The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: a101960 on 26.08. 2008 15:33

Title: Ignition timing
Post by: a101960 on 26.08. 2008 15:33
I have reason to believe that my ignition timing might need resetting. Just occasionally when pulling away the engine spits back through the carb. The spit is not violent and it occurs randomly. The engine has done 1000 miles since a complete rebuild. Is to possible to re-time the ignition without removing the cylinder head? I have one of those MCA graduated TDC indicator tools, however the A10 plug holes are set at an angle. The motor does not seem to pull as well as it should and it seems to be reluctant to go much over 60 mph. The exhaust and mechanical noise increases without any more real go. Maybe I am just being a wimp and perhaps should I just open up and see what happens? The carb mixture looks to be more or less right (n/s cylinder is a bit weaker than the o/s). It always starts first or second kick, and it idles smoothly and reliably.
Title: Re: Ignition timing
Post by: A10Boy on 26.08. 2008 15:54
If youve only done 1000 miles since a rebuild, you dont really want to push it over 60 yet. Just a thought, have you changed the oil yet, if not you should do so.
The timing can be checked in the usual way, but since the plugs are a different colour, perhaps you should check they are the same grade and change them if not. The missfire could be worn pick ups, dirty slip ring, dirty points, failing condenser, dirty earth brush, etc.

You probably know this, but dont try the old "pencil in the plughole" way to measure timing, its a sure way to break the pencil inside the cylinder.
Title: Re: Ignition timing
Post by: a101960 on 26.08. 2008 16:34
The engine oil and filters have been changed every 250 miles. There is also an oil filter in the return line that has also been changed every 250 miles. I have also used Redex (upper cylinder lubricant) since the rebuild. This was a BSA recommendation. (Paranoid? what me?). I will however check out the brushes, but the magneto was completely rebuilt as part of the engine rebuild and has a nice big fat blue spark. The different plug colours are very likely to be due to induction bias. I am currently trying to find out what the angle of the anti bias spacer is. The plugs are both the same (Champion N3C) which is the recommended plug for an ally head.  I have done all of the usual things like swapping the plugs and leads from cylinder to cylinder. So you think that at 1000 miles the engine might still be a bit tight. How many miles does it take to free up the engine then?
Title: Re: Ignition timing
Post by: dpaddock on 26.08. 2008 17:27
The occasional spit back from the carb is probably because of a weak mixture, and it happens usually more so when the engine is not yet up to full heat. It's nothing of concern, really, especially that it's not violent and/or regular, as you state.
Induction bias is discussed elsewhere herein in previous postings.
I use a plastic straw from a can of WD-40 for my timing stick.
Title: Re: Ignition timing
Post by: RichardL on 26.08. 2008 19:32

If I recall correctly, the angle of the plug hole is listed in one of my books, which is at home while I fiddle at work. If you have flat-top pistons, arriving at the correct setting is a bit of simple math I would be happy to contribute, if it helps. First, I need to find that angle. If you have dished or domed pistons it is a bit more complicated. I have domed 9:1 and have been playing with the geometry for using one of the TDC indicators.

OK, back to work.


Title: Re: Ignition timing
Post by: a101960 on 26.08. 2008 19:58
Thank you. My pistons are more or less flat top, but they are slightly raised maybe a 1/8" or less. There are very small valve seat recesses.

Title: Re: Ignition timing
Post by: tombeau on 27.08. 2008 08:08
If I get a chance tonight I'll whiop off the carb and measure the induction bias gasket for you tonight.
Re popping things dowwn plugholes I always use a paper straw, on the grounds that if it does get stuck in there it should burn up pretty quick.

I don't know if these figures are commonly available, but B.M.L.S posted them on (credit where its due)
They are degree disc figures for A10 ignition timing.
I never really trusted a depth gauge at an angle on a domed and coked up piston

11/32 in = 33.6 degrees;
3/8 in = 35.2 degrees;
13/32 in = 36.7 degrees (RGS).

Title: Re: Ignition timing
Post by: a101960 on 27.08. 2008 10:30
Thank you everyone for your responses. Some very useful ideas and suggestions. It occurs to me that the intermittent spitting back that I commented on might well be caused by the by the leaner mixture in the n/s cylinder.
Title: Re: Ignition timing
Post by: A10Boy on 28.08. 2008 15:22
How many miles does it take to free up the engine then?

