1975 merc850 bottom plug,#4 cylinder is white?

[khes142]

title says it all. plugs 1-3 all look brownish after use. cyl 4 plug,the lowest cylinder, always looks white.the motor runs so-so.starts great but performance should be a lot better. any ideas?

 

[Yepblaze]

Re: 1975 merc850 bottom plug,#4 cylinder is white?

A like new plug might indicate water getting into cylinder while running, or cylinder not running at all.


Apparently some blocks can crack around the porting under the side cover and ingest water.

Some times it happens, yet has little or no effect on the way it runs. But water in the cylinder can be bad and cause corrosion. If real bad, you might actually see a droplett of water on the plug when removed.

Of course that's not an absolute, and there could be other reasons such as extreme lean condition from a failed lower seal, or an ignition issue not allowing the cylinder to fire properly. But if the cylinder is dead it will become apparent pretty quickly as you throttle up under load.

 

[khes142]

Re: 1975 merc850 bottom plug,#4 cylinder is white?

thanks for the reply...what can you really expect from a 32 yr old motor??

 

[Motor Boater Bill]

Re: 1975 merc850 bottom plug,#4 cylinder is white?

There are much older motors running just great, but any motor can, and eventually will, die. It's more of a question of when will it get to the point that it isn't worth fixing, and that depends a lot on whether you do the work yourself or have to pay to have it done--in which case it doesn't take much to consider a motor "not worth fixing." All that said, water in the cylinder can be one of those conditions that pushes a motor on over into the "parts motor" category.

 

[The-Machinist]

Re: 1975 merc850 bottom plug,#4 cylinder is white?

First thing to check is compression, If that checks out the rest is just details

 

[Scaaty]

Re: 1975 merc850 bottom plug,#4 cylinder is white?

Water jacket/liner has gave up the ghost around that plug (cracked/warped)...water will now get in there and rust the rings and etc next...fixable..