Ignition timing

[a101960]

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.

[A10Boy]

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.

[a101960]

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?

[dpaddock]

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.

[RichardL]

A101960,

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.

Richard

[a101960]

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.


 

[tombeau]

Hi,
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 Britbike.com. (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).

Cheers
Iain

[a101960]

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.

[A10Boy]

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.

[a101960]

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!

[snowbeard]

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!

[jfligg]

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

[snowbeard]

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.

[RichardL]

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.

Regards,

Richard

[snowbeard]

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