goirish,

It sounds like you are trying to pull or flip the prop through compression. Big engines will try to kill your hand if you do that. Here's what you do.

Choke until wet, then flip over 5 or 6 times to get the cylinder wet.

Hook up the battery, but never flip the prop through compression.

Turn the prop up against compression just like you normally would if you were flipping normally.

Then slap the prop or twirl the spinner BACKWARDS so the engine bounces against compression going backwards. If will fire instantly, kick back and start forwards.

Once you get the technique down, it's 1-flip starts most all the time.

Note: if the cylinder is wet, when you try to pull the prop through past top dead center, the engine is going to fire before you get to TDC. This will tend to start the engine backwards.

If you set the prop level when it against compression and were going to flip it normally, you hand would be open palm upwards so you could pull the prop 'over the top' past compression. This will get your hand killed. Put the prop up against compression just like that, but do not flip upwards, turn your hand over and slap the prop with your hand going underneath the crankshaft. I always use a leather glove, but have never been hit using this technique. You do have to slap the prop down & under pretty well.