If you have more than one carb you may have to balance them. If they are out of balance the cylinders fight each other causing vibration. The best way is with some form of Mercury Manometer. The manometer measures intake maifold vacuume pressure. You want the pressure to be the same in all of the carb manifolds. In the carb manifolds, or on the front of the carbs themselves, you will find small tubes that you can push a rubber hose over, or little threaded holes that you can thread a fitting into. You attach the hoses from the Manometer to these tubes or fittings.
With the fittings attached and the Manometer connected, start the engine. It is a good idea to have a large fan blowing on the engine to help cool it. With the engine warm, adjust all carbs to read about the same manifold pressure. Normally, this is within 2 cm Hg of each other. Just get all the carbs reading within 2 cm of each other. The Manometers are marked in 2 cm increments. ( Usually there are only three screws to mess with. ) You match carbs 1 and 2 to each other, then 3 and 4 to each other, then match carb banks 1/2 and 3/4 to each other. The adjusting screws are between the carbs and hard to get to unless you have a Special Tool. You have to work fast. Once the engine gets too hot, it will be hard to get good readings. If this happens, let the engine cool down then re-test. As always, a shop manual is very helpful.
You really must have the Manometer if you have CV type carbs. However, I have noticed that if the CV carbs are in balance, the engine will rev up smoothly with no stumbling, when you whack the throttle open hard, from an idle, with the engine in neutral. If it stumbles, they are probably out of balance. Be careful and don't over-rev the engine. If you do this with an engine that has normal slide carbs, the engine will die. This is normal. With too big of throttle opening at idle the vacuume disappears. With no vacuume, no gas is sucked from the carb. No gas... No run!
Some carbs have no Manometer fittings. If you have one of these, you will have to invent a way to make sure the throttle slides rise and fall together. Take the air cleaner off and stick your fingers in two of the carbs and feel the slides move ( Engine off, I would hope !). Listen to the slides hit the throttle stops. Look at them. Put pencils under the slides and see if all the pencils move the same as the throttle opens. Lots of funny ways to do it. Maybe even try looking in the shop manual ! If the carbs are normal slide carbs, that is, with a solid connection between the throttle cable and the throttle slide, you don't absolutely have to have a Mammometer...but it sure helps. They do make a Manometer that fits on the carb bell, but I'm not impressed with the one I have.
I am suprised at the amount of E-Mail about my little Mammometer typo! I think I shall leave it just because it seems to bother so many men. Strange, no women have complained... only men! I mean really guys, it does kind of look like the right tool, doesn't it! But I don't want to disappoint anyone so here is a PDF file of a patent for a real Mammometer. There you go guys... have at it!