It is isn’t a issue to have a spark plug fire on
the exhaust stroke as long as it fires on the
compression stroke. A failing plug that doesn’t
fire on the compression stroke but will in free
air, ignites the fuel on the exhaust stroke as
there is no pressure, because the exhaust valve
is open. This will result in a backfire through
the exhaust. The result can just be a loud bang
up to damaging parts of the exhaust including
blowing it off the motor. Not a common
occurrence but something I have experienced,
more than once, with this type of ignition.
Tubs
.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.