I have never had a weber with as much power as dual SUs, but I have never had a new
weber either, so I wouldn't know, I do have a weber I rebuilt on my 520 project, as far as I am concerned, it's a piece of shit, as I have had dual SUs on this engine, and it had way more power than it has now on a much heavier rig.
I think I said this before, when you look down the throats of the carbs while it is running, both the pistons should be open/raise the same amount at an idle, if one is higher than the other, they are not in sync, and I would turn the screws on top of the carbs I have marked in the photo below, these screws are what you turn to sync the carbs, it only takes a little to make a big difference, a half turn is a lot.
Ok I have edited another photo, this one has 3 places I have marked with a white square also, the upper highlighted square on the back/right carb in photo is showing the screw used to make both carbs move at the same time when you press on the gas pedal, you adjust this by twisting the throttle shaft by the firewall and make sure that both carb shafts move at the same time, the other 2 lower highlighted squares are showing the fuel mixture nuts on the bottom of the carbs, these are how you adjust the fuel mixture(lean/rich), the leaner you run the engine, the harder it will be to start cold without a choke.
Here are a couple photos of my spark plugs, the first one is the set I took out yesterday, I believe that the carbs might have been a little lean, but I have both carbs about the same as they all look the same.
The next photo is a little more to my liking, the plugs are not as white, but they are not black either, you can see in the photo that the 2 outside plugs are a little darker, they were a little richer than the 2 inside plugs.
If your plugs are black, you are to rich, and as I said before, the leaner the carbs are, the harder it is to start the engine when it is cold without a choke.
“The difference between genius and stupidly is that genius has its limits” Albert Einstein