this may sound really really stupid but I did it once.....
..... do you have a plug in the silencer??
I was in the middle of rejetting when a box from tucker rocky came in the ups truck so I stopped for a moment and opened the box ..looked at my new socks, my new flywheel puller, my new chain and sprockets, and my new uni silencer plug (aka butt plug)
I opened that up and stuffed it in my silencer and went back to work installing the carb...well I tried to start it up when finished and no go ..so back and forth for the next 4 hrs removing and reinstaling the carb...got so pissed off that I went inside for a while came back out and was scrathing my head thinking wtf... I looked down and NFW I pulled the plug out and kicked it and ...ring ding ding
good luck