If you're comfortable with your ignition timing do the following:

Turn off & disconnect the LPG tank
Bleed propane from the fuel system- (push in little nipple inside brass fuel coupler which connects to LPG tank- make sure the coupler is pointed AWAY from you as the LPG is very cold & will frostbite any part of your skin or eyes it comes in concact with)

Remove air cleaner tubing to mixer, depress throttle 1/2 way down & spray starting fluid into mixer. Crank engine over while spraying either (have fire extinguisher handy in case of backfire). Engine should run on either for a few seconds at least.

If your ignition timing is correct, engine should fire. If engine won't start or even try to start, then your ignition system needs to be gone over.