Hello, I'm Chris.I will do my best to answer your question fast and accurate using the info you have provided. Thanks for visiting Just Answer.
Does you mower have a throttle control or does it always run at full throttle?
Have you changed and or cleaned the fuel filter?
It runs full speed all the time. Where is the fuel filter under the carb?
Your concern sounds like the carb is clogged up or the fuel is restricted. If there is no visible fuel filter in the fuel line from the tank to the carburetor then you have a filter inside the fuel tank. It should be cleaned by back flushing or replaced. As for the carb you will need to remove it noteing the cables and bolt locations then remove the fuel bowl and spray clean it all out using brake or carb cleaner. After a engine sits over time they often accumulate moisture and trash in the fuel bowl. Below is a picture from my website access to honda showing a carb breakdown. Since I dont know your model number it may differ a little.
I also had a problem last year with the engine running out of control like the throttle was stuck wide open. any ideas