Can you see that it is drawing fuel into the carb? I know I have had a couple of engines that were a bear to start the first time as they wouldn't draw fuel up to the carb. I had to remove the fuel line off the carb blow in the vent and then put the line back on while pinching the line to keep fuel in it. Then I had to give it a good shot of fuel into the throat of the carb to get it to fire. Once it fired for the first time and got fuel into the carb it was good. For some reason some carbs just don't want to suck fuel until the inside of the carb is wet with fuel.

Don't know if this is your problem, but it is worth a try.