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Spark Plug Problem

L

lost_and_confused

New member
"Hey everyone

I have a pro

"Hey everyone

I have a problem with the #1 plug blowing apart, Not pulling out of the threads but coming apart. The engine is a 1968 Mercruiser 120hp 4-cyl.. The motor run great for awhile and then it back fires up through the carb, I then power down and then back up runs fine again and then the #1 plug comes apart. The threaded part stay in the head and ceramic comes out. It did this twice and always to the same cylinder.

Thanks for any help."
 
What does the combustion chamb

What does the combustion chamber look like when you remove the threaded shell?
 
"as far as I know it looks cle

"as far as I know it looks clean. After looking at the remains of the blown plug, it looks like the plug is getting really hot because the spark plug boot is melted the the plug. This could explain why the plug is coming apart but not why it is doing this in the first place."
 
My only guess would be either

My only guess would be either timing is WAY off or a bad exhaust valve contributing to the "back fire" or you are running very lean on your fuel.
 
"I'm thinking lean, too, a

"I'm thinking lean, too, and #1 is the first to go. That's where a good description of the combustion chamber would be helpful. Too much timing advance could do it too; should be easy enough to check."
 
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