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I have just seen a concentric installation where after cleaning in detail and the pilot screw set as rich as possible all air leaks in tract checked and slides sycronised (or not ) The rear cylinder only runs at tickover with a hand covering the bell mouth otherwise the front cylinder lugs the rear round. The way the rear picks up even after a prolonged lugging when the hand covers the carb suggests it's not ignition. When the throttle is opened it still is reluctant to chime in. They are difficult carbs.