bullgrizzle

hi the problem is as follows, the advance /retard valve arm goes like a sewing machine when the engine is running. whilst in france bike started losing power on hills or flat road. the conclusion was the advance retard valve which was changed on repatriation to england. although the valve arm is still going like the clappers she now runs,but a bit ragged. is there another cause for this problem perhaps on the vacuum side as if you pinch the vacuum tube to the valve ,the fluctuation of the valve stops and you can of course open the throttle and increase revs cleanly but due to vacuum line being closed it wont retard and stalls i cant find anyone who has had this problem so any help appreciated.