spark plugs 4 A10

[fbm]

Hi there, I have a BSA A10 650cc, can anyone tell me what should be the gap size of the spark plugs for eg 0.5 or 0.8 14mm, I needed an urgent answer thank you.

[groily]

Not sure about the 'should be' of it, but I have always used 20 thou, say 0.5mm. Will work at 25thou though, as they will at a bit less than 20. Assuming the mag's OK, natch. If it isn't . . . . you may find it needs a smaller plug gap cos it lacks the oomph to make the great leap from one electrode to t'other. I'm talking iron head. I also use 20 thou v happily on alloy heads of other marques with mag ignition. Groily

[dpaddock]

groily's correct - 15 to 20 thousandths inch is fine. Anything over 20 stresses the mag.

[LJ.]

Just had me plugs out to check... They were sand blasted clean and then took a 14 mile ride, and this is what they look like when I got back, any ideas?

The gaps are 19 thou and magneto points 12 thou (checked both sides of cam ring and Okay) There was a bit of back fire when blipping the throttle, but other wise the bike seems to run fine. (1961 A10)
White is left cylinder and dirty is right side... Plugs are NGK BP6HS.

[bsa-bill]

looking at the plugs the left hand side one ( the white one ) looks to have a slightly longer snout, this is surely a perspective thing, never the less it suggests something, why not try a hotter plug in the right hand side, not an ideal situation I know but worth doing I would say.
The other thing I would try would be to make a biased washer to fit between the carb and the head, it may be that your engine is breathing heavier on one side than the other pushing more mixture into the side that is black, thick fibre or tuffnell washer can be picked up at autojumbles and filed to make them wedge shapned, fit it so that it is going to squirt more mixture to the side that is weaker, I have done this to my flash and it did help

All the best  - Bill

[a10 gf]

A much discussed problem, now very clearly depicted, the plugs showing that much difference. Your findings and solution will be valuable reading.

You may want to be 101% sure that it's not caused by some difference between the plugs, by trying another fresh pair, see if the result is the same, (very unlikely that this would be the problem, it's only to eliminate any unexpected possibilities).

[RichardL]

LJ,

I hope this is in no way insulting, but have you eliminated the possible basic causes that might be considered before we go all expressive with the more exotic ones?  For example, valves, guides, head gasket, etc? I actually think e's idea that the plugs are unequal seems more likely than uneven spritz from the carb. (Bill, please don't shun me for contradicting. I need and value your advice). One thing that I think would be very interesting would be to clean them again, put them in the opposite cyclinders, drive a bit and look again. If the trouble stays on the same cylinder, and your leads will allow it, you could try switching leads (at both ends, of course) and see if the trouble follows. I suppose the dry carbon opts for overly rich and that lead issues would likely result in some quite uneven running, but I kinda' like the strategic troubleshooting approach.

Richard

[fido]

Remember that it's the colour of the insulation cone you use as a guide to rich or weak running, pale brown being the ideal. Soot on the electrodes themselves could mean it is burning oil on one pot.

[LJ.]

An update to my plugs problem as mentioned further down the thread...

I had the carburettor off to look at the ports and see the route of mixture flow. It appeared that a gasket was over hanging inside blocking part of the flow to one side, it was only a very small amount though. Needless to say that I replaced the gasket and ensured that no overhang was visible.

The conclusion is that *I* now know this area to be Okay and rectified of previous owners handling.