Hi All - have a closed system (jrt pump to heat xchanger to logs) and I am seeing carbon (black stuff) in my antifreeze.  trying to figure out where carbon-exhaust sees antifreeze in the engine. oil is fine; no loss of power but have not compression tested....thxs kenai

the jet pump water goes thru the exchanger but doesn't contact the coolant/antifreeze used by the engine and all of this is 'upstream' or before the cooling water runs into the front side of the logs.  so knowing there isn't any interface in the logs with antifreeze; looks like the carbon contact is in the heads or engine; // does the intake manifold have any contact with carbon?...thxs kenai

If logs are cooled with lake water from heat exchanger, how could exhaust get into the freshwater/antifreeze side?
Heat exchanger should have a "radiator cap" ~10 lbs maybe? the exhaust system would have to be in excess of 10 lbs of back pressure (if there was antifreeze in the logs) to have exhaust entering the logs.

Most likely a blown head gasket between chamber and water jacket. A pressure check of the (antifreeze/freshwater) side of the system will tell.
The head may also be cracked, but that is not as likely.

Good luck
Daniel

Has to be a exhaust leak into a coolant passage somewhere on the logs

I'm behind this 100%. The only way it's possible is blown head gasket. The log's simply CANNOT do this...... 

Ray