Report "My Toro 21" walk-behind self-propelled mower works well for about 30 minutes. After that, it stalls, sputters, and acts like it isn't getting any fuel. Will barely take an idle, but quits after trying to engage. If I let it sit for a day, it will run great, only for the next 30 minutes. Have cleaned the air filter, and and run a wire through the fuel line looking for a block. No luck. Model # is 20692--- can you help?"