I had a similar problem, and there was a flapper that had linkage to the carburetor, as the air increased, it would close the carburetor to lower the speed. As I recall grass or debris had been sucked in the top of the engine and had over time accumulated in the area of this flapper. To get to this device, you had to remove the top cover (in my case a metal cover).
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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