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Twin Magneto


b.l.hayward@tesco.net

Website User
Non-VOC Member
Hi All, During my basket B rebuild (which, as you all know, takes years!) I had the mag reconditioned and I think the cam ring (on which the contact breaker lobe operates) has been put in the wrong way round. Being a V twin the length of the 2 arcs between the lobes on the cam ring is different, obviously, and it must go into the mag body the right way. Would love a pic of the cam/breaker-end of a mag which works to either confirm this is the problem or that I need to delve deeper. Thanks in anticipation. Brian
 

BlackLightning998

Well Known and Active Website User
VOC Member
Won't the parts diagrams or other reference books do it for you

Hi Brian,

Won't the exploded diagrams on the spare parts list or other publications give you want you want?

I have the orginal Magneto off my Series C in the garage that came back with the bike (I'm on Pazon electronic now) - if I can be of any help I'll happily go photograph bits for you - just point me in the direction of what you need?

Cheers and good luck.

Stuart
 

john998

Well Known and Active Website User
VOC Member
Twin Mag

Hello, I do not have a picture to hand but from forty years on the peg that locates the cam ring is at the top. The largest low point on the ring is on the right. Hope this helps.
It is from memory possible to fit the cam ring/ bearing carrier 180 degrees out.
 

BlackLightning998

Well Known and Active Website User
VOC Member
I heard that one recently......

I heard recently of a WLS member who had his Mag "professionally overhauled" only to find the bike would not run.

After a very long time trying to work it out they found the carrier was 180 degrees out - frustrating or what.....


S


Hello, I do not have a picture to hand but from forty years on the peg that locates the cam ring is at the top. The largest low point on the ring is on the right. Hope this helps.
It is from memory possible to fit the cam ring/ bearing carrier 180 degrees out.
 

Phil Mahood

Well Known and Active Website User
VOC Member
It is surprising how often I see this. The cam ring itself is located by a pin in the back. It cannot be installed in the wrong position. However, the whole contact break end housing can be installed 180 degrees out. To rectify, just take out the two screws (one with points cap retainer post), spin it around and re-secure. The post should be to the front of the bike. Looking at the cam ring as the mag would be mounted on the bike, the first lobe (rear cylinder #1) should be at about 6 o'clock. Then there is a long gap to the next lobe at about 11 o'clock. The bike will not run properly, or at all, unless things are this way.

If the cam ring housing is out 180 degrees, it is possible to time it so that the rear cylinder will start. However, there is extreme fire danger. The other cylinder will fire when the inlet valve is open sending a flame out the front carb. Many a twin has been imolated this way! Beware.
 
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b.l.hayward@tesco.net

Website User
Non-VOC Member
:confused: Ok gents, thanks for the inputs and herewith an update. Decided to check the timing just in case (was part of the rebuild done by another!) and found that the rear (No.1) cylinder had the points opening MORE than full advance when on full retard (ps forgot to say that I have a manual mag - cable adv/ret). Geronimo! - the solution was simply a retime - NOT! Retimed using the bottom lobe on the cam and obviously No1 was ok. But No2 would fire at just before BDC of the induction stroke and would equal flames thru the carb on No2 - not good. So it looks as though the end plate of the mag is indeed 180 degrees out. BUT I can't turn it because of the boss for the cable adv/ret - it would foul the big oil filter chamber nut. SO why can't I time No1 using what is currently the top cam lobe and switch round the plug leads? After all, the distance between the lobes will be correct then.
 

Phil Mahood

Well Known and Active Website User
VOC Member
Hi, Brian.
Your idea, while logical will not work. This is beacuse there is another part in the mix - the slip ring. This is a ceramic disk with a copper strip. It can be seen under the high tension pickups. The slip ring is key'd to the armature such that the whole will not work unless as described. The copper strip that conducts the current will not be phased with the breaking of the points. The bottom line is, there is no getting around rotating the end housing 180 degrees.

I am not intimately familiar with the manual advance mechanism details. This was not standard on Vincents with the KVF mag. The KFTT competition mag had this, but that is physically a different item. Perhaps you'll need to fit the standard set up with an ATD. Can anyone else help?
 

timetraveller

Well Known and Active Website User
VOC Member
Dont give up. Even if you do decide to go for another system eventually. It is all part of learning how things work to sort out the problem. It will also improve your confidence in your ability to sort out things for yourself. We all had to do that fifty years ago. Nothing wrong with the BTH but why not consider a coil ignition system if you really want easy starting? It needs a decent charging system but you need that anyway for lights etc. You could buy the bits for a coil ignition system and one of our car alternator kits for about the cost of a BTH and you can get bits for the ignition system if something goes wrong just about anywhere in the world. :)
 

b.l.hayward@tesco.net

Website User
Non-VOC Member
Ok...time for an update - sorry for the delay in posting but life got in the way!
Decided to give Dave Lyndsley a call and he gave me a couple of bits of useful info:
1. The only real difference between a KVF and the more ubiquitous K2F is the cam ring which operates the contact breaker. The armature, winding, contacts and slip ring are all the same. It's just the asymetrical positioning of the lobes on the KVF cam ring that causes the spark to occur at the right time for the V (the position of the standard slip ring contacts means that the pick-up brush is sitting on the contacts even though the contact breaker opens earlier for No.2). Hence why No.2 spark is weaker - the armature turns through fewer degrees of rotation.
2. If the cam lobes/end cap have inadvertently been set up 180 deg out then it's perfectly feasible to time No.1 on the TOP lobe - as long as the timing for No.2 is then where it should be.
My KVF has been modded for manual A/R and the slot in the rear of the cam ring to move it with the cable means the cam ring can only go in one way. Plus the position of the A/R cable abutment means that the end cap cannot be turned thru' 180 deg.
So I timed No.1 on the top lobe and checked No.2 timing - spot on! Petrol in the tank, flood the 276's, retard a tad, hefty swing and the beast burst into life for the first time since 1970, at least! MoT'd last Wednesday and sent of for my historic vehicle (free) road tax yesterday - bliss!
Many thanks to all.
Brian
 

Len Matthews

Well Known and Active Website User
VOC Member
Another option would be to dispense with the manual advance end casting, fit an automatic (plain) one and fit an ATD, Good enough for the makers,good enough for me!:)
 

b.l.hayward@tesco.net

Website User
Non-VOC Member
Er....why would I want to tear down something which is now working fine? - I gave up creating work for no good reason MANY years ago. Besides , my twin 5 inch clocks on top of the forks look really nice! :eek:)
 

Len Matthews

Well Known and Active Website User
VOC Member
Glad to read that you've solved your problems. Come to think of it, fitting an ATD would be hassle, pipes off, timing cover off,etc. not to mention expense!
Bad suggestion, sorry.:eek:
 

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