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...

Might be engine mountings.

My A12GX also has a slight vibration when taking it up past 5500rpm.
The gear lever does'nt vibrate but when holding your hand the whole time on the gear lever you can feel that it has very slight vibration. I was informed that it is because it is not aftermarket balanced and normal for a stock factory engine.

mine buzzes pretty good, i think it gets worse the lighter your flywheel is.

Yeah, my flywheel was lightened by the way.

i think it has to do with the lack of a harmonic balancer, and the vibration is partially "balanced enough" crank components... if you had all the rods and everything weight matched and had everything balanced well then it probably does that less..

Did you get the flywheel balanced after it was lightened? Because the flywheel acts as a dampener, just like a harmonic balancer.Take some of the mass away from the flywheel & it'll let the engine buzz.
It could also be that the flywheel is now itself so out of balance that it is the cause. You could either pull that flywheel out & have it balanced, or get another one lightened then balanced, & swap them over.

Yes. It was lightened, but not balanced. As you said, i'll get my spare flywheel lightened & balanced and i'll swap it over. Thanks alot.

I also then suspect it to be the flywheel.
Mine was skimmed quite a few times.About 4 times and now I suspect that it was also enough to get it out of balance a little.
I have a stock standard spare A12 one, but are having trouble desyphering the picture that Dodgeman posted about lightening it to factory race specs.

Aren't you all lightening the flywheel on a lathe? That should evenly remove material from the flywheel. A-series flywheels don't normally need to be balanced, unlike a few other engines.

A15s are normally pretty smooth at 5000 rpm. Does it happen in every gear at 5000 rpm? Or only in 2nd? It could be a road-speed problem...

check my youtube videos... my car is incredibly loud inside, not particularly buzzy, not compared to other 210's i saw on youtube, and my exhaust has a squeaky hanger