Small Engines (Lawn Mowers, etc.): surging of engine, bolt torque, air leak

Question
I have a five horse Brigs and Stratin motor on a rototiller.  Anyways it would not start and it had been sitting for a while so i cleaned the fuel bowl and the small holes that suck the gas into the carburetor.  Then i put it all back together and now it surging, like the rpms go up and down, up and down, all with in a second.  Is this an air leak or not enough fuel?  P.S. the only way to make it run constant is to have the choke pulled out half way.

Answer
Difficult to say.  Have you checked the engine top no-load RPM?

If the engine rpm or governor spring is weak it will cause the engine to surge.

I'd check all the gaskets and bolt torque to make sure there is not an air leak.  Also check the engine mounting bolt torque.

The reason is may smooth out with the choke half on is that you are changing the engine RPM.  
Are there any adjustments on the carburetor?
Let me know.
Eric