Outboard motor sneezing is an audible result of a lean fuel mixture resulting from problems in the carburettor. Described variably as a sneeze, cough or stutter, an outboard motor or any engine will lose its ability to run smoothly if it does not receive enough fuel. In small engines, the culprit is the carburettor. On modern automobiles a fuel injection system has replaced the carburettor. However, small block engines, such as those used for outboard motors, still use carburettors as a part of their fuel system. When the carburettor fails to provide the correct mix of fuel, the outboard motor will no longer run smoothly.
Improper Fuel Mix
When an engine receives too much gasoline, it is described as running rich. The engine is flooded with gasoline and not enough air to allow combustion. However, when an engine receives too little gasoline, it is described as running lean. When the injection port from the carburettor becomes damaged or clogged by foreign material or general wear, the engine becomes starved for gasoline. Pressure build-up along the gasoline line will intermittently cause gasoline to spray into the engine causing the stuttering or sneezing effect from sudden combustion.
Adjusting the Mix
Carburettors contain a screw assembly that allows the owner to adjust the mix of fuel manually. Unless the carburettor has been rebuilt recently, these screws should never need to be adjusted. However, if you have recently taken it apart for cleaning and the outboard motor has begun to sneeze, check your owner's manual for the correct screw positions. Typically, screws should be out 1-1/4 turn from flush.
Improper fuel storage is the biggest cause of gunky build-up in a carburettor. Gasoline is a highly volatile liquid and even storage for a few weeks can cause significant evaporation. In particular, evaporation from the float bowl of the carburettor leaves behind a gum-like substance that can break off and clog up the injector needle as well as other parts of the carburettor, reducing the amount of fuel that can reach the engine for combustion.
Even if your outboard motor ran fine for several hours or even days after bringing it out of storage, the cause of sudden sneezing could be carburettor gunk breaking loose and gumming up the works. The only way to fix this is to take apart the carburettor and use a low pressure aerosol carburettor cleaner to remove the build-up from all the parts.