Here you can see both carbs installed on the bike. The right carb has a 5mm brass screw of the correct thread pitch installed in the new vacuum port with a small aluminum sealing washer underneath. You could also use a vacuum sealing screw and washer to fit various Honda 4-cylinder models. Honda part # for the screw = 93500-050080A. The crush washer is # 90452-323-000. This mod may also be performed on the center and right carbs of 1972-73 GT750. Do not over tighten the vacuum port plug screw. Provided you've used an aluminum washer under it, it will not leak or fall out. You now have a factory-like arrangement for accurately synchronizing the carbs to obtain the smoothest engine performance possible. Any other method is pure guesswork. Warm up the engine enough so it’ll idle before proceeding, then shut it down. First, put maximum slack in the throttle cable by running the adjuster at the handlebar all the way in. Then run the idle speed screws out enough to allow the slides to bottom in the carbs, and adjust the throttle cable slack at the carb tops so that virtually no slack remains at that location (there will still be slack at the handlebar). Attach a carb synch tool to both vacuum ports, and set the petcock to "prime." Start the engine, and use the cable adjuster at the handlebar to bring the idle up to around 1500 rpm. Note the vacuum readings. If vacuum is higher on one side, use the cable adjuster on the higher side carb’s top to raise the slide slightly and balance the vacuum signals. The rpm will increase slightly – that’s okay. When the vacuum signals are even, snug the adjuster locknuts at the carb tops, blip the throttle, then check to be sure the vacuum is still equal. Shut down the engine, and run the handlebar cable adjuster back in all the way to provide maximum cable slack. Now turn the idle speed screws back in enough to allow the bike to idle. Start the engine, and adjust the idle speed and carb balance using the idle speed screws (not the mixture screws). When the rpm is where you want it and both carb vacuum signals are equal, shut the engine down. You're finished. Reinstall the vacuum line between the petcock and left vacuum port, turn the petcock to "on," and adjust for correct throttle play at the handlebar cable adjuster. The play should be as small as possible without causing the idle to rise as the handlebars are turned full lock. If it does, loosen the handlebar cable adjuster as necessary to eliminate that. Lastly, check/adjust the oil pump cable with the engine not running, using the pump cable’s adjuster to align the match marks at full throttle. Don't forget to install the plug screw in your new vacuum port. If you're working on a three-cylinder bike, the balancing procedure is the same, just one additional carb. Adjust 3-cylinder oil pump as per the shop manual for your bike.