There's a technique to kick starting big singles. First, as Chucky said, no throttle. Do the kicking bit with the throttle shut.

For the next bit I'm assuming you've got the timing right and that it's not stuck / defaulted to full advance. The timing for starting should be around 5-10 degrees BTDC. If it isn't, for whatever reason, it will kick back. It's unusual for modern(ish) Jap stuff to have a timing problem but for the sake of your leg it's worth checking.

Next you need to get the piston in the right place for starting - slowly push the kickstarter down until you can feel it come up on the compression stroke. Now pull in the compression release and ease the piston just past the top of its stroke (you can feel it move). Release everything, let the kickstart come back to the top and come down on it with your whole weight. You need to get the piston moving as quickly as you can - a wimpy kick won't work. Keep the throttle shut!

The idea behind getting the piston just past TDC before kicking is that it then has two complete revolutions to get some speed / momentum going before the firing stroke (and at the same time the kickstart reaches the bottom of its stroke and disengages from the engine, so saving your leg). Half strength kicking or starting with the piston in the wrong place are the main causes of kickback.

If it doesn't start within 2-3 kicks you've probably flooded it. Switch off ignition, switch off fuel and choke, open throttle fully with compression release pulled in and kick the engine over 3-4 times to remove excess fuel. Then switch everything back on and start again.