Pete C wrote:

On Tue, 11 May 2004 18:51:39 +0100, The Natural Philosopher
wrote:


Its an oldy, but apperas to have bags of compression. Simply was in use,
left up for a winter a couple of years ago, never started since.

Its a ltllet 2-0stroke japanese engine - probably about 25cc or so and I
can;t see any spark - the pllug smells of petrol and we even dribbled
some mix into teh p[lug hole, to no avail.

I whipped off the pull handle and discovered something I hadn't seen
since me dads moped curca 1960. A magneto and some contacts. I adjusted
the gap so it was at least closed some of the time and open some of the
time, but still no visible sparks. I removed the wire off the switch
that stops the thing in case this was faulty, and still no sparks.

First of all, is there some way I can identify that there really are no
sparks? I stuck a car strobe light in series with the plug, but I think
this runs off the LT side of the car not the HT...so probably proves
nothing.

Its been years since I fiddled with a magneto, but ISTR that if there is
fuel and a spark these things generally kick a bit and fire if not run.
This ones doing neither.

Is it down to thimgs like stripping out the points and claening them up?

Is there anyway I can use an analogue multimeter to diagnose the problem?


Hi,

Try putting the meter on resistance range and attaching it across the
points, it should read some low ish resistance when the points are
open, and a short circuit when the points are closed.

Also measure the resistance between the plug lead/coil HT output and
the cylinder head of the strimmer, it should read a higher resistance
but not an open circuit.

Then measure the resistance between the electrode of the spark plug
and the cylinder head of the strimmer, this should be an open circuit.

Next set the meter to DC volts and connect it across the points, turn
the engine over slowly and you should see the needle kick slightly as
the engine turns over.

Now take a neon screwdriver and put it on the HT output from the coil
while turning the engine over and holding the end of the screwdriver,
it should flash as the engine is turned. Repeat this with the end of
the HT lead.

Now take the plug out and use a straw or thin stick in the plug hole
to identify when the piston is at 'top dead centre', by rotating the
engine slowly and feeling when the stick is pushed out the most. Or
there may be some alignment marks somewhere on the shaft and body of
the strimmer that indicate this.

Then adjust the points until they are only just open, using the meter
to check for this.

Now adjust the gap on the spark plug so it's not too wide. Put the
spark plug on the lead, hold the body of the plug onto the cylinder
head and turn the engine over. With a bit of luck it should spark.

The only other thing on the electical side would be to check the gap
on the spark plug is the right width, or the spark may not be strong
enough. This might be printed or stamped on the body of the strimmer
or maybe a suitable value can be got off the web.

Hope this helps,
Pete.




Enormously Pete, thats a good suite of sensible things to try.

Thanks very much indeed.