In the original 72 240Z maintenance manual, it is stated that the SU carb idle adjust nut be turned until the top stop position is reached. Then the nut should be backed off three turns. Then the nut on both carbs should be adjusted 1/8th turn at a time until the 2-1/2 turn position is reached. Does this jibe with other member understandings?

That's a good rule of thumb. Once both are set at the 2 1/2 turn setting, there is a simple way to set the correct idle mixture. First, make sure the valves are adjusted and then with the air cleaner off and the engine running, cover one of the carbs' intake horn with the palm of your hand. This will cut off any fuel/air to that bank of cylinders. The engine should run on the other bank pretty smoothly. With your hand still in place, adjust the idle on the other carb. If the engine is loping, open it up. If it is running smooth and fast, slowly close it down to where it just wants to die. Adjust it back out about an 1/8 turn. Once that one is adjusted, remove your hand and rev the engine to get things back to normal with both banks. Repeat on the other carb. Also make sure the carbs are synched before you do this.

Prior to this procedure, I assume that you have already adjusted each of the individual carb idle screws with the unisys gauge?

That's right. This should be one of the last adjustments made after valves, carb synching using your synchrometer and float adjustments.

Could you give me some direction on float adjustment? My guess is changing the position of the float tab that contacts the float bowl fuel valve.

If i remember correctly the hight of the float should be 9/16 of an in. To high and you'll starve your engine to low and fuel will spurt like gyser out of the vacum line nozle on the top of the fuel bowl lid.

According to the factory manual: 1 - Remove the float chamber cover and place it float lever side up. 2 - Lift up the float lever with the tip of your finger then slowly lower the float lever. Stop lowering the float lever at the position at which the float lever seat just contacts the valve stem. 3 - The float leve is correct if the distance from the top of the float lever (which is actually on the bottom now since you have everything upside down) to the inside top of the float chamber cover is 14-15mm (0.5512-0.5906 in.). Adjust to this dimension by bending the lever slightly. The level of fuel in the float bowl should be just above an imaginary half way point - about 23mm (0.906 in) below the inside top of the chamber cover when it is in place. A couple of things to keep in mind. Be extremely careful with the cork chamber cover gasket. If it is old and compressed, replace it. Also, when you pull off the chamber cover, be careful when you turn it over as the little pin that holds the float in has a habit of sliding out. It is a bugger to find if it falls on the floor.

the idle adjustment on a SU round top is on the side of each carb. it is what you adjust to ballance the two units . the adjustment "knob" that is under the intake and is about 3/16" thick and has knerled edges , thats the mixture adjustment next to that one is a knob that is like a large thumb tack this one dont mess with it is set at the factory. The mixture knob is the one that you turn up all the way to its stop , then back down about 2 1/2 turns . Then with engine completely warmed up you can set the mixture. That is if you have ballanced the carbs with a uni-syn and have properly tuned the rest of the engine . Spark advance , valve adjust , plugs and so on . If you want me to walk you through setting the mixture just e-mail me it really is simple.