<-=- Club Bevel Heaven -=->

I'm slowly working my GTS back to being a complete motorcycle and have just discovered that I am can feel a quite powerfull suction If I cover the exhaust outlets on the heads of my GTS with my hand and crank the engine with the kickstart. I've yet to work out on which cycle this is happening and will check further tonight.

Intuition tells me that this shouldn't be and that either the exhaust valves are sticking open or the valve timing is somehow way off.

Does anyone have any other thoughts on this?

Your valve timing could be out but thats pretty easy to check. Just check the dots are lining up on the bevel gears around TDC. They are hunting teeth so they only come around every so often. Very quick easy check is piston at TDC and the cam lobes should be at 8 and 4 o`clock when on compression stroke, or 10 and 2 at exhaust stroke.
Cam profiles have an overlap, that is, a period when both inlet and exhaust valves are open. This overlap varies depending on state of tune. It`s possible what your feeling is nothing more than the overlap. Although I can`t be sure as I`ve never held my hand over an exhaust port while cranking. Bit hard on mine as it`s a kicker only..........