I don't understand your use of a syringe. I use an empty bottle from motor oil with a spout style filler cap like you get on a bottle of 90 weight gear oil. Then I push the fuel line onto the spout and connect it to the carb fitting. I added a hose clamp on the spout after my line slipped off one time......
 

Yea I once had a setup like that but it's no longer. I am just filling a syringe and squirting fuel in the line till the bowl is full.  A full carb should run a minute or so but this runs a few seconds and dies. Its like if I put enough gas in the carb it doesnt have a choice but to suck a little in the cylinder. I know it sounds just like a stopped up carb issue but I can blow compressed air and carb cleaner through every passage.  Coukd they motor be sucking so much air that it simply doesn't have the fuel to even out the ratio?  Hell idk I'm just spit balling..Lol

Come on experts spit out some thoughts for me.. I'm at a stand still here.  I would be willing to send my carb to one of you if you would try it on yours just to rule out a bad carb?????  Haha

If you really want to I could throw it on mine to test. Worth getting the float bowl gasket and making sure the float height is correct before sending it off though.

Do you know the float height?

Page 81 in the manual; www.yamaha-enduros.com/index.php/service...-and-parts-catalogue

Got it running..

Bravo.

Hurray

WooHoo!