Usually when mower engine rpm bounces up and down its from dirty carburetor. Some times fuel additive will remove the debris closing off the jet. Sometime it gummed up you can us carburetor cleaner spraying directly down the Venturi (hole in th carb.) Honda carburetor you can some times remove the drain plug in the bowl and spray the cleaner into the bowl and let it run out hopefully removing dirt that causing the fuel restriction. Then some times you have to remove the bowl, spray directly in the jet and spray the jet hole, needle and seat Area.
If all that fails the complete disassembly of the carburetor and thoroughly cleaning last resort. Done all the above many times to correct iradic rpm problems on small gas engines
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