Hi,
The Briggs and Stratton is usually a four stroke, i haven't met a two
stroke Briggs and Stratton, although i see no reason to think there
aren't any.
The magneto type of ignition system is really surprisingly reliable
in my opinion. There are two basic arrangements, the contact-less
type called 'Electronic' and the older more traditional points type.
Only physical deterioration or damage is likely to prevent the
electronic type from working, sometimes damp may have to be cleared
and once cleared should not prevent the ignition system from working
assuming there is no damage or excessive deterioration to the lead or
the cap and plug.
With the points type there are two common reasons for failure. One is
corrosion on the contact surfaces, these can be accessed via a hatch
or cover on the flywheel and cleaned. Two, the moving contact can get
stiff, and not move properly, sometimes staying open. Just check that
its free like its supposed to be.
Another problem that crops up occasionally is that the inlet valve can
get stuck open, or sometimes not close properly. this is usually due
to rust on the valve stem making it stiff. The cam will lift it, but
its the spring that has to return it. You would think that a bit of
rust would soon clear with the valve going up and down, but it doesn't
always clear, sometimes it sort of gets 'impacted' and requires actual
intervention to free it. A finger on the plug-hole will tell you if
there is compression when the engine is turned, but keep the lead out
of the way. Curiously i have never met this problem with the exhaust
valve. Dunno why not, either of them could be left exposed to the air.
I usually check for ignition by spark length from the end of the lead
to the engine block, i expect about three eighths of an inch to be
about right, it may run with a lesser spark, but if you cant get more
than an eighth of an inch then theres something wrong there.
I dont find plug gap critical, about my thumnail thickness seems right
to me.
PETROL.
As has been mentioned, petrol does deteriorate. Try to use very fresh
petrol if you get problems starting after a period of inactivity. I've
never had problems with older petrol after starting with fresh petrol,
but often they wont start using petrol thats been sitting around for
a few months.
Some small engines use a spark killing method which is operated by the
oil condition. Especially on generators which may get run for long
periods without the oil getting checked, this is one way of getting
the oil changed if it becomes so grotty that the extra wire from the
magneto kills the spark. This arrangement may have prolonged the life
of many small engines, although i have never seen it used on a Briggs
and Stratton, but you never know. Of course once you know about it,
the wire could just be lifted, but thats a bit of a liberty.
The most usual cause of problems is water in the fuel, or in the
carburetter, and even after clearing the water, theres the corrosion
which can block jets, and needs cleaning out. Ive always been happy
to use bits of wire although some say you should only use compressed
air. I use compressed air too, but sometimes thats not enough, and a
bit of wire will usually clear out a stubborn jet.
Best of luck with it,
John