The petrol does not have many places to go.
1. In to the engine, where it can get used for running, or down the drain which is the crank case, or out the exhaust.
The float and shut of valve ( needle and seat ) could be buggered, but it may have injectors.
2. Leaks before it gets to the engine.
3. Leaks from the tank.
Check the oil level. If it is thin and smells like petrol, or the level is high, you have too much oil, too much fuel, or too much water in the crank. If the oil is white, you probably have a blown head gasket. But if your engine is air cooled, scratch that.
When you run it on run or reserve, you are hopefully creating a vacuum to draw the fuel from the tank. When you run it on prime, the fuel just free flows. But you did not have the wherewithal to provide any details. You may have straight on off petcocks. Who the hell knows. No one unless you care to devolve some info.
UK