I posted here in November regarding a petrol lawnmower that was refusing to
start. It's a Tecumseh BVS143 4-stroke engine on an unnamed box, so nothing
special.

Anyway, before it was running for about 1-3 seconds before cutting out. I
eventually traced it down to a perished spark lead, and today I replaced the
electronic coil/lead unit with a good condition used part. Earlier this
year I also changed the oil (it was filthy) for new 15w/40 and cleaned the
spark plug and air filter. Today I put in new petrol (bought today) and
oiled the air filter.

The five lines of print that came with the new coil said to adjust the
magneto gap between magnet and laminations to 0.30mm. But I don't have
their gauging tool, so I googled the thickness of a piece of 80gsm paper to
be 0.10mm, and used three sheets. It started, and continued to run, but it
didn't have much power to tackle thick grass and was misfiring every 4-5
cycles. I did leave it to run for a while to clear out any clag, but it
seemed to be running clean after the first minute or two.

So I tried several paper thicknesses:
1-2 sheets - cuts out after 1-3s
3-4 - runs, some degree of misfire
5 - misfiring 60%+ of the time, but enough to run without load just about

So is there an easy way to adjust the coil gap? I'm guessing it must be
possible to get hold of a little kit of plasticard spacers of relevant
thicknesses or something similar - any suggestions from where?

Also, anything else I should be looking at for solving the misfiring? I
didn't wring out the air filter, so maybe it has too much oil? What else?
I haven't looked at the fuel filter or the carburettor (can't work out how
to find these) - would these be the cause, or is it likely to be just the
coil gap?

Thanks
Theo