I just finished putting together a 1914, the engine was redone by a reputable T machine shop, so I am out of the loop on what was done, but it did look good when I pulled the head and bottom. I set up the electrical end and just tried to start by hand, boy is it stiff. No luck so I jacked her up still no good. My son and I finally hooked up a strap and pulled down the road. She finally lit running on battery and mag, but I went back to the shop because the bands were not set right. Now that I can stop it when pulled does anyone have a good answer why it will not keep running, I know it is getting gas, how much is the question, pull it again starts up sounds like a T should but quits.
Most (all ?) T carbs have a drain at the bottom of the carb bowl. Open it up and see how fast the gas drains out.
20 HP should be about 1 gal/hour. Put a bucket underneath and if you drain out a gallon in less than an hour, fuel flow should not be the problem. (probably best to do this outside ;o) )
Less than that, check tank, sediment screen & fuel tubing.
It sounds like a feel problem. Perhaps the tank sediment screen is plugged, maybe the fuel line from tank to carb. Another possibility is a plugged vent hole in the tank gas cap. Also, it could be a problem in the carb itself.
The first thing I'd do is check fuel flow.
Sam also open carb 1/4 turn. And,check gas cap to be sure vent hole is clear.
Try opening the spray needle more. A tight engine will require more fuel also don't forget to advance the spark till it's running sweet. Also check for vacuum leaks between the carb, intake and block.