Why is it impossible to start with a pull start? That's a small engine. Should be a couple pulls and running. Is it hard to pull or hard to start? Two different problems.
Hard to start probably means it isn't primed properly, the tune is completely out of wack, glow plug is shot or the glow heater battery is almost dead.
Hard to pull, it's new, lots of compression, lots of pinch in the sleeve or it's flooded. For compression/pinch problems, try loosening the glow plug 1/4 turn to start, then tighten it back up once it's running. Also, you could try heating the engine up with a hair drier. If it's flooded, stop pulling. Remove the plug, turn it upside down, rip the cord a few times to get the fuel out of the engine. Start over.