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You are going to need to remove your carburetor from the engine, clean and rebuild it.
Before anything make sure you diagram all your linkages or take some pictures so there is no question when you reassemble it. Once it is off the engine you are going to want to completely disassemble it then soak all the metallic parts in a good quality carburetor cleaner (most auto stores carry this) for 12-24 hours. Make sure that all the jets and emulsion tubes are clear. Running a small wire through each hole will make sure of this. Once everything is clean the carburetor can be rebuilt with a new carb kit.
Here is a good page that describes with pictures how to go ahead disassembling your carb and what to clean. Make sure you read through page two as well as that page is more importantCarburetor dissasembly page