1998 Kat 600 Closed plug gap??

[skiboy88]

My 98 keeps on running on three cylinders, then I pull the plugs to find that one is completely shut, gap the plug and it runs good for a while.. What is hitting this plug?

[Teh_K]

Hm, haven't heard of this before... If the piston is hitting the plug (*cringe*), then it would close the gap immediately and would run like crap almost right away. Maybe the grounding tip of the plug is weak or something and gets bent over time from the combustion in the chamber?

I'd get new plugs and see if it still happens.

[The CyberPoet]

The only thing that can hit the OEM-sized plug and close the gap virtually instantly is something riding on the piston crown, such as a piece of metal (bolt, etc) dropped into the cylinder, a really horrible build-up of carbon (very hard) or actual engine parts broken off from the piston (or lower and connected to the piston).

What you would need to do at this stage is get a bore-scope (or borrow one) and look inside the camber to figure it out, OR remove the head to look at the cylinder/piston (lot of work by comparison). Since a lighted borescope tends to run $120 and up, removing the head is the cheaper option unless you know someone who already has a borescope.

The other thing you might want to do is verify that the spark plug that is in that cylinder is the correct part number -- not all spark plugs are the same length, and if you somehow ended up with the wrong size, it could be hitting simply because it's too long for this engine.

Cheers,
=-= The CyberPoet

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[rexazz2]

Had this happen on my Mercury Outboard
It seemed to be random , sometimes on the way to fishing hole , sometimes hauling water skier, trolling or never
Then it started happening all the time
Took it into Mercury dealer and they said bad rod bearing !!!
Unloaded it as is

[hardlydangerous]

change that plug NOW
the ground strap may just be that weak...

if you put in a new plug and it does it again you have a VERY serious carbon build up problem or something.