I'd also agree. Only the stretch of the belt on the fixed idler side will effect the timing. If the belt stretches by 1mm over that length the cam pulley will be 1mm further back around at it's circumference than it would be before the stretch. The movement of the moveable pulley or diameter of the moveable pulley will not effect it. Make sure you turn the engine over in the correct rotational direction when setting or checking belt tension, otherwise the wrong(tensioner) side of the belt will be transmitting the 'pull' from the driver pulley. Nasher
The shortening and lengthening of the chain is a by product of changing sprocket sizes (both front and rear) to change gears. The derailleur arm is spring loaded to take up any slack that results. The arm is on the slack side of the chain and doesn't affect the gearing in any way.
looking at the photos of the engine shown by @Birdie and those of the ST2 type engine, the diameter of the casing relief at the adjustable bearing seems to be greater (definitely on the vertical cylinder). what would that show, though? maybe there should be a bigger bearing on the adjustable side?
And a smaller diameter adjustable idler also means more belt contact around the main pulleys than a larger diameter adjustable idler - just saying.
So the longer arm tensioners arrived today. Need to get the proper bolts for them as they have a wider slot with a shoulder on the bolt to fit it. But they work perfectly! Loads adjustment now, old above, new below.
