To understand this problem, you must understand a little about how a carburetor works.
When you place the choke in the on position, it creates more suction on the throat of the carburetor which creates more vacuum to pull fuel out of the main jet and up through the emulsion tube which mixes air and fuel. The idle circuit could also be in play during this start up. An engine needs a slightly richer or more fuel than air mixture when it starts.
As the engine is running and the choke is slowly released to open, less vacuum is created and less fuel is then drawn out of the main jet as the engine is brought up to speed.
What happens is fuel becomes a solid state as it evaporates and this plugs up the emulsion tube and the idle jet. If that happens the symptoms of only running on full choke will be present. At this point the carburetor needs disassembled and thoroughly cleaned of any fuel deposits that have accumulated. Then it will need to be re-assembled and tested to ensure that the carburetor cleaning did its job in fixing the my lawn mower will only run of full choke problem.
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