This is some crude accumulated in the top of the water
jacket partially blocking the flow of coolant. Needless to say
you would not want this stuff in the water jacket near the spark
plugs. Crude obviously rises with coolant bubbles.
This engine was overhauled about 2 years ago so I expect
the coolant was not changed in that time period.

Paul Lamar

Bill Freeman wrote:

Paul,

This fits with my suspicion of the engine sitting for a while
with a low coolant level.  The crud forms at the surface where
the air is available for corrosion and to oxidize 'stuff' into
'crud'.

Bill

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