V12 Vibration under load: Any Ideas?

[NigelHCooper]

I have a series 3 V12 2+2 1973.

Recently replaced all suspension bushes, had wheels balanced, checked and replaced UJs at rear, replaced failing vacuum hoses.
I am suffering annoying vibration when under throttle.

I am now convinced (because of the other work) that the engine is causing the vibration. (Only other spiining part is the propshaft and UJs seem fine.
Engine revs freely at standstill but when driving vibration builds with speed. (Clutch in it dissipates). It's a bit lumpy and hunts when idling but responds well to throttle.
All plugs new and sparking, carb diaphragms all relaced, it may be running a tad rich.
No nasty noises, just vibration.
I assume that the opus ignition is working ok (it works or doesn't surely?)

Could this be a carb problem?....Valve? (not looking forward to that diagnosis, so anything else first...)

I get the feeling it is a single cylinder but as there are 12 it manages to still run. How many can fail and still leave a running motor?

What should I check next?
The car used to run smoothly and I am running low on ideas.

[DaveK]

Nigel,

I was going to suggest maybe engine mounts but you seem to think that the problem is with the engine itself. Does the V12 have the rear engine mount like the 3.8 / 4.2? if that is incorrectly adjusted it can produce vibration. What condition is the crank damper in?
Also gearbox mountings?

Just throwing up other ideas because I really have no idea about the V12.

Dave

[NigelHCooper]

Thanks Dave, I think it's the engine but will check your things first.

I have read a lot since posting, all sorts of things could be at fault.
Fuel system... generally is there enough? (carbs are usually not the issue but fuel relief back to tank and blockages can starve delivery)
HT leads
Opus
Distributor (although that seems fine, no arcing or discolouration)
valves (not wanted!)