[aussie merc]

11 inches of vac is a major indicator you say its been there for ever sounds like someone stuffed up on the build personally i would remove the timing cover and the heads then check cam timing and valve lift somethings out!!!! keys do split ang gears do slip also people use cams that they think are ok [looks good] but when you actually check them there literally little more that a wobbly stick Its still running shows how tolerant flatheads are to being out of sinc an ohv would have quit long ago