Sooty Plug (not plugs)

[Mosin]

I've been using my bike quite a bit over the last few days and clocked up quite a few miles. As I was nearing home yesterday I noticed that it was popping slightly occasionally when I closed the throttle. I have whipped the plugs out and discovered that the r/h one is much more sooty than the l/h one (which is completely fine). Both are NGK B7ES and are gapped identically. I cleaned them and swapped them over and took the bike out for another run to try and establish if it was the plug which was at fault. Once again the r/h one sooted up.

The engine has recently been re-built and I know that the ignition timing is spot on.

This all seems quite unlikely so any hints or tips on where to start my investigations would be most welcome.

Cheers,

Simon

[groily]

If it's soot from running richer that side - shouldn't be oily muck if the motor's all rebuilt? - could it be another case of induction bias (see comments on here about the bias washer -  carb spacers designed to angle the carb more towards the weaker side)?
Popping on the overrun is probably an air leak somewhere in the exhaust - at the head, silencer joint or anywhere else there's a joint. I had some of that when the joint connecting my siamesed pipes wasn't that good - bit of Firegum and a new clamp cured it. My lh pipe also has a very slight tendency to work out of the head a little, and needs a gentle tap every now and then. Popping tells me when to get the rubber mallet out. Nothing seems to have cured that just yet. Some folk say some silicon sealants can actually do a reasonable job.

[MG]

Simon,

could also be a carbon track around the mag slip ring. Take one of the pick ups out, earth the cut out contact and clean the slip ring with a piece of cloth slightly soaked with brake cleaner through the opening for the pickup by turning the engine over with the kick start (easier with the plugs removed).
Some of the brushes available wear rather rapidly and through the conductive track around the slip ring the spark will jump over to the other cylinder which is in the exhaust stroke, this would also explain the banging from the exhaust.

Cheers, Markus

[Mosin]

Simon,

could also be a carbon track around the mag slip ring. Take one of the pick ups out, earth the cut out contact and clean the slip ring with a piece of cloth slightly soaked with brake cleaner through the opening for the pickup by turning the engine over with the kick start (easier with the plugs removed).
Some of the brushes available wear rather rapidly and through the conductive track around the slip ring the spark will jump over to the other cylinder which is in the exhaust stroke, this would also explain the banging from the exhaust.

Cheers, Markus

Sorry, I should have mentioned, I've dispensed with the Mag and am running a Pazon surefire ignition so this eliminates any potential mag problems. One thing which did occur to me was tht because the Pazon uses a "wasted spark" system, I could just swap the HT leads over and run the bike for a bit like that. Assuming that the problem then stays on the r/h cylinder this will eliminate the ignition system from the equasion altogether...

Interesting thoughts about inductive bias Groily. However I was under the impression that where this ocurred it tended to be the l/h pot which ran rich - exactly the opposite of what I have got happening. Please feel free to correct me if I am wrong though....

[Russ]

I am just reading BSA Twin Restoration by Roy Bacon and on page 48 he says, Quote "Check the gaps on the new rings and do fit the taper ones the right way up or you will have a plug oiling problem". I know Simon said he has a "Sooty Plug" but thought this may be worth mentioning as he has just had an engine rebuild. Keep in mind that I am a complete novice and don't know what the taper on a ring is refering to.
Russ.

[groily]

Don't see why it should always be the lh pot if there's bias Simon, but I dunno! Someone will though, that's for sure. I thought the bias washer was something you could fit either way round to angle the carb either way a fraction, but as I haven't seen one even this past 35 years and my current 'A' seems not to be afflicted . . . .
On the ring taper question, unless compression rings are marked 'TOP' it's v hard to tell which way up they should go . . . If they are marked (usually next to the gap, usually to the right of it looking from on top, and legible when new if not for ever) then you'd think the bloke that put them in might have noticed . . . but an error's a possibility. Not a very welcome one I would think, as it spells work.

[Mosin]

Today I fitted an induction bias gasket as recommended and went for a nice run of about 60 miles or so (having first thoroughly cleaned both plugs). I've just whipped the plugs out to look at them and was pleased to see that whilst they are both now sooty, at least they are fairly equally sooty! This leads me to suspect that all I need to do is adjust the mixture and I should be laughing! However, I have had a complete and total mental failure. Can one of you kind gents please remind me whether I need to be screwing the mixture screw inwards or outwards to weaken the mixture? I know I should know, but I find the more I think about it, the more I am getting my brain in knots.

Simon

[lawnmowerman]

Simon,
Where did you get the induction bias gasket or did you make one up. I think I may have the same problem. I have an ally head and no heat block tufnel washer - not sure if they are only required on iron heads. My studs don't seem long enough to take a spacer washer.
I need to rule out any mag problems first and then a bias gasket will be the next thing to try.
As far as the mixture is concerned, AFAIK the mixture screw is for tickover only - you may need to drop the needle a notch to weaken the mixture through the main jet. Also worth checking it has the correct size main jet and needle - there are various lengths and different tapers.

Jim

[Mosin]

Jim,

The bias gasket came from Cake Street Classics. I rang round literally dozens of places and they were the only people who had one. I had read a lot of stuff about them and a lot of people told me that they didn't work at all. All I can offer is my own experience on the matter and that is that fitting one seemes to have evened up the carburation between the cylinders perfectly. My Shooting Star also has an alloy head and it has gone on no problem.

As an update, this morning I dropped the needle by a notch as you suggested before riding the 25 miles into work. The first thing I noticed was that the bike was pulling much better. When I got to work I quickly whipped out the plugs in the carpark and both looked perfect. Job's a good 'un!

Thanks,

Simon

[bsa- bill]

I made one  with a flat wood rasp from a tufnell washer, takes a while tough, bit like starting a fire by rubbing two twigs together, can't say it worked 100% but maybe did have some effect.

[lawnmowerman]

Glad you got it sorted Simon.  I will bear Cake St in mind if the maggie proves ok.
Don't think I will make one Bill - I remember that tufnell is pretty hard and I dont have access to a milling machine.

Jim

[a101960]

What way round does the bias gasket fit then? If for example the near side cylinder was running lean would the gasket be fitted as in A or B ? I ask this because it is not obvious to me how this actually helps the problem. By that I mean the thick side of the gasket although turning the carb towards the weaker cylinder it also moves the carb further away from it. I am completely  lost in understanding what is supposed to be going on here.

John

[Mosin]

The thicker side of the gasket goes next to the cylinder which is running leaner.

I have no idea how it actually works and to be honest, as long as it continues to work, I am happy in my ignorance and perfectly contented to believe that it is magic!

Simon

[muskrat]

By pointing the carb more towards the weaker side the incoming charge is less restricted (straighter path). Not sure if it helps the weak or hinders the rich.
I used one with good results on the '51 years ago before I swapped to the alloy head.
Cheers