1981 Johnson 70...black soot below cowling

[jpw1488]

I just finished a decarb treatment with seafoam and then ran the motor for a couple hours on the lake today. I’ve got more than usual black soot on the upper leg around the exhaust relief ports. Looks to be coming from under the powerhead though. Motor runs great. Idles good. WOT 5400 at 30 mph on new 1960 Alweld CC. Wondering if that’s anything to be concerned about (exhaust base gasket) or just a result of the decarb. It’s always left a little soot there, just a little worse lately. No soot under the cowling or exhaust side plate. Thanks.

 

[Paulywog0667]

When was the last time you checked the gaps on the plugs? The fuel will leave behind a soot/wet soot if the plug gaps aren't giving good spark.

 

[spybot]

I think that once you do a seafoam decarb you NEED to change the plugs.
On a side note regarding seafoam. I used it once but i will never use it again. 1st it is no good for the gaskets/seals that are in the outboard. Only found that out after i had used it.
2. I was getting 125 psi on my 4 pots some o e said use seafoam its great for your engine. Me being new to outboards thought this must be what you use on them. Once i done the decarb and changed the plugs i took my boat for a drive. She was not the same perky wee 85hp. There was something lacking. So i did another compression check. This time i had a difference of 17psi between the same pots. The seafoam had eaten its way through the carbon build up and what everelse was holding my old motor together. Had to rebuild that motor all because of seafoam.
Some may argue that it did its job but in my eyes it killed my motor.

 

[oldboat1]

^^ Interesting. good advice, imo.

 

[jpw1488]

Thanks for the replies.

Checked the plugs, they look okay, but will consider replacing them.

I had that thought as well about removing carbon which may be sealing the old gaskets. Wondering if maybe worsened an existing base gasket leak. This motor has never been decarbed to my knowledge, and I’ve yet to get it to stop smoking after several seafoam treatments. I removed the lower cowling over the exhaust housing and wiped everything clean and didn’t see an obvious problem.

Another thought, I sprayed the powerhead with some Wd-40 to help keep mud wasp nests from sticking. Wondering if engine heat maybe burned that WD-40 off and that’s what was causing the soot on the external upper leg.

I ran it again for a bit the other day and no issues. Motor seems to run great. Starts right up. Idles good. I’ve really got it in good shape otherwise - new coils, wiring, powerpack, stator, timer base, carb rebuild, water pump. Bottom cylinder compression is lower than top and middle. Top 125, middle 125, bottom 105. Bottom cyl compression is what prompted the decarb. Bottom cylinder has been lower for at least 10 years. Blew a head gasket last year on the bottom cylinder and compression now is a little better than it was before the gasket blew. I had the head off a couple times, sort of glazed but no major scoring. Didn’t see broken rings or other damage with an endoscope. Bottom piston has just a touch more side to side play in the cylinder. Compression goes up with a little oil added through spark plug hole.

 

[oldboat1]

Might resurface mating surfaces for the cyl. head and the cyl. head cover. May be able to reuse gaskets. Torque to specs in circular pattern, starting in the center. Clean and regap plugs, or replace.

Or just have fun with it if it's running well. If you live in an area where the water gets hard, store it with fogging oil over the winter.