Just a note for you on the original question, the mixture screw setting for the late model 550 carbs. There is actual no adjustment function on that screw. So you turn it the proscribed number of turns out and then leave it there - you don't screw it in and out in the hope to make gradual improvements. That screw is actually an immediate On/Off valve, not a valve that lets out more as you open it farther. That is not true of the pre-'77 carbs, in which a similar screw has a very different function and does act as an adjustable valve.
The way you can tell the two different types of valves apart is to look at the end. If the valve is adjustable it will have a tapered (pointed end like a needle) tip that will allow more fluid to flow past it as it is retracted. The other type of screw will shave a stepped end, cut at 90 degrees to its length. The seat for this type of screw is a straight hole and so when this valve is opened the proscribed number of turns the shoulder clears the top of the hole and fluid flows but retracting it more does not increase the size of the opening and so no more fluid flows. When you see the specification for tuning this type it will typically say something like open 1 1/4~1 1/2 turns. The reason is that it takes 1.25 turns to open the valve but if you open it more than 1.5 turns it puts too little pressure on the retaining spring and the screw might vibrate loose and fall out. The range they give you is not a range in which the thing is tunable - its more like a bolt torque spec.
