The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: coater87 on 27.06. 2017 11:57

Title: bad points gaps
Post by: coater87 on 27.06. 2017 11:57
 Here is the story.

 Setting points gap I get 12 and a solid 20 (21ish) on the other side.

 I have to get this closer. The only one I would send the mag to is Andrew at Priory, but he is exactly half a world away.

  I have machined parts for a long time, I cannot see one side of a cam ring wearing that much more than the other without "help" like someone ground on it. Or it was made like crap to start with.

 I have read that shimming one corner of the cam ring housing to "cock" the cam ring into running more true is one way (I dont like it, thats got to be hard on the bearing)

 Removing the cam ring and sanding the high side and shimming the low to recenter the cam ring -sounds do-able, depending on the grade of steel the cam ring is made from. If its something like heat treated 4140, you might as well try to carve granite with a feather duster. *conf*

 Someone even mentioned some kind of escentric screw under a welch plug somewhere to center the cam ring. I have no idea what he is talking about.

 Tricking the bearing off center using different thicknesses of insulating paper. This also sounds do-able, but by my feeler guage I have to move 5 thousandths total. That seams like a lot for a paper movement, plus thats an insulator. It sounds like a good idea, but I dont think its all that great. I could introduce weird problems down the road.

 Its also possible my three piece armature is not true, but without a complete tare down I have no decent way to check this.

 The mag checks out electrically with the tools I have, and spits blue sparks with a flick if the armature. I would like to try the mag out before I send it half way across the world.

 What is the best way to get a better gap without ruining what I already have?





Title: Re: bad points gaps
Post by: JulianS on 27.06. 2017 12:11
Worth check a few things before starting grinding.

Remember that the really important thing is that the firing point is the same on each cam lobe. Grind wrongly and the timing will be miles out between cyinders.

Is the points securing screw straight.

Are the bearings correctly shimmed.

Are the bearing inner races square on the shafts -the soft brass damages if they are not pressed on true.

Are the bearing insulators properly fitted - if some of the petals break off the bearing can sit to one side .

Are the  outer races square in their housings.

Is the points end cover warped.

Suffered all these over the years.

Lucas gave a 3 thou tolerance on points gap.
Title: Re: bad points gaps
Post by: duTch on 27.06. 2017 12:19

 julian rounded me up but anyways;-

Lee, first up, Andrew @ Priory has publicly stated a while ago that he is no longer able to do work for international orders, so forget that idea.
 12 to 20/21 is a big difference.
I have read that shimming one corner of the cam ring housing to "cock" the cam ring into running more true is one way (I dont like it, thats got to be hard on the bearing)

 Depending on the particular issue, it's not the housing that is packed, but the ring itself- well that's what I did by packing under the ring with layers of tinfoil..and got it down from 12/16 to 12/14.

 I recently had my Maggy rebuilt, and Darin who did it said he packed one side with some kind of 'O' ring *dunno*, he explained it with a diagram with circles and arrows 'n stuff (but no 8x6 glossies), but I didn't quite get the full drift; but he got it within about one thou *smile*

 Is it auto or manual advance?... if auto maybe easier to cope with a static packer, if manual maybe the housing is worn...maybe I'll allow those more knowledgeable to take over before I cause trouble

Title: Re: bad points gaps
Post by: JulianS on 27.06. 2017 12:32
The eccentric screw is used for adjusting the internal timing of the magneto , it will not balance the points gaps.

First photo shows were it fits.

Second shows the part. The plug expands the thread to fix it in place.
Title: Re: bad points gaps
Post by: coater87 on 27.06. 2017 12:37
 OK, Andrew is out *sad2* *sad2* *sad2*

 That starts to limit things quite a bit.

 Im not in a hurry here.

 My thought Julian are that I will never get a good 180 degree firing with the gap this far off. One lobe is going to fire sooner, and the other lobe later because it has to go further around the rotation before the lobe breaks the points. Or am I thinking wrong on that?

 Its an auto advance BTW.

