Author Topic: Magneto cam ring  (Read 1392 times)

Online Butch (cb)

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Magneto cam ring
« on: 06.10. 2014 11:32 »
So whilst the bike was up on the table having the valve clearances checked I had a peer in the magneto. It's an auto advance type. I must confess this is the first time I've pulled the cap off in 2750 miles - since Roger at Cake St put it all together for me. To my eye and despite devouring all the info forthcoming here it always looks a bit magical in there to me.

Found the points retaining clip adrift, and a tiny plastic button (insulation I presume) which it should bear against came off in my hand. I just stuck it back in as it appeared it should go. Long since lost my magneto spanners and so had to file a little off a 6mm spanner I had - one to add to the riding toolkit now I guess.

Checking the points gap I found it on 012 thou one side and maybe 009 on the other. But the bike has been running great so what to do? Well I've opened it up slightly to maybe 009-010 and 013-014 which I presume will have advanced things slightly.

I remember Roger saying that the ring was far enough out when he checked it that he'd gone at it with a Dremel (I think). Is a 003 thou difference within the realms of acceptability on a Flash?   
Warning - observations made by this member have a 93% unreliability rating.

Of Bikes; various, including ...
'58 Iron Head Flash Bitza


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Re: Magneto cam ring
« Reply #1 on: 06.10. 2014 12:50 »
 A three thou imbalance on a Golden Flash is not unusual. It is possible to reduce the imbalance but, if you say the bike runs great, I'd leave it alone. The problem is not always the cam ring itself but sloppy machining of the cam ring housing and the body. I have come across quite a few customers with the same problem, then they buy a new cam ring, only to find it hasn't solved the problem. If you want to be fussy you could check the timing on each cylinder. Even with a points imbalance it is possible for the timing to be correct as the points gap does not determine when the magneto fires, what is important is the impact point, i.e. when the heel of the points hits the cam ring which, ideally, should be 180 degrees apart on a twin cylinder.  A tell-tale sign of a really bad imbalance is blue exhaust on one side suggesting the blue side is running retarded (or weak which is another story).

Online RichardL

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Re: Magneto cam ring
« Reply #2 on: 06.10. 2014 13:22 »
Testifying on what Beezermac said, when I rotatated the cam-ring housing on my K2F by 180 deg., the points barely opened, if at all, on one side. With the points out, I was able to easily see the eccentricity between the housing and the armature shaft. In this condition, the bike would not start. Put back the other way and she started-up easily. Not that Beezermac (a magneto specialist) needs my agreement for validation, but I would go along with the idea of being happy with "running great". Creating more advance risks preignition, which ain't good for nothin'.

Richard L.
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Online Triton Thrasher

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Re: Magneto cam ring
« Reply #3 on: 06.10. 2014 17:44 »
The gap and timing would have been off, originally and Roger will have remedied the timing only.

I sort of suspect that the larger gap should be 0.012" and the other gap should just be whatever it ends up as.  Can you ask Roger?

Or, of course, check the timing.

Online groily

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Re: Magneto cam ring
« Reply #4 on: 06.10. 2014 22:28 »
Agree with Beezermacc whole-heartedly - in our imperfect world the 180°:180° (or close as you can) is a better objective than perfectly equal gaps, even if you could get them. New camring or otherwise. But one thing that is certainly beneficial is 'snappy points opening' to avoid arcing and undue wear on the cb. If the gap is too small on one side, then there is a risk of some of that. Slow speed tests on the bench show that an undersize gap on one side (especially if coupled with wear on the heel of the moving point causing slothful opening) will cause that cylinder to under-perform at minimum rpm. The spark may in fact become a series of sparks, stretching over several degrees. The effect is very noticeable on test and is similar to having a worn pivot for the moving point, which introduces a different hassle but a similar result (plus wobbly firing intervals). While a decent K2F ought to produce pretty well 100% of its sparks across a Lucas-spec 5.5mm 3 point test gap at 130 mag rpm advanced (cold), the effect of a smaller gap or 'lazy opening' can be to impair that considerably. Usually good sparks will still occur well within the range of the kickstart cranking speed on a twin - but lazy opening or other vices on lower-geared kickstart bikes (like Velos) can be a pain that does have a noticeable effect.  Cheers, Bill