I am a certified outboard marine tech and have owned my own part time business at home doing this and also worked for a $tealership for a few years. Contrary to what many mechanics do with customers, I have always tried to give my customer a simple suggestion to try before bringing a motor to me for a simple carb issue. I will normally tell them the little trick I am going to share with you and if that doesn't take care of the problem, then I tell them to bring it in. Get a can of concentrated fuel injector cleaner just like what you would buy for your car. Most of the time, a bottle of this will do anywhere from 15-20 gallons of fuel. I tell the customer to just mix a tank at about twice what it calls for and try it. Typically, this would be around an ounce or so per gallon. Mix this in with a tank of fuel and run the engine as you normally would. I have always used the injector cleaner instead of regular carb cleaner, as I feel it is refined a step further.
Another thing to look for is to keep an eye on your oil level carefully. If you notice the oil level starts rising, don't run the mower anymore. What is probably going on is that your inlet needle/float assembly is stuck open. When this happens, the fuel continues to gravity feed into the carb and the float will not shut it off when full. The fuel level will continue to rise and eventually, it will run out from the "throat" of the carb and into the cylinder/s. When this happens and the engine is not running, it will drain into the cylinder on top of the piston. Since you have a small ring end gap in the piston rings, the fuel will find it's way thru the gap and drain down into the oil. As this happens, it will thin the oil down and can actually wash down the bearings, as the oil has thinned out too much to lubricate.
This was a common problem at one of the $tealerships I worked at as they sold 4 wheelers and over half of the ones we sold had this common problem. We would typically have 4-wheelers stacked in crates for 4-6 months at a time before they were ever uncrated. When we did uncrate them, they ran fine for a day or 2 then developed this problem. I called the manufacturer and discussed the problem. Come to find out, they would start a new model year somewhere around March and not release them until August or Sept. At the time they were assembled at the manufacturer, they would put around a quart of gas in them, test fire them for 1 minute and then shut them off. Let's say this happens in March and we didn't uncrate them until November. This is nearly 9 months that the fuel has been in the carb and has began varnishing by then. As the fuel evaporated from the carb in the months, the varnish would happen faster and start causing problems. Because of this, I either added injector cleaner to them after starting them up, or the ones that already had signs of problems had a carb cleaning done before ever being sold.
What I would suggest with any lawn mower, tiller, chain saw, outboard, jetski, or anything that is going to be stored for several months...Add fuel stabilizer to the fuel tank and run the motor for at least 10-15 minutes before storing it so the stabilizer can mix with the fuel and be distributed thru the engine. Don't do the old "grandfather trick" that has been passed down for years. This involves shutting the fuel valve off and letting the motor run until it runs out of fuel. Truth of the matter...you didn't run it out of fuel. You ran it down so the fuel level was so low (lean), it could not run anymore. The reduced amount of fuel in the float bowl will now evaporate faster, leaving varnish deposits in the process. When this happens with a 2 stroke engine, you also managed to rob the internals of your engine of the oil that lubricates the engine and protects it from rust thru it's winter storage.
Last tip...Trash that metal fuel storage tank and buy plastic instead. The metal will cause condensation at the bottom of the tank due to hot/cold changing temps from day to night. This condensation can cause the tank to rust and contaminate your fuel system. Fuel also tends to varnish faster in a metal tank than what it does in plastic. The plastic also has less condensation as plastic is a better insulator than metal.