  .010 or .011 total run out just feels like a kick to the bean bag at this point.

 I looked at another deader-than-Lincoln mag I have, and I see the welch plug thats on the breaker housing. Has anyone ever messed with this thing, or is it a myth that it is used for cam centering? *conf*
Title: Re: bad points gaps
Post by: coater87 on 27.06. 2017 12:42
 Sorry Julian,

 we are posting at the same time.

 What do you mean by internal timing of the mag?  *conf*


Title: Re: bad points gaps
Post by: JulianS on 27.06. 2017 13:24
My simple understanding goes like this - the points should be just opening when the primary winding voltage is at its maximum. This is the point at which the magnetic flux in the mag reverses. The grub screw is used to move the cam ring so that the points are just opening when the reversal takes place. This gives maximum spark.The builder of the mag will set it and moving it may result in a weaker spark.

If you turn your mag by hand you will feel the magnetic pull increase then  pull will stop and the points should be just opening.

The mag relies on many things being just right to get a balanced points gap and firing interval- the armature must be true - the inner races must be square on the armature and perpendicular to it. The outer races and cam ring must be concentric and fitted square and the end cover must be square to the body - the bearings must be correctly shimmed plus more items I am sure.

I would certainly check the firing interval of the mag before deciding what to do next, a balanced points gap does not necessarily mean 180 degree firing interval and 180 degree firing interval does not necessarliy mean equal points gap.
Title: Re: bad points gaps
Post by: groily on 27.06. 2017 17:55
I think most would agree the interval is more important than the gaps, but a difference like that is too much. It would be good to know what a strobe shows in terms of the firing interval, regardless of the points gap?

If all the things mentioned already by Julian et al are OK (non-concentric bearing race on the cb end is probably the most common area of error due to damaged insulator or butchered & peened-over housing etc etc), if the camring still has some definition on the up ramps and looks to be good, and if there is no serious slop in the ring's fit in the housing, then a perfectly good way of shimming is to use self-adhesive stainless steel shim tape, which comes in all sorts of  'thinnesses' and works a treat (sometimes needs the housing warming to get the camring to slip in). A grub screw in the right spot can also help with a fixed camring, but is a bit more drastic.

Slop in the pivot upon which the opening point sits can also have terrible effects on points gaps and firing interval (and make the latter very variable at different rpm) - a new pivot post can make all the difference. What looks good at hand-turn speeds or tickover may well be truly awful at high revs.
Dremelling the camring ramps can get the interval right - but best not to unless/until all other possibilities have been eliminated and you can see what happens across the rev range, otherwise the camring can be fubarred.

If there's run out on the armature shaft, it won't be the cause as that will affect both sides the same. The eccentric pin will not affect this problem - except where it is off-position and the camring is cock-eyed because it isn't sitting fully home.

It's a funny thing, but BTH mags don't suffer from the same problem very often - it's the hardest thing to get right when it's wrong on a Lucas twin or 4 cylinder mag and can take longer to resolve than a complete strip down and reassemble.
Title: Re: bad points gaps
Post by: coater87 on 27.06. 2017 21:57

 I slacked off the fasteners for the points housing, literally tapped it with the plastic screwdriver handle and retightend. By doing that and resetting points I am at a strong 13 and a strong 11.

 Thinking about this, I should be able to put a timing disk on the crank, and one on the magneto and see what degree both cylinders fire on.

 We know they fire at points break, so if I mark both lobes on the mag, I should be able to see where i am at left to right.

 Does that sound right?
Title: Re: bad points gaps
Post by: duTch on 27.06. 2017 22:22

 Sounds like a good start
Title: Re: bad points gaps
Post by: Triton Thrasher on 28.06. 2017 06:46
Don't just look at the lobes.

Find where the points release a cigarette paper or similar material.

Why 2 timing discs?
Title: Re: bad points gaps
Post by: muskrat on 28.06. 2017 08:53
Just put one on the crank and use a strobe light to see the difference. 32 to 35 degrees btdc is good.