Merc 850 could be a 6 cylinder from the 60s or a 4 cylinder from 1973 - 1977
It takes three things to make an engine run, Compression, Fuel, and Spark. remove any one and it won't run. First thing is check compression, if the engine is in poor mechanical shape, you may wasting time and money. Next would be spark, easy enough to check for. If that is there then proceed with the fuel system. Is fuel getting to the carb?