That can be caused by several sources.
If it has a gravity flow fuel system, could be the carb flooding, needle and seat not fully closing letting fuel drain into the engine while it's shut off. Symptoms are fuel loss or empty tank after storage, liquid locked engine when trying to start, engine flooded when trying to start, black smoke when first started.
Or if it has a fuel pump, the diaphragm can leak fuel into the engine. May or may not show symptoms, depending how bad the leak, but can cause rich running, or fuel starved under load.
And a rich running condition can do it. Choke not fully opening, flooding, clogged air filter. Symptoms are black smoke, poor fuel economy, poor performance.
Whatever the problem, it needs to be addressed ASAP, not good for the engine!