Norton Big 4 Spark Plugs

[charlie]

Sean it looks as if your magneto is a 977B. Geoff is this the same as the magneto on your engine? If not which one do you have? This might help clarify the differences, I do have some data for these mags.

[vhgmcbuddy]

Definitely a 977B. What I can’t be certain of is whether this is the original magneto. Hopefully, as Geoff has a manual for the Norton Big 4 industrial version, this will show the correct mag and illustrate how the timing is carried out. I am trying to avoid taking the mag apart as it has only just been rebuilt and is working perfectly, but if no one has any info on the 977B and how (or even if) the timing can be adjusted, this might be my only course of action.

[vhgmcbuddy]

Took the plunge and decided to try and remove the impulse coupling to see if there was any way of adjusting the timing. When I originally stripped the Trusty down, I could not work out how to get the impulse coupling off as there is no nut to undo, or that’s what it looks like at first glance!! I then realised that the answer had been staring me in the face all along. The magneto shaft has two flats on the end (see the picture in my previous post), which didn’t seem to do anything. The grey matter then kicked into gear and I began to wonder if this was actually a nut, rather than the magneto shaft. So I locked the impulse coupling solid with a screwdriver and found a spanner which fitted on the flats. Began to gently turn it anticlockwise and much to my relief it unscrewed allowing the impulse coupling to be removed (the mystery “nut” is third from the left in the first photo). With the impulse removed, I could see that there was no way of adjusting it, other than rotating by 180 degrees. So this particular type of magneto was obviously factory set. This led me to take another look at the chain sprocket. If you look at the attached photo, you will see that the sprocket has 15 teeth. As this is an odd number, it means that there is a tooth inline with the bottom slot that fits over the lugs on the impulse coupling, but the top slot is inline with a root, so this allows for half tooth adjustment by rotating the sprocket 180 degrees. When I refitted the magneto back on the tractor, I must have fitted the sprocket 180 degrees out from where it was originally (sods law in action!). By rotating the sprocket half a turn, the points now begin to open 3/8″ before TDC. Haven’t had time to test run the engine yet, but hopefully with the timing now set correctly, any other starting issues will be down to the carburettor.

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[charlie]

Sean I hope that is the solution to the problem, something for us all to remember, nice clear photos.

[Author]

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