"jim" wrote in message
...


noticed the
spark plug had actually broken off clean. Anyone see this happen before?

I have never seen the top of a spark plug break off all on its own. I see
it a lot when something hits the plug, but never while you were just running
along fine.

I have seen the inside portion of the ceramic insulator break off, usually
as a result of running to lean (and hot) or excessive pinging. If the
inside breaks off it could cause the entire insulator to be pushed out,
leaving just the metal outside with a hole in it.

Any damage by running on 3 cyclinders for about an hour?

Maybe.

If the inside portion of the spark plug broke then it would have fallen into
the cylinder. It is very likely that this would cause some damage, from
scoring the cylinder walls, getting jammed into the cylinder head, or
lodging into the exhaust valve seat. This could have very serious
implications. It is also possible that it broke into small enough pieces
that they just went out the exhaust. Any damage from this would have
happened in the first few seconds after it broke.

As far as running for an hour, it could be really bad for the engine. The
other three cylinders will now be running at full load, and they could
overheat as a result. If you made it back and there wasn't any further
reduction in power, obvious overheating or other indications of bad things
then you are probably okay.

As far as duplicating this in the future, I would be very careful. If the
center insulator is blown out of the spark plug then you have a hole from
the cylinder into your engine compartment. Even though the cylinder is not
running it is still drawing air/fuel in from the carburetor. Instead of
compressing it, however, it pumps it out into your engine compartment,
filling the engine compartment with an explosive fuel mixture. If you have
adequate ventilation you might be alright but there have been cases where
people have tried running with one spark plug out only to have their entire
boat blow up on them.


Rod McInnis