The next time you get a chance to look at a stock engine on an older (pre-'seventies) Volkswagen Beetle, find the exhaust manifold by tracing back from the pipe(s). You will see two tubes, one on each side, that go up toward the carb and are clamped onto the intake manifold for a considerable amount of its run.These are the heat risers. Their job is to transfer heat to the intake manifold, so that the fuel/air mix is preheated; and the heat is even transferred to the narrow passages of the carb to keep them from icing up. Oftentimes, inexperienced owners of such Volkswagens start to notice that their car stalls when coming to a stop after extneded high-speed driving. The problem is infrequent at first, but gradually occurs more and more often. Soon the car will stall almost everytime it comes to a stop. This is happening as the heat riser tubes gradually fill with carbon from the exhaust gasses. An experienced VW owner gets the carbon out; but if they don't, eventually the riser fills solid. Once the riser is COMPLETELY blocked, the sweating problem you describe becomes EXTREME in a warm, humid climate. Some people (notably the late automotive writer John Muir) claim it can actually damage the air-cooled Volkswagen engine.
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