Automatic K2F cam ring - how to install?

[roadrocket]

Hello folks, asking for a friend:

The cam ring in a K2F with automatic A/R is loose and makes the ignition timing wander off. At the moment the ring is held by adding pressure from the cover. But how is the ring supposed to be installed in the housing? Should it be by a press fit directly in the housing or some kind of packing between housing and cam ring? As it is loose now, would it be proper to arrive at some kind of packing?

[Triton Thrasher]

It should be a close fit.

People do as you say “pack them,” sometimes to one side to even up the timing between the cylinders.

[Swarfcut]

Auto magneto cam is a friction fit into the housing. Over time with vibration, etc the alloy wears and the ring becomes sloppy. Loctite will hold it in place, or if badly worn, a piece of shim stock to fill the gap. The camring will not end up truly central, the error depends on the amount and position of the wear to the housing, so be prepared to compromise on the points gap to give the best running performance. The ultimate cure is a new, expensive, unworn housing and a new camring, and at current prices  a cheap electronic conversion starts to seem quite attractive, and will give good accurate sparks on each cylinder.

Swarfy.

[roadrocket]

Thanks for that gentlemen!

[RichardL]

I believe you can rotate the eccentric adjuster to where there is no rotational slop in the cam ring. The adjuster is behind a blank cap and, maybe, out of view on the barrel side of the mag. Then, if you carve a piece of hard felt to fill the lubrication channel on the outside of the ring, you might snug-up the ring a bit by pressure from the felt. In any case, the intention is to have the felt (or some other wicking material) in the slot for ring lubrication. (Personally, I think this lubrication method is mysterious, possibly dubious, engineering).   

Richard L.

[groily]

(Personally, I think this lubrication method is mysterious, possibly dubious, engineering).   

Richard L.

I agree Richard. Not very effective sometimes, is for sure. However, on manual rings, having the greasy bit of felt does help with the wear between outside of ring and inside of housing. But even so, as TT said in the context of advance/retard controls the other day, it's all a bit ramshackle. Have to be careful though, if moving the ring in the housing too much, as you don't want to go too far away from the best position of points opening : magnetic flip point of the armature.

Where there is wear on a fixed ring, yes, it can be compensated for with shim stock, and the felt can also help with location. But rather than worry overmuch about the points gap - unless it's a good few thou off - I'd worry more about the firing interval between the sparks.

An error there is a recipe for lumpy tickover and possibly a lot worse at high rpm.  Remembering that any error at the mag is doubled up for the crank because the mag runs at half-speed, a difference of just 2 or 3° on the mag means one side is running 5+° off the optimal ignition setting.

If someone said to most of us that "You know what, it's quite fine to set the timing anywhere between say 30 and 40° BTDC" we'd probably laugh and swear blind 'no way mate'. But the truth is a lot of parallel twins are being operated day-in day-out with errors that mean the 35° setting (or whatever) is actually a good few ° off on one side.

I know it's a complete pain - but if unsure it really is useful to strobe both sides separately to see what's happening across the rev range.

And if faced with the problem but not inclined to make the sort of horrible investment Swarfy mentions, the least one should do is to split the difference between sides (when you know what it is).

[RichardL]

Groily,

I suppose, then, that it's a lucky accident that my rickover is excellent (or seems so to me) despite having set the eccentric adjuster for what seemed like the most physically stable position of the cam ring.

Richard L.

[groily]

There's a bit of leeway Richard, so no problem! You can't move things very far with the eccentric, so as long as the ring isn't over-advanced so the points open before the magnetic flip, all will be OK. Other things being equal, a few degrees more retard than is perfect won't do a lot of harm, and you'd only be able to tell probably by doing dynamic tests at very low speed. Lucas said a K2F should produce 90% of all sparks across 5.5mm three point test gaps at 130rpm of the mag  . . . if it needs a few more revs, as long as it's within the range of a strong right leg on the kickstart, all will be well! And a really good mag will do the spark thing from under 130 anyway. Best I ever saw was about 100 rpm. I'm sure yours is in the 'really good' category!

[Swarfcut]

Groily, I never considered new magneto parts a horrible investment, more of a distress purchase when all else fails. However I can understand you interest in keeping  the magnetos of this world alive and sparking.

Could you elaborate on splitting the difference to get the best from a miss matched firing cam ring? I thought adjusting the points to give correct timing on the more advance cam lobe was the only way, as endorsed by Brightspark Magnetos.

Swarfy.

[RichardL]

"...as endorsed by Brightspark Magnetos."

Swarfy,

Is this subdued tongue-in-cheek Brit humor? I'll leave it to Groily to explain my question, if he chooses.

Richard L.

[groily]

Ha! That old humour thing  . . . !
I only meant 'horrible' in the sense of £€$s Swarfy, with cam housings at £x and rings at £y and the result an empty bank account! A very excellent investment in fact, but not one a lot of folk want to make unless they are in really dire straits. And at the moment, new camrings are increasingly unobtainium, which isn't helping.

When I was talking about 'splitting the difference', I was simply referring to setting the ignition timing.
If a mag is say 2° off between cylinders on the camring, the actual firing points will  be 4 crankshaft ° different between cylinders.
So all I meant was that
a) having established the amount of error by using a strobe on the engine or another good method, and
b) if not wanting to have to spend £150 odd to replace the bits  . . .
then
c) instead of setting the timing by reference to one cylinder only now you know the other won't be that close, set it so that each side carries half the error.
Nothing to do with setting the camring or points or any of that. That side of it ought to be done as per Lucas etc. Sorry to have confused  . . .

While a degree or so won't make much odds, especially with low compression motors like my own A, with HC pistons, alloy heads and other goodies, an unseen variation in timing is a common reason for pinking and rough low speed running. If one side is at a true 'per book' BTDC setting and t'other ends up 5 or 6° more advanced  . . . or retarded  . . . won't be a recipe for smooth. So although like everyone else I've spent half a lifetime with Rizlas between the points and shoving pencils down one plug 'ole only, it is so worth getting a degree disc on the crank and checking both sides with a strobe.

Because mags are old tech and a bit 'ramshackle' to use TT's phrase again, they don't stay 'perfect' like an electronic system should. Indeed, from new they probably weren't 'perfect'. They can be got close, but it can be a labour of love, and it can sometimes require the 'horrible' investment. The new CNC machined housings available are pretty good, and camrings I've had have been accurately machined - but even then we're working with a contact breaker with a few 'warts 'n all' features, and a tendency to wear.

I would say the hardest thing to get half-decent on a K2F is the firing interval, which is why I bang on about it.
BTH KC2s are more consistent in precision of construction in these areas, and rarely cause anything like the same hassle. But their pick-ups stick out at 90° to the body, which makes them a tricky fit on many machines  . . . which is a shame frankly. 5T and 6T Triumph owners seem to get on with them very well, those who are lucky enough have that option.