"newshound" wrote in message
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On 09/01/2014 13:32, Bob Minchin wrote:
Agree about the bulge.

Not sure how "modern" stuff like this makes its spark, in "old" systems
with a flywheel magneto there was a set of points to break the induced
current in the primary, and the capacitor served the traditional purpose
of suppressing arcing. In which case removing it temporarily would confirm
if it was the problem without any risk. But maybe it does something else
in a solid state "magneto"?

i'd imagine the capacitor is part of the 'regulation' for the alternator's
output,

the spark will be produced a typical one piece ignition coil found in small
engines, it's next to the outside of the flywheel, follow the HT lead back
from the spark plug to find it, tho' it should be obvious... looks sort of
like a capital letter A, made of laminated iron, think transformer core,
upper half is smaller than the lower half, around the bar of the A is the
winding.... almost always potted and black, lower half of the A goes towards
the flywheel and the bottom of the legs of the A are profiled to follow the
flywheels curve.

no points, condensers or other external triggers, it just fires off a spark
when the magnet in the flywheel goes past it,

only connections will be the fat HT wire to the plug, and a thin wire going
to the kill switch, other side of the kill switch is to earth, so earthing
the thin wire kills the spark,
very little to go wrong with them... bad earth of the iron core, sometimes
there's an extra but short small wire to a ring terminal that goes under one
of the bolts, corrosion could have made that high resistance,

The gap between the lower part of the A and the flywheel sets the timing,
it's a few thou, but unless you've been playing and have moved it well away
from the flywheel (iron A has elongated mounting holes) it should still
produce a spark,

Of course the winding may be open circuit, or the joint to the HT lead duff,
but usual test is replacement after you are sure the spark kill wire is not
earthed inadvertently.

Just one thought, are you still running the original spark plug and using
the plug to test for the spark?

few years ago i bought a cheap chinese 1/5 scale radio controlled buggy with
a 22cc petrol engine in it (typical strimmer type engine)
knock off of the FG 1/4 scale marder buggy that sold for over a grand.

it ran for about 2 tank full's of petrol then died, i pulled the spark plug
out, grounded it, and saw no spark, then proceeded to dismantle the
flywheel/fan cover, checked the engine kill switch was working and not stuck
shut (momentary push button, hold it in until engine is fully stopped) but
it was fine, so i took the ignition coil off and checked it's resistance,
re-set the gap, still no spark,

Then i got a belt whilst pulling the starter cord and holding the HT lead to
hold the plug hard against the head, bloody cheap **** 'torch' brand spark
plug had gone, apparently it was the internal resistor in the plug that goes
due to crap manufacture,
Put an NGK plug in, and it still running fine 5 years later.