To expand on that just a little bit: yes, the variability suggests that there is a pressure leak somewhere, as d_jones says, so check that. However, the fuel leaking from the carburetter would most often mean that your regulator is letting fuel through when it shouldn't - possibly because of some dirt in it. You do have a fuel filter in front of it, I hope? See
this useful fact sheet for plumbing schematic and other information. The regulator may also have a weak spring, or someone may have loosened its adjustment screw a bit much. You might want to turn it 1/4 turn clockwise, and see if that stops the fuel leak.
Note that you can't just start these engines, and expect them to run at full throttle right away. An easy way to start them is to run the starter with the engine at full throttle, and
no glow driver attached for about ten seconds (to get some initial pressure into the tank, and fuel to the carb), then throttle back to a fast idle position, attach the glow driver, and start. After that, you should let the engine run at a fast idle for half a minute before attempting to throttle up, because it needs to fully build tank pressure, stabilize the fuel flow, and warm up a little bit.
You can now tune the main needle and idle screw as per
the instructions. What the instructions don't say, is that the regulator might need some adjustment, too. You can check that, once the high speed and idle are good, by checking the transition from full throttle to around 4000 rpm, and back to full throttle, just like you checked the transition to and from idle. If this medium speed position seems rich, turn the regulator adjustment screw clockwise, if lean, counterclockwise. Just a little bit at a time, no more than 1/4 turn, and then 1/8 turn increments once you're near the correct setting. As with all the other adjustments, always remember that with the YS engines, it takes time from you make the adjustment until it "takes"; give the engine time to stabiIize after any adjustment.
If you have to adjust the regulator, you should go back and redo the high speed and idle adjustments, and then check the medium speed again.
Oh, and yeah, if you've got the plumbing set up as in the fact sheet, above, you won't be able to pump fuel out of the tank, because the pump will draw air from the carb through the regulator. I put a clamp on the fuel line between the tee and the regulator when I need to empty the tank.