matter of loosening the lock nut, followed by loosening the contact breaker securing screw and turning the adjustable contact point in or out with the help of a feeler gauge? It looks like there is very little room to turn the lock nut and adjustable contact point, also the spanner must very small.

[Rex]

Yes it is a small nut and from memory it's 4 (or possibly even 5) BA. That said, you can often pick up little original sets of mag spanners for just this job, and some even come with a little screwdriver blade for the screw too.

[R]

12 thou with the points fully open is the figure I've always worked to.
And thats for a variety of magnetos/machinery.

I've never actually measured what it might be at precisely tdc - that figure may be compatible with 3 thou,
depending on what the slope of the cam is.

Have they quoted what the spark timing number is, either degrees btdc or a distance before tdc.
FULLY ADVANCED. That is the important bit when all is said and done ...

[Rex]

I don't think they'd invented the degrees before BTDC method when that BSA was made. 
If they didn't use the "pencil down the plug 'ole" they probably did the quick-and-dirty method, namely full retard at TDC with points just opening.

[Billington]

Hi R, my BSA Motor Cycle Instruction Book (All models 1930 to 1936 inclusive) state that ignition setting for my bike should be “Piston at T.D.C Ignition fully retarded, contact breaker points open .003 inch”.

Two of my BSA books state the above, but another BSA book  states “with the contacts full open, should be considerably more or less than will just hold a 12 thou’ blade of a feeler gauge(0.012 in.) they should be adjusted. ”

I’m someone who tries to do technical tasks by the book, however I think in this case I may just have to learn to live with contradiction. It may be that they are both correct under the right circumstances.

I beginning to think that I should adopt the rational that if it’s running fine it probably is fine.

[iansoady]

On the other hand, it never hurts to gain faimiliarity with tasks like this. Like others, I've always timed on full advance using piston distance BTDC - degrees are probably overkill for these older engines.

Are your books proper BSA manuals or something else? You should be able to get copies of the genuine article from the BMS scheme at the National Motorcycle Museum.

[john.k]

The 003 was the thickness of a piece of note paper......you put a piece of paper in the contacts ,rotated the motor,and when the paper could just be pulled out ,that was considered to be good enough for the "break" point of the contacts where spark occurrred............as mentioned the correct full opening was .012" to .015".....................................as someone once said at a vintage bike run......"Do get yourself a set of feeler gauges ,they only cost sixpence"......."Sixpence....sixpence?...when I were't lad,there wer'nt sixpence in't whole of Yorkshire"

[Billington]

Hi iansoady, Here are the book I use:
1, B.S.A Motor Cycle Instruction Book All models 1930 to 1936 inclusive
2, B.S.A Motor Cycle Instruction Book 1932 Models
3, B.S.A Motor Cycle Replacement Parts 1932 models
4, Book of The BSA (reprint of 1947 edition)
5, BSA Motor Cycle Maintenance And Repair Series by D. W. Munro (A Practical Guide Covering All Models From 1931)

Book 1 and 2 both suggest the piston needs to be at T.D.C Ignition fully retarded, contact breaker points open .003 inch.

Book 4 states “with the contacts full open, should be considerably more or less than will just hold a 12 thou’ blade of a feeler gauge(0.012 in.) they should be adjusted. ”

[R]

It does sound like there are 2 ways of arriving at the same thing.
I'd stick with the 12 thou and distance btdc, its a tried and true timing method for many a machine.
(Does it quote the distance btdc ??)

I beginning to think that I should adopt the rational that if it’s running fine it probably is fine.

And you also have under your thumb the advance/retard lever, so if its not running fine you have the means to make it do so...

[cardan]

Book 1 and 2 both suggest the piston needs to be at T.D.C Ignition fully retarded, contact breaker points open .003 inch.

Book 4 states “with the contacts full open, should be considerably more or less than will just hold a 12 thou’ blade of a feeler gauge(0.012 in.) they should be adjusted. ”

Hi,

Taken together, these two bits of info tell you how to set the points on a magneto so a bike will run well. The first piece of info tells you about setting the magneto timing - the point at which the plug sparks - and the second tells you the points gap when fully open.

But there is a serious problem with the first piece of info. It is no-doubt correct for a new, unworn, magneto as originally fitted to the bike. But by specifying the gap when the points are opening, it assumes the slope of the opening ramp on the points cam is exactly as it was when new. I wouldn't want to assume that 70 years down the track unless you have a new-old-stock points cam.

If the magneto were on the bench (off the bike), setting the points to a maximum gap of 12 thou presumably gives best operation of the magneto. (Even this assumes that the INTERNAL timing of the magneto - the position where the points open relative to the maximum flux point of the magneto armature - is still correct after all these years! You can check this by setting the mag to full advance, and checking that the points open close to the point where there is maximum 'tug' when the armature is rotated.) It's a good idea to set your points gap to 12 thou or so.

Because of the wear thing, I'd ignore the advice to set the points at 3 thou TDC fully retarded. The magneto will spark just as the points open, and you want this to happen at 35-40 degree before TDC with the magneto fully advanced.

If you do this, the bike will run great, and should take full advance for most normal riding. You may even find that the points gap is 3 thou at TDC with the magneto full retarded.

Cheers

Leon

[Rex]

40' sounds a lot for an old SV clunker running on modern juice though.
IIRC my old Bonnie was something like 38' on 5 star petrol, and down to something like 34' when unleaded etc were introduced.
On balance, I think for a bike like this I'd stay with the "set at TDC retarded" method and find the sweet spot with the A/R lever. These old girls run hot at the best of times and I wouldn't want to add to that.

[cardan]

Yep, 40 degrees BTDC sounds a lot, but, for example, the book value for a 1920s side-valve EW Douglas is 45 degrees BTDC! In an overhead valve engine the plug is right in the combustion chamber, adjacent to the valves and above the piston, whereas in a side valve the plug is usually way off to the side somewhere...

Certainly "points opening TDC fully retarded" will work on the side of the road. But keep in mind the Standard for magnetos is 30 degrees of adjustment at the magneto, which equates to 60 degrees on the engine. So for an unmolested mag "TDC fully retarded" will give a spark at 60 degrees BTDC with the a/r lever fully advanced. i.e. it will run best with the mag lever set about half way. Good enough to get you home.

Cheers

Leon