#4 plug different than 1, 2, and 3

[neversinksmith]

hey gang. just picked up a 73 merc 850 4 cyl. after examining the plugs I noticed that 1, 2, and 3 looked perfect but on the #4 plug the porcelain was half dark(like the rest of the plugs) and half eggshell colored. should this lead me to believe that the crank seal on the bottom of the powerhead is going? thanks

 

[achris]

At that age, easily. While you have the power-head off replacing the seals and bottom gasket, do the exhaust covers too...

 

[gm280]

I agree with achris as well. And if you do go into the exhaust cover, clean out some of the built up carbon there as well. Because I'm very sure you will have some build up!

 

[neversinksmith]

this old girl is going to get a complete tear down and rebuild so I plan to replace all seals gaskets and bearings. I just wanted to know if this WAS in fact the reason for the #4plug looking different.

also, ive seen this condition on plugs before on 2 and 4 strokes where the porcelain is half dark and half light. what causes this?

 

[Chris1956]

If the crank seal was bad, the plug would be rusty on the business end. The bottom plugs are a PIA to change, as you need an open end wrench. Maybe it is a lot older than the other plugs?

BTW - if that motor was used in saltwater, the water jacket cover bolts will usually break. I would recommend you confirm the need to replace the water jacket gaskets, prior to trying it, for that reason. Usually the crankcase and transfer cover bolts will cooperate and come out.

 

[neversinksmith]

just used in the Hudson. that might be worse than salt water...

 

[merc850]

Possibly the PO changed the plugs but was to dumb/lazy to figure out how to change Nr.4, I have a Direct Charge 850 and half the plug is darker and I think it's due to the high deflector on the piston, the seals are new and the plugs don't have water on them. Don't panic.

 

[neversinksmith]

If the crank seal was bad, the plug would be rusty on the business end.

now that ive never seen.

 

[Chris1956]

When the crank seals leak, water does get into the cylinder. You will see droplets, discoloration or even rust on the spark plug, depending on how long it has been run that way and how old the plugs are.

 

[Faztbullet]

Make sure you bore it as cylinders will be worn around ports due to side thrust from pistons and also do the restrictor mods...

 

[neversinksmith]

where can I find instructions?

 

[Chris1956]

I think Fazt means the plastic idle restrictors that fit into the drilled holes in the transfer ports. You just push them in, and they are a friction fit.

 

[Faztbullet]

Here is link to mod and pictures,
http://forums.iboats.com/forum/engi...page2?t=288700&highlight=bleed+restricter+mod

 

[achris]

now that ive never seen.

I have, lots of times.... Usually because the engine's been 'put away' for a while. If the engine gets used lots, a leaky seal (or exhaust gasket) will result in plugs that look really clean, 'cause they been steam cleaned....