Ericson 29: Sooty spark plugs

[learking]

Just when I thought this sooty plugs problem was cured I discovered it was still with me. Background is that I bought the 1973 Ericson 29 in May 2006. Basically, the boat was in good shape but needed some TLC. This was my first inboard powered boat having owned outboards since 1987. By now the boat is in really great shape. Less than a week ago we were departing our harbor, on Lake Ontario, and ran through some nasty floating islands of organic material which managed to clog the engine water intake. When I realized the engine was overheating I shut it down, still in the narrow channel, and dropped an anchor, then another. Went overboard and cleaned the water intake. The engine started right up and temperature quickly cooled to normal. Being conservative we headed back to the slip. The engine had no apparent damage and ran great but I wanted to do a compression check before heading out again. The next day there was no water in the oil so I began removing spark plugs and checking compression (after first closing the through-hull fitting: thanks to Moyer's manual I knew about that). Expecting to see clean plugs I was disappointed that they appeared sooty as ever. The engine had less than 10 hours since new plugs were installed. Compression was low but consistent. (65,65,70,65). No water in the cylinders either. Whew. About the only thing I've done to the engine to cure the sooty plugs problem since purchasing the boat was lubricating the choke cable which helped the idle speed immensely. After doing that the engine ran so well I thought the problem was fixed. Any suggestions what to do next? I remember reading somewhere on this site about some carburetors having air leaks in the halves which don't quite fit properly. The previous owner had installed a new carburetor. Can't seem to locate that discussion now, however. Another is the possibility of an exhaust restriction causing the problem. There appears to be copious amounts of water spewing from the exhaust, even more after I replaced the impeller last month. (Two broken blades but found all the pieces!) Hey, maybe the fact the engine is 34 years old might have something to do with it?! Any suggestions to cure this, one of the few problems I now have with this boat, would be appreciated very much.
Thanks!
Gardner Soule

[Don Moyer]

Gardner,

Your compressions are a bit on the low side and could contribute to a low
order of chronic plug sootiness. As long as the plugs do not foul out
enough to create a miss, and the engine is otherwise performing well for
you, the compressions do not in themselves indicate the need for an overhaul
at this time.

I would follow up on your suspicion of a slight restriction in your exhaust
system. The Atomic 4 likes a very low exhaust back pressure in the range of
1 to 1.5 psi and back pressures in the range of 2 to 3 will frequently allow
a rather normal amount of water to pass through while negatively affecting
the engine. If you had such a low order of elevated back pressure, it could
account for your low compressions in that it would cause the same soot your
seeing on your plugs to be forming on your rings.

The most common place for restrictions to develop is in the hot section of
the exhaust where the engine cooling water enters.

Don

[adab1402]

an additional thought

dons points are well taken ,you mentioned an air (vacuum) leak in the carb body ,that would usually result in a leaner mix ,causing the opposite of sootiness ,if your not sure your choke is opening completely take it off and check it out ,a mirror can be your best friend . fair winds ken