As they sit, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems.
Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.
More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carb. It Must be removed from the engine, cleaned very well, blown out with compressed air and reassembled using a NEW carburetor rebuild kit. ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carb too. Dirt and water from a dirty fuel tank will also plug up the carb. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carb kit.
The float could be defective as well. It could either have a hole or small pin holes it it.
Or it could just be misadjusted.
Or there could be something stopping it from causing the needle valve to make a tight seal to shut off the gas when there is enough in the bowl.
In any case, you should not have gas in the muffler.
Worse yet, if the carb is not working properly, the gasoline will find its way into the crankcase of the engine.
It will mix with the oil and also increase how much oil / gas mixture is in the engine.
Too much and you can have all sorts of problems
The oil / gas will push past the rings into the cylinder.
It will foul the plug, and not provide enough lubrication in the cylinder. It may run, but you may do damage.
Check to see if you have gas in the oil.
Check your oil level.
Remove and either clean /dry off the plug or better yet replace it.
Get the carb working correctly per above.
Change the oil.
Yes, you could have a timing issue, but lets check this first.
Has it sat for a while. How long.?