Leemom3:
The first-day-lesson you'd learn in a gasoline engine class is:
1) They need fire to run.
2) They need air to run.
3) They need gas to run.
Let's presume the first two are ok and go to the third. The reason for this is something you said about filling the tank 100%.
Just as you can hold a liquid up in a straw if you put your finger over the end of it, you can sort of do the same thing with your engine if the tank is not vented. The cap is acting like your finger over the top of the gas tank and holding back fuel from the carb. See (3) above.
I can foresee a situation where you filled the tank to the top, attached the cap, (I'm supposing the cap has a little twist vent on it) but didn't open the air vent. Once you opened the petcock to let gas into the carb, the vacuum held back the gas.
A little of it seeped into the carb in time, and you were able to start the engine, but you may have been cranking with a practically dry carb all this time.
If no fuel wasn't the case, maybe too much was the situation, aka, flooded. To keep the engine from flooding, you could try pulling the choke out, yanking ONCE, pushing the choke in and pulling a few times. If no start, pull the choke out, yank once again, push it in, and then try again. Even trying a 'half choke' may prove to be the trick.