Breakthrough finding,
If I blow air through the little black tube with the engine running, engine revs higher when given throttle than if I let the black tube remain untouched. The only side effect of blowing through the tube is that I see gas trickling out the intake when I blow through the black tube.
It seems it would be very tough to blow through that little black tube while the engine was running, don't know why you would do that, but lets try to use that info to help us. Doing that could only put pressure inside the bowl of the carb, which I think would force more fuel through the carb than the vacuum from the engine alone. One would think this extra supply of gas would make the engine rev higher as you describe.
Thing is, the clear tube that comes from the bottom of the carb is a vent, inside that vent connects to a tube that rises up above the fuel level in the carb (or it is supposed to be above the fuel level). If this vent is above fuel level and not clogged then the air you blow through the black tube should just come out the vent tube, if the fuel level in the carb is too high then you would get gas coming out of the overflow tube (since the fuel level is up above the top of the vent tube). If you get fuel out of the overflow tube when you blow through the black tube then you definitely need to adjust your float. Actually, if you ever get fuel coming out of the vent tube you need to adjust your float or fix a non sealing needle valve. I would check the vent tube to make sure it is not clogged, someone had said their vent tube came clogged from the factory. With the carb off and drained of gas try blowing through the VENT TUBE that comes from the bottom of the carb, if you can not blow through it then you need to open up the vent tube so it can vent your carb. I would look into where that fuel is coming from when you blow, if it is not coming from the vent tube then wherever it is coming from is probably an air leak....
One note about the black tube, some people say pulling this tube from the air filter made their engines run great, I tried it (before I adjusted my float and jetted the carb) and it made no improvement for me so I put it back. After I tuned the carb and had her running great through the entire throttle range with good idle and top speed over 35MPH I decided to pull the black tube again and see what effect it had. Much to my surprise I dropped off about 10MPH from the top speed and the engine was not running as well with the black hose disconnected. A short while later I put the black tube back into the air filter and she started running great again and the top 10MPH were recovered. So it seems, at least the way my carb is tuned, the black tube does