Two Stroke Engine's Question. (mowers or boats)?

why a lean condition causes an increase in RPM:

Engines run on a mixture of fuel vapor and air (everybody knows oxygen is required for any kind of fire). Too much of either, and you get no combustion. The ideal mixture for gasoline is said to be approx. 14 parts air to 1 part fuel (at sea level -- of course it depends on the oxygen content of the air). You can probably tell, I'm no scientist.

For a variety of reasons (most of which are unknown to me), these engines are metered to run slightly richer than 14:1, with "rich" meaning more fuel, less air. One reason might be that the lubricating oil is mixed with the fuel, so if 100% of the fuel is burned, there's no oil.

So there you have it, as the carb starts to run dry, the mixture improves (gets "leaner").

Note that if you have a multi-carbed engine, the top carb(s) will run dry first, and those pistons get dragged through dry holes as long as the others are able to keep running -- for that reason, I do *NOT* run my 3-cylinder outboard dry. Instead I stabilize the fuel with Seafoam. If for any reason I won't be able to go out & run the motor every few weeks, I'll drain the carbs. If I know it'll be a few months before I get back to it, I just remove 'em & store 'em in the garage.

This increase in rpm is due to a lean condition created by lack of fuel. If this lean condtion was prolonged, engine damage would be expected, but as the air fuel mixture soon reaches the point of not enough fuel to create combustion, the engine dies from lack of fuel.

Yep, the other two got the answer.

The engine leans out and increases the rpms

the fuel mixture leans out as it tries to draw the last little bit of fuel out of the bowl,