To synchronize depressions the procedure is as follows:
- close ALL air-screws completely and adjust the linkage so that all carburetors are just completely closed.
- screw the general idle-screw so that the engine can just idle
- Carburetor per carburetor, open the air-screw of the barrel which has the greatest depression until the two depressions are the same.
- Adjust the linkage between the two carburetors of each bank of cylinders to obtain the same depression on the four barrels. It may be necessary to retouch the air-screws but always acting on the same screw
- adjust the linkage between the two banks of cylinders. Again it may be necessary to retouch the linkage between the two carburetors of a bank and possibly air-screws
- after each adjustment, give a short throttle and let the engine come back to idle then adjust the speed of the engine with the idle-screw
At the end, each of the four carburetors must have one of its two air-screws fully closed.
Then adjust the richness of each barrel with the gas-screw to obtain the highest engine speed but on an 8 cylinders, it is necessary to have a very fine ear or a lot of experiment, from where the utility of lambda probes.
I installed lambda probe nuts on the exhaust of each cylinder of my Westfield (2 liters Opel 4 cylinders engine with two Weber 45 DCOE) and I confirm that 1/8 turns on the gas-screw makes the lambda vary significantly. It's a very fine tunning.
I do not know in the US but in France there are almost no mechanics who still able to do that, the good repairers of motorcycles, when they agree to work on a car, know how to do.
It is already long to properly synchronize a four-cylinders, I do not imagine what it is for an 8. Good courage.