Outboards that have multiple carburettors need to the carburettors synchronised in order to run smoothly. The process for doing this is different for two stroke and four stroke outboard. Synchronising the carburettors on an outboard will make it run more smoothly and efficiently. It can also prevent a two stroke outboard from "sneezing" periodically. On a two stroke outboard the process involves checking that all the butterfly valves in the carburettor open at the same time by the same amount. On a four stroke outboard the process involves checking that each carburettor's intake runner has equal vacuum to the others.