The ListerSR2 fitted to Pentargon is an air-cooled 2cyl diesel engine, produced in the late '60s, and designed to be used with plentiful supplies of fresh cooling air. The "M" version was developed for use in [working] boats, from where it gravitated to narrowboat use particularly in smaller models from 30ft to 36ft. The SR2M was fitted to a number of Springers of those dimensions at that time and mainly worked best when best understood. It was rated at 13BHP delivered at 2000rpm. When run at 1000rpm it develops about 6BHP.
"When Best Understood" is not used here glibly. Although the SR2 is very reliable and very well designed, perfect for the uses to which it was being put, most boat owners did not understand the fine points of using this fine engine to its optimum capability.
Being fitted in a confined space under a deck, means a lot of thought had to go into provision of cooling air for the engine and the extraction and dissipation of the [much-heated] air after it had done it's work. On Pentargon, Sam Springer excelled himself, but a "law of diminishing returns" applies.
The cooling system can only remove a given number of units of heat in a given period of time. On Pentargon, the whole engine lump slowly absorbs heat hour after hour even though in theory the air-cooling is working exactly by the book. There comes a time when all the components of the engine reach critical temperatures (over and above the designers intentiions) and start to act funny. Engine tuning goes off but almost imperceptibly and the motor eventually begins to run as it were 'with the brakes on'. If pushed beyond that point the exhaust smoke becomes very black which is a critical warning that it is about to cut out.
This the is "THE BLACK SMOKE SYNDROME"
During the first couple of years of current ownership the "BLACK SMOKE SYNDROME" was never experienced. Pentargon was rarely driven for more than about two hours at one time and never at other than about tickover revs (maybe 600-800rpm). However the trip through England included some delays which had not been factored in and at times the boat was pushed at rather higher revs and for longer cruising periods . The "Syndrome" was not fully understood originally, leading to unexpected engine cut outs often in very embarassing circumstances. But eventually a pattern emerged which allowed us to understand that the"Syndrome" could be anticipated, monitored and prevented!