On Sat, 30 Apr 2016 16:40:11 -0700 (PDT), wrote:

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I can see that the main flywheel-like thing has a smooth shiny surface that passes within 1mm of something covered in plastic that is about 2" diameter and an inch thiock from which the HT lead goes to the spark plug. That must be the coil, is it?

Yup. ;-)

I presume that there is no contact-breaker. And this would also mean there is no condenser, yes?

Yes. Probably something as simple as this (you may have better info in
the manual you mentioned):

https://dl.dropboxusercontent.com/u/...ing_5_pole.pdf

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I will try cranking it over with the plug out in the dark and see how bright the spark looks within the next 24 hours. If said spark looks feeble, I will try buying a new plug. If that makes no diff I will try buying a new coil (or at least, the thing I suspect is the coil)!

;-)

It doesn't remotely resemble a motorcycle coil that I've seen

No, it may not (unless your motorcycle uses the same system). ;-)

The thing most people consider a coil is a cylindrical object with an
HT lead (or connector) poking out the end and a couple of screw or
spade terminals (one to ignition or ballast resistor and the other to
points or electronic equiv).

(However, my motorcycle familiarity is limited to 1950-1070 British bikes and 1990-2006 Harley Davidsons. Oh, and more recently, a Honda 125 scooter, which was IMO, the most practical machine of the lot!!

My MZ had what I think you would recognise as 'a coil' but the BMW
one(s) was something between that and what you may be seeing on the
B&S engine.

If you imagine a magnet built into the outside of the flywheel, these
coils are normally wound on a 'square U' shaped iron lamination where
the ends of the laminations face the flywheel and are shaped (concave)
to match the circumference of the flywheel. As the flywheel rotates
the magnet passes across the ends of the ignition coil core, magnetic
flux passes though the iron and generates a current in the coil
windings. I'm guessing it may do the same as the flux collapses and if
so I'm not sure which end of the process generates the spark that's
used (it doesn't matter as the flywheel is keyed to the shaft and the
coil only adjustable re it's distance from the flywheel). ;-)

FWIW, for setting that gap I've seen suggestion of passing a couple of
sheets of paper between the coil and the flywheel and with the two
screws loosened, gently hold the coil against the paper / flywheel and
nip up the screws. Rotate the flywheel with the paper and then check
there is clearance throughout a complete revolution. If the flywheel
isn't completely circular, do this at the highest point or if you do
it where the magnet is, you won't have to apply any effort to hold it
in place. ;-)

It is possible the could could be set with too large a gap (see the
manual for the spec on that) and it will make a big difference
(especially if the system is marginal).

Cheers, T i m