I have an older Stihl 041 AV saw that would not start. To troubleshoot the fuel supply, I poured some gas mix directly into the carburetor - it fired up immediately, but stalls when that fuel is burned. I removed and checked the fuel lines, and they are clear. I filled the tank with fresh mix. Still no luck. Could someone briefly explain how the fuel is supplied to the carburetor, and recommend some steps to troubleshoot the fuel supply problem?