Timing disc

[groily]

In theory camring housings ought to interchange Richard.
But in practice, mixing and matching can show up some funnies, as can rotating through 180° on those housings that can be fitted either way up. I imagine at the factory they had the various castings in parts bins and put them together at random - can't see how they could have had dedicated parts for each instrument. But I'm sure they were all tested to ensure the spark interval was near as possible 180:180°.

The most common reason for error, assuming the camring isn't pitted and rusty and generally fubarred, is that the bearing isn't seated squarely.  But the heel on the opening point may behave differently on one lobe from t'other sometimes for the tiniest reasons of geometry, there can be slop in the ring's fit, especially with manual ones, etc. And sometimes, even a poor register on the end housing where it sits on the mag's main body. Where there is a bit of slop, it is worth releasing the retaining screws and wiggling, tightening, and trying again.

As Lee says, a very little goes a long way and a tiny bit of 'adjustment' can make a big difference. With fixed rings, there's nowt wrong with using a bit of shimming - there are some good self-adhesive stainless steel shim tapes available in various thicknesses. With manual rings it's a bit more sensitive as there are parts of the circle you can't shim easily, plus there is a wear factor to think about. The use of a locating pin or grub screw which bears directly on the camring as opposed to nestling in the notch provided is also sometimes a cause of error as it can force the ring out-of-concentric.

Any error at the mag is doubled up in term of crank degrees, so (being picky) anything more than a degree and a half odd of imbalance at the mag isn't good enough, especially on higher compression engines. (Some say that the bits could show 1-2° of error from brand new.) In cases of modest error splitting the difference when setting the timing  will produce a pretty decent result but is a fiddle to do and needs a strobe because a spoke down the plug 'ole won't do it. Performance at idle will be that much smoother if things are 'right' and - just my opinion - but a lot of unsuccessful faffing around with idle settings on carbs could be avoided if the firing interval on the mag were checked first.

Personally, I think an uneven firing interval is about the biggest PITA to try to sort out on K2Fs and KVFs. It's even more annoying when what looks good at one speed  goes  pear-shaped at others. It is very often the case that wobble on the pivot post for the moving point creates different behaviours at different speeds, quite often with the spark not being a 'clean' one, but a series which flash over several degrees of rotation (and not always starting where you want!) FWIW, BTHs seem better at staying to spec in this area.

It's very easy to disappear far too far up one's own rear end on all this, but it is worth making every effort to get the mag close to right before fitting and then to check the timing on both cylinders. Some of the pinging tales we hear about on the higher-spec motors are partly (or more than partly) caused by this sort of imbalance.

[RichardL]

It's very easy to disappear far too far up one's own rear end on all this, but it is worth making every effort to get the mag close to right before fitting and then to check the timing on both cylinders.

Groily,

Thanks, very kindly, for sharing all this good information. Very educational. Additionally, the bit above is quite therapuetic, letting me know I'm not alone in the struggle and it is not for naught.

Richard L.

[RichardL]

... All this in daylight, ....Now I see why the commercially available disks have light graphics on a dark background.....

   
I have a fair notion that there's a readily available product known as 'white paint' which may help...
 

Dutch,

As I see it, white paint is good for marking the exact angle to which one is trying to set the timing while rotating a distributor. Here, the point is to analyze the angle achieved after locking down the ATD. When I tried to plasticize the paper used for the disk, by spraying with clear acrylic, it soaked through and spoiled the contrast. 

Richard L.

[duTch]

 oH....sorry, .I think I actually meant *black* paint, and guess I overlooked it's made from paper thinking more like mine that I made from scrap 2mm ally and attached permanently behind the spring

[muskrat]

G'day Richard.
I've done the same as duTch (who was first?). Alloy plate marked in 5 deg increments behind the spring. Not hard to do the same on a s/a primary.
Cheers

[duTch]

 

.. (who was first?)..

  Musko, I may have loosely pondered it,  but I'll give you full credit for first..

[RichardL]

Gents,

First, the good news (for me). Swapping in the cam ring housing from my basket-case almost completely cured timing asymmetry. Now it's within a couple of degrees, I'll take a risk and say "at worst." Now, the dilemma. Based on my gravity-pulled fag paper method, timing is about 32 and 34 degrees (5/16 to 11/32). Idle is probably smoother than ever. Popped a bit given throttle while warming up, but that stopped. If I wasn't trying the timing light method for the first time, I would have called the timing perfect and put her together for the season's riding. But NOOO, I have to try the timing light, which is saying about 38 degrees fully advanced and about 10 degrees at idle. Fag paper with no advance was 5 degrees, the same as the difference between light and fag paper fully advanced.  If you have read this far, maybe you would like to offer an opinion of what's going on, or, for that matter, just make up my mind for me.

