Self retarding timing

orbinrangle · Post by **orbinrangle** » Fri Sep 12, 2008 10:40 pm

Having fun with my 'overhauled' 18S. Had to reset the timing three times but it never stays good for long before it retards itself again. Can't put too much pressure on the little brass nut behind the points or I'll knacker it. Tried a drop of thread lock to no avail. Should there be a locknut there?

Save me from this misery somebody.....

SPRIDDLER · Post by **SPRIDDLER** » Sun Sep 14, 2008 9:45 pm

Robin, I'm not clear how you manage to retime the ignition at the points end of the mag??
Are sure that it's not the sprocket on the end of the mag or on the exhaust camshaft that's slipping?
Must admit that I have only ventured far enough into my mag as that necessary to fit new points and can't remember exactly how the points assembly is fixed onto the shaft (keyway?? taper??) so I may be sparking up the wrong tree here.
Still, you seem pretty miserable. (And ignored!)

itma · Post by **itma** » Sun Sep 14, 2008 10:15 pm

I must admit I too am puzzled

clive · Post by **clive** » Sun Sep 14, 2008 11:10 pm

I am not sure what you are doing but will have a guess. Firstly I assume this is an N1 mag rather than the later SR1 mag with the auto advance. Secondly that you have set the points gap and then timed the engine by using the sprocket at the camshaft end. ( I find if you try to do up the sprocket on the magneto it always moves while you are doing it up.)
Thirdly that you have rechecked the timing after you have done it up in case the sprocket at the camshaft end slipped whilst you were tightening it up.
If all was well and then once you ran it you found that you needed to adjust the points gap again I would suggest that the thread has gone where the bottom point fits into the rotating arm. This is quite a fine thread in an alloy arm and I have had a couple go, but then I am heavy handed on locking things up tight.
Hope that helps

SPRIDDLER · Post by **SPRIDDLER** » Sun Sep 14, 2008 11:30 pm

You're right Clive, the points gap will affect the timing, but Yeah, come on Robin. Try and keep up!
Do you mean that after running the engine you have re-adjust the points gap or re-adjust the ignition timing?
I agree - I also adjust the ignition timing by moving the camshaft sprocket rather than the mag sprocket. Even then it always takes me two or three goes to get it absolutely spot on though; it's difficult to stop the setting change when tightening the sprocket. There's only a gnat's todger of sprocket movement between, say, 7/16" BTDC and 1/2" BTDC.
Still I guess it's enough for Mrs gnat.

orbinrangle · Post by **orbinrangle** » Mon Sep 15, 2008 2:13 am

Clive, I think you must be right. I adjust the points in the approved manner but feel I cannot tighten the nut up enough (it pushes on to a taper) and after ten miles or so I find the timing retards itself and I have no more advance adjustment left on the lever. New parts needed I suspect.
This is my first British classic so I am dull with it. But I'll get there.......

SPRIDDLER · Post by **SPRIDDLER** » Mon Sep 15, 2008 8:54 pm

Robin,
I've just come across a drawing of the c/b assembly which shows a tab washer fitted under the brass nut which secures the assembly to the mag shaft....

orbinrangle · Post by **orbinrangle** » Tue Sep 16, 2008 5:35 pm

Ah. That'll be why it unwinds itself then, allowing the points to open. Suppose I'd better find one.
Thanks
Robin

SPRIDDLER · Post by **SPRIDDLER** » Tue Sep 16, 2008 7:35 pm

Err, I think we're at cross purposes here, Robin, but I can't think how to clarify. If you can get sight of a drawing...perhaps there's one on Christian's Archives?

Biscuit · Post by **Biscuit** » Wed Sep 17, 2008 12:51 am

If the sprocket is given a sharp tap with a smallish hammer it won't move as you tighten it. Learnt that as a teenager 60 odd years ago.