It depends on your build tolerances, but given that they are fairly slow revving and the barrels are hard cast iron, I would have thought that 1000 miles would be too soon for full revs, more like 2-3K.
As you know, if the rings dont "bed in" properly, the barrels will glaze and oil consumption will be high.

Maybe its me, but I would err on the side of caution.
Title: Re: Ignition timing
Post by: a101960 on 30.08. 2008 20:46
I ordered an induction anti bias gasket from Cake Street Classics which arrived this morning, and it was fitted to the bike this afternoon. I have been out for a 50 mile ride this evening, and BINGO! both plugs are now exactly the same. So I can tell you that the bias problem does exist, and the gasket is the cure. Incidentally the engine pulls a lot better now. A result!
Title: Re: Ignition timing
Post by: snowbeard on 08.09. 2008 20:55
so it sounds like you're good, no need to retime it eh?

I was going to give my whole convoluted plan for timing without having to remove much at all, even the rocker covers, but it seems unwarranted here.   glad to hear it!
Title: Re: Ignition timing
Post by: jfligg on 09.09. 2008 01:02
Hi Guys  I have always used one of those Top Dead Center tools that  screw into the spark plug hole.  Should I stop using it?  Does the angle of the rod that you measure with make that much difference?  Thanks Jeff
Title: Re: Ignition timing
Post by: snowbeard on 09.09. 2008 03:44
please anyone correct me if I'm wrong, but those are the preferred method already?  because you don't have to deal with the inaccuracy of infintessimal vertical travel as the crankshaft reaches the top of its turn, you measure more accurately in the travel of the stroke, then once you've stopped it on either side you split the difference for a calculated, but more accurate TDC.  it also doesn't matter the angle, as long as it hits the same spot each direction.

my silliness to add was that my stop was a bored out plug with a bolt, so not airtight.  in order not to have to take the rocker covers off to watch the valves, I put a slit glove fingertip over the plug to tell me when that piston was on its compression stroke.  it would balloon up and squeal on the comp stroke, so I knew where I was.  I also had a stick in the plug hole to watch, but measurements were taken on the wheel.
Title: Re: Ignition timing
Post by: RichardL on 10.09. 2008 02:22
Snowbeard, the distance of the piston from TDC is intended to be the actual firing point with timing fully advanced. If I recall correctly, there is always some lead of the spark before TDC in order for the exploding fuel air mixture to have time to reach its maximum energy of combustion. I think the glove fingertip idea is funny and probably works, but I just put my thumb over the hole and wait for it to blow.

Jeff,  I am still planning to figure out the right values to compensate for the angle of the spark plug with the TDC tool. Not serious math, but, in my case it's a little trickier in that I have domed 9:1 pistons.


Title: Re: Ignition timing
Post by: snowbeard on 10.09. 2008 04:01
in the research I did I found that 32 degrees was the BTDC timing.  I used the center stop to set the wheel at TDC and then back it up 32 degrees. 
I'm pretty sure I found it on this site?

it works well enough for me, I can't tell if I got any shift from tightening the bolt, I checked it a couple of times after the final set,  but with my advance full forward I don't think it has the power it has at half back.  I haven't noticed pinging, so I think it's doing well!  I do most of my riding around town so I'm not full out usually, and at the lower revs (I assume since I don't have a tach) it seems to pull harder mid advance
Title: Re: Ignition timing
Post by: RichardL on 10.09. 2008 08:31
Mine is auto advance and the timing is set with the mechanism jammed at full advance. I suppose that with manual advance exact tnot as critical with the automatic advance. Believine this, I must have tried a dozen times before I got itto what I think is right. Yes, it does change ith tightening, but I must be careful what I, as I can't recall if all engines used the tapered pinion shaft or if earlier models use a woodruff key. For the tapered shaft, it is essential, I believe, that the pinion get set on the taper by using the least torque necessary before tightening the small screw through the center of the points block. I tried tapping the pinion onto the shaft with a mallet, but that is risky and, I think, tended to move the point a little on its own.

Must get going. Bye.

Title: Re: Ignition timing
Post by: snowbeard on 10.09. 2008 17:07
both mags I've been into (a 54 that's on it and the 57 that came off) had the woodruff key on the points plate, so I didn't really bother loosening that.  both of mine had the tapered pinion for the drive gear, and I think I actually used the little bolt on the points to hold everything steady on tightening, but don't do that, it's too small and delicate.

it took me at least six tries too, eventually I learned how much it would slip and tried to counter it.  hence having to recheck it after a few times.