Now, I'll add something amusing, laughable or, for some of you, old news. I got really tired of struggling with the points bumper sliding down the lobe when setting the timing. Trying to create just the right amount of friction with the taper is iffy and, I think, risks breaking off the points locating tab if the taper gets too tight. I had tried wooden shims between points post and cam ring, but they didn't stay in place. After years of screwing with this at every timing, I finally had the idea shown below. Twelve of you will now tell me I am the last to think of it. Anyway, I cut a slice off of a "Pink Pearl" pencil eraser and jammed it between the two posts and the cam ring. Then, I wedge a really big screwdriver between the points posts and very slowly turn the screwdriver with pliers on the square shank.  Works at treat, I think. Anything amusing or laughable here, or just old news? (Just noticed the picture also shows the dangling clip lead attached to the fag paper.)

Richard L.

Edit: Has slowly occurred to me that I can measure the actual BTDC in inches as a way to validate which method to believe. Dumb. That's for tomorrow.

[duTch]

 To save space, I'll do the usual cull for what I need, and for detail scroll^^up^^ and read....

 

Now, I'll add something amusing, laughable or, for some of you, old news.........I finally had the idea shown below. Twelve of you will now tell me I am the last to think of it............

 On the contrary, I think you may have thought of it first...to be honest....  ..I'm lost at the rubber bit (just keep it out of the oil tank-doesn't erase so well after)...how about the ATD/AAU (whatever the 'auto advance unit' is called), on the other side....? I just wedge that in advance with the nut/pinion/taper close to locking and then get the points just opening, and bang the nut/pinion on...fiddly, but didn't think it was overboard- maybe I'm doing something wrong 

[RichardL]

Dutch,

I take you to mean you use friction from the taper (you say, "close to locking") to stop the points from sliding down the lobe slope while finding and holding the "just open" timing point. I got tired of trying to find the point where the taper doesn't tighten up and make it difficut to turn the points. The eraser rubber gives a continuous smooth friction that doesn't lock up. Once the timing point is found I tap the taper home and sneak up up on tightening the nut.

Edit: Should I mention that the rubber is removed after timing?

Richard L.

[muskrat]

G'day Richard.
I'll give you points for the eraser trick. 1/2 a wooden cloths peg works for me, also in the auto advance. I also like your weighted fag paper. I never thought of that, just used fingers.
I always find a discrepancy between the paper and strobe methods. I believe the strobe 1st. The thickness of the paper holds the heel further in so it travels a tad further till it lifts hence giving a slightly retarded result.
Cheers

[duTch]

 Nah Richard, just me being confused between the taper for the ATD and the breaker plate with a locating lug.... and to be honest it's been a while since I had that stuff around so have conveniently forgotten it- since when I did the 'Orabanda-special' slotted stud holes.
 Now just need to get it close and lock it off,  then do a little rotation to set it where I want it ...

 

.Edit: Should I mention that the rubber is removed after timing?

Richard L...

 No,   but I'm still baffled how a wedge ends up in the oil tank

[RichardL]

 No,   but I'm still baffled how a wedge ends up in the oil tank

Dutch,

You either have a really good memory or just saw a photo from several years ago.  To fill in for  anyone not here for avout 10 years, I had posted a photo of my  advance locking peg floating in my oil tank. It rolled off my seat and into the tank, only to be discovered later when I went looking for it.  Thanks for the memory, Dutch. It's a good one.

Richard L.

[BSA_54A10]

People stress way too much about the timing.
Google one of the spreadsheets that plots piston drop vs crank angle, there are a squillion of them out there.
When you get the spreadsheet, plot piston drop vs crank angle on some graph paper.
When you do this you will see just how sensitive the crank angle is to piston position.
Now remember unless you are doing this with the head off, you can not guarantee that the measuring stick is actually vertical .
Unless you are running 11:1's and trying for 9,000 rpm, the timing is nowhere near as critical as most would have you believe.

When you get your timing disc fitted, take notes and work out where your particular bike runs best.
Somewhere between 30 & 40 degrees is usually fine and if using piston drop with a measuring stick and a fag paper, about as close as one could expect.
There are very overenginered under stressed engines and don't really complain very much till you are getting into Super Rocket teritory .
If you have a little think about it, a manual advance engine will only actually be timed exactly right over a very small rev range   

[Greybeard]

No,   but I'm still baffled how a wedge ends up in the oil tank

Dutch,
You either have a really good memory...

Nah, I remember the 'Wedge in the oil tank' incident as well. Who could forget?

[duTch]

 

.....You either have a really good memory or just saw a photo from several years ago............

 Dunno about that either, some things are unforgettable fodder for friendly heckle, as epitomized here;

 https://www.youtube.com/watch?v=0yXinPYK-Jk