This is from an article in Classic Yammy written by the guy that was involved in the actual recall and distribution of the recall kits that Yamaha did form their recalls....it explains the problem as they discovered it back then.
"Of course I also cannot recall everything that happened during those hectic days but I remember that one of our race mechanics, Rod Tingate, suspected that the problem was in the oil circulation. He cut the oil tank and glued a Perspex window on it. After running the engine it was clear that not oil, but just foam came from the return pipe.
Oil temperature was therefore the main problem, we were thinking. Only years later we realized that it was something different. Japanese Motorcycles in the early 1970’s were light years ahead of cars. But we only had “old” car-designed 4-stroke oil!
Several oil-engineers have assured me later that if modern 4-stroke motorcycle oils had been available in those years, Yamaha would not have had any recall.... And also Honda would not have had their camshaft and camchain-tensioner problems as another example. But if... does not help."
"Of course I also cannot recall everything that happened during those hectic days but I remember that one of our race mechanics, Rod Tingate, suspected that the problem was in the oil circulation. He cut the oil tank and glued a Perspex window on it. After running the engine it was clear that not oil, but just foam came from the return pipe.
Oil temperature was therefore the main problem, we were thinking. Only years later we realized that it was something different. Japanese Motorcycles in the early 1970’s were light years ahead of cars. But we only had “old” car-designed 4-stroke oil!
Several oil-engineers have assured me later that if modern 4-stroke motorcycle oils had been available in those years, Yamaha would not have had any recall.... And also Honda would not have had their camshaft and camchain-tensioner problems as another example. But if... does not help."
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