Hitchcocks Forum

A question: if I increase the points gap by 5 thou how much will this advance the ignition timing? I don’t need to know the precise amount of ignition advance (if that could be calculated – which I doubt!) but if a 5 thou adjustment to the points would have any significant effect on timing. Thanks.

As you say, it could be calculated but life's to short.
From experience, I reckon you would definitely notice and I think an Enfield would let you know by kicking back more than usual.

I suppose there would be a modicum of advance but i'm thinking more in terms of dwell being shortened..

If a coil or associated electrics are not up to scratch then in theory the spark could be "thinner" not sure how else to describe it..

Unlike a multi cylinder engine, a misfire on a single as you apply power can be very distracting..

Just theory of course, real world probably won't make much difference.. gwilly

Good point Gwilly, it may well show up a not-so-good capacitor first.
Out of interest, why would you want to?

In order to calculate how much difference it will make to the timing, we need to know how steep the ramp is on the points cam. I doubt if anybody alive knows that, so I guess the only way to find out is to actually try it.

As Thack says it depends entirely on the angle of the cam, at 45 deg 5thou extra would give 5 thou advance, but the slope is normally far less so a change of 10 thou advance or more is likely. Don't forget that the points speed is half engine speed so now double those figures, 10 thou and 20 thou is now on the cards.

I have two bikes with magneto ignition at the moment and over the years have owned many more, I have always used the points gap as an easy way to fine tune the timing.

I don't think you need to worry about dwell; compare with - for example - a V8 which has to accommodate 8 lots of dwell in every distributor revolution, we will always have more than enough, so you can use any of several usual methods to set your timing just as before after points have been altered.
I have been experimenting with a makeshift manual a/r lever as outlined in my recent posts "Ignition timing,iron engine", I am certainly convinced that advance and retard on a bullet is even at best a vague compromise. It only alters under rev changes, but not load as a traditional vacuum assisted system does; so maybe a handlebar a/r lever to supplement the centrifugal system will be the solution. Hey, maybe I'll mark it "Early" and "Late" as Mr Royce did on his excellent motor cars!!

Perhaps there's an opportunity here for Mr H to produce a kit. After all, he does a solid rear end, old fashioned carbs and other "retro" bits. Why not manual A/R?

You could measure it with a strobe timing lamp, and a timing plate: complicated. It's an old dodge, try it and see. A bigger change is had by fiddling with the A/R springs and weights, or at least making sure they're doing their job proper.