Backfire or sneezing through the carburetor
Backfiring usually indicates a lean running condition, stumbling or sputtering indicates its running rich. I'd pull a spark plug and see what the color indicates to be sure though. If it's doing it at a specific rpm range, use the kill switch while it's happening if you are using brand new plugs and you'll know for certain. If the exhaust or intake has been modified and the shop put the carburetor back to factory settings it would make it lean.
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