What year is your motor??
Verify your getting good spark on both cylinders.
Is there any chance your flooding the motor out, to much choke?
If you pull the plugs and if they are soaked then dry them off, then reinstall.
Try to start without choking the carb.
You may need to adjust the air fuel mix on the carb. At the start of the forum page there is a sticky which has a link and sink by FrankA.