Lawn mower problem
 

My mower is reving constantly, it will not stay at one speed. I have drained the gas tank and cleaned out, cleaned and gapped the spark plug. No adjustments on the carburetor. Help!

My mower did the same... found that air filter was not tightly fastened to carb. body. To check this out..when mower is running try moving around the air filter body to see if you get the result you are experiencing.

Mine too
 

Troy mower with a honda motor. Bought the blue gas additive from lowes and added the correct amount. Problem solved within 15 minutes of motor running.

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.

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

The flapper under the flywheel cover is the governor, it works on flywheel air pressure. Some have mechanical governor also.