Air inlet on fuel tank cover blocked. (tiny hole in cap blocked)
Idle adjustment on carburetor/mixer incorrect.
Too much or too little "drag" from the trim head. Remove wire from spark plug before you hand check, and with a cold engine. You might get a serious cut if it decides to start.
Most of the time it is:
Air filter dirty.
Screen in side fuel tank clogged.
Air inlet on fuel tank cover blocked. (tiny hole in cap blocked)
Idle adjustment on carburetor/mixer incorrect.
Too much or too little "drag" from the trim head. Remove wire from spark plug before you hand check, and with a cold engine. You might get a serious cut if it decides to start.
Crud build up in fuel system or leak in primer.