For an A15, is it common for the engine to vibrate at about 5k rpm?.

Well, mine does. Vibration not in sense as though the engine's gonna die off due to low idle, but its a kind of high frequency vibration. Even the gear lever rattle's terribly.
This problem was not obvious before, but now, its quite bad. I even tried advancing & retarding the dizzy, it makes no difference.

Anythig to do with the mounting perhaps?...visually it looks fine for me...