A7 Magneto problem?

[RichardL]

Sooo, if I understand this, you let the probe float and observe TDC, then, you run it down to the BTDC timing depth and lock it, feeling for the touch when you rotate the crankshaft. Andy, did you error when saying you marked it "above" TDC, because I don't know how you get there. I am wondering if the crank can be turned gently enough to just kiss the probe and hold it there without nicking the piston or bending the probe. Have you had any trouble with that? I also wonder if the angle of the probe causes a false reading with respect to distance from TDC, hypotenusally thinking. (See the next post.)




Richard

[RichardL]

Here's a rough representation (all in metric) of the problem I see with the probe angle. As you see, it represents a 70 mm piston with the probe touching the top at a 45 deg. angle (just an approximation), so, where you pushed in 3/8" (9.525 mm) you're really measuring 6.7352 mm.

Is this something to think about?

Richard

[BSA500]

Ok try to explain this I think.You turn the engine by hand then rear wheel for small adjustments until you have found tdc.This is then marked with a fine pen.You then slide the probe out and mark a line 3/8 inch above the tdc mark.You put the probe back in and turn the engine BACKWARDS beyond the new timing mark this is to take out any backlash in the timing gears.The engine is then wound forward until the 3/8 inch mark is lined up this should be the the static timing mark to set the mag up.I have worried about the angle problem as well but using this method I have usually managed to time up the engine ok.But if I happen to have had to remove the head I set it up the timing the measuring the piston movement.I usually have trouble judging the points just opening bit and as in this case the points are a little to advanced.
Andy

[Nitti]

Hello,

I also have one of these "ignition setting micrometers", and its very useful indeed.

When you have found TDC, its just to back the piston, and screw down the micrometer scale to the right value, it measures in 1/10 mm, a bit over the top of accuracy for most of our engines I guess, but it feels good to be on the spot.
The cost for the device is about 55 USD.

Nitti

[bsa-bill]

What is needed is a fine adjustment on the Magneto, a very thin plate between the magneto and the timing case with a micro adjustment between the plate ( fixed to the timing case ) and the magneto (adjuster fixed to the magneto ), the three holes in the magneto would need to elongated.
I know for a fact that this has been done ( although I cannot remember who it was that has done it but he is on a internet list)

Some enterprizing trader might take this up???

All the best - Bill

[RichardL]

Andy,

With your explanation, the "mark above" from your previous post makes perfect sense. Thnaks for clearing that up for me. Does the probe in your device get locked in place or does it move with the movement of the piston? If it locks, I am still a little curious about the "feel" of the piston coming in contact, but, I think that is something one must learn for one's self. I will be able to time my rebuild with the head off, so the whole question might not matter to me, for now.

Richard

[BSA500]

I found a picture of one on ebay and it does not lock but move with the piston.....

[fido]

I think if you use the device shown you should calculate for the plug hole angle. I use a pencil poked down the plug hole and try to keep it as near vertical as possible to reduce the angular error.

[RichardL]

I may have already tortured this, but I'll stick my neck out anyway. Suppose you have concave or domed pistons, then, the point at which you make contact with the top of the piston at TDC may be at a very different angle than where you make contact BTDC. This could dramatically affect the BTDC measurement. This said, I must acknowlege that it's working for Andy. I wonder if his pistons are flat-tops?

Richard

[BSA500]

They are 8.5:1 comp pistons with a flat top,luckily the rod rests on these tops and not the slope around the edges.It shouldn't work if you think about it but it does so I don't think about it!!

[RichardL]

I am now desparate to not think about it, but I know that as soon as my head is back form the shop I'm going to measure the plug angle and spew out a formula for compenstation, revealing an engineering obsession I could or should sometimes do without.

Richard

[groily]

Well, basic SOHCAHTOA trigonometry will enable anyone to calculate the extra length needed on a slanty rod to get the miraculous 3/8ths vertical, provided we know the angle at which the plug enters the head. Compensating for this issue is one reason I like the home-made fixed-length tool, which can be best made when engine is on bench if one can be bothered to put gasket and head on, torque it down, make tool with tip that just touches piston - domed or otherwise - when piston's in the right spot as set with the head off and measured accurately. Having said which I haven't made a precision one for an A10 yet, preferring a rod threaded down a dead plug.
Re the talk of getting adjustment on the cb end of the mag to enable fine tuning after locking the pinion, there are a few words in a few places, maybe even here, I can't remember. Anyway, it's perfectly possible to achieve minor adjustment of the cam ring in its housing, by removing the small locking screw that locates it (careful, it has a wee plug over the top of the screw head in the hole in the housing and it's hard to get out), and replacing with a larger thingy with an eccentric head which will push the notch in the cam ring a bit this way or that. A friend has done just this by drilling and tapping the hole to a larger size, and has 5 (crankshaft) degrees either way, which should be enough if one hasn't made a total ******** of setting the thing. The mag can stand that sort of variation without losing spark intensity as a result having the points open too far off optimal position of the armature. He runs a V twin, which with a mag is a sensitive beast and the palaver may have been worth it for him. For a BSA twin, with pretty long stroke and not exactly Formula 1 state of tune, I reckon it's great fun but not really worth all the hassle! Groily

[RichardL]

I can't believe I was able to Google "SohCahToa" and find what it means. It's been a long time since my trigonometry class, but I think I would have remembered that word if the teacher had used it as a memory cue. For example, I still honor the memory of ROY G. BIV, or GUMP (pilot for: gas, undercarriage, mixture, prop).

Richard

[groily]

I'm amazed you could find it too, Richard. Sounds more like an oriental volcano. It's just one of those dumb things I've remembered since for ever. Never even thought about it until I got an old lathe and had to start thinking about turning tapers. Then suddenly, all that school stuff started to seem relevant and I wished I'd listened. So help me if I ever want to work out the area or volume under a curve, cos that I know I can't do! And ROY B. GIV? Bill

[RichardL]

Colors in the visble spectrum and in a simple rainbow from longest wavelength to shortest: red, orange, yellow, green, blue, indigo, violet.