I am just about to do the valve shims on my 1978 Gs1000 I had symptoms etc. I took the head off and no damage was visible. I have significant power loss and the engine will not rev (bogs) even with bigger jets. The engine Idles nicely off choke when warm. However cyl 3 has compression but is not firing consistently. The gs has covered 28K miles with no service checks? Any advice please,will be gratefully accepted.