Title: Re: Ignition timing
Post by: groily on 10.09. 2008 22:47
Methinks it might be worth - if the timing pinion slips while the nut is being done up - lightly dressing the taper on the armature shaft and the cone in the pinion on the bench to make sure they are both burr-free and clean. Also, a very clean thread (worth alway running a die down it) and a very clean nut, preferably a new one. A little cleaning, fine emery paper will do, can go quite a long way, even though the thing doesn't look much different for it. Ideally, you should be able to press the pinion on the taper gently by hand to the point where it engages enough to take nut and spanner (wrench) without going for a walk. In which happy case it's perfectly OK to hold, gently, the cb centre bolt with a 4BA (or 1/4AF)  box or socket spanner, while the pinion is lightly pushed on at the other side and a very clean and free-running nut run up the thread to hold it. It's hard enough getting the pistons and points in the right places by whatever means, without having to guess what error will introduce itself while you do the thing up. Although some people do reckon a light tap works, I prefer not. Even a gentle whack can make the thing bounce, losing the setting at the cb end in the process. Plus put a new tiny burr on both male and female taper. But I agree, it's a miserable job, trying to do one thing up without disturbing another, which you can't see properly while you're doing it.
Sometimes I think it would be good if mag pinions were keyed to their shafts and a dot provided on the pinion to align with the adjacent one, as per cams. Wonder if anyone has done that? The mag would then almost certainly be model- and marque-specific. But, boy, would it make it quick and easy to put together. The downside would be lack of ability to compensate for wear on the cam ring etc. Oh, and the problems that could follow on from an armature rewind! Maybe they outweigh any possible gain, so shan't be attacking anything with sharp weapons any time soon . . . . just day-dreaming really.
Title: Re: Ignition timing
Post by: RichardL on 12.09. 2008 05:21
Yep. As I've said before, it couldn't be any more awkward trying to set the smallest possible opening of the points while the cam rider rides up on the steepest portion of the cam.

One thing I did to make myself less crazy, while making multiple attempts at timing, was to spray the backside of the horseshoe washer with some spray adhesive so it would stop falling off every time I loosened the pinion.


Title: Re: Ignition timing
Post by: snowbeard on 12.09. 2008 05:42
horseshoe washer? *dunno2*     

I think my tach drive nut is right on the timing gear itself, I didn't have a washer when I replaced mine...  but it is a manual advance, so just a steel and fiber gear on the pinion...  maybe a washer would help eliminate some slipping tho?!
Title: Re: Ignition timing
Post by: groily on 12.09. 2008 10:55
no horseshoe washer with a standard manual set-up- it's on the ATD under the head of the centre bolt and goes against the washer with 2 locating holes in it which is under that . . . As Richard says they flop about a bit! But not sure quite how tacho-drive goes as haven't got one.
Title: Re: Ignition timing
Post by: groily on 14.09. 2008 16:36
Had to redo my ign timing this a.m., when I found that the new points fitted to my SRM recon magneto had been fitted at the absolute limit of adjustment, and there was no way of getting a bigger gap than 8 thou without trying to modify the mounting for the fixed point. (it swivels on the centre bolt fixing and is held fast by a small screw in a slotted section of the plate below the point). Lousy item, which I didn't really want anyway, but I was told that the backplate and points I supplied couldn't deliver the same timing on each cylinder. Strange, I thought, if the cam ring was serviceable (which it was) and the bearings were new (which they were by then), but OK, you know more than me, replace as necessary etc. Anyway - replaced the thing this morning with an old brass Lucas one off another spare mag, put a new set of points on it, and things have been transformed. Better in every way, including they're adjustable!.
But the thing is - the mag pinion went on without force or any slip / risk to the position of the cb unit at the other end, which didn't even need holding during proceedings.
Good to see while I was in there that the mods I had to make to the toothed belt dynamo drive conversion (steel taper and centre for cheesy alloy pulley whose taper self-destructed in a matter of a few hundred miles) were holding up well and that the belt had not stretched in several thousand miles. All's well in the end - but that's 2 SRM things I haven't been that chuffed with, and 3 if you count the supply of allen screws instead of longer studs for attaching the alloy sump plate with drain plug, which might just be me being faddish about screws into aluminium. Silly little things which I will mention next time I'm on the phone to them, but they've absorbed some time fixing/improving stuff that shouldn't really have needed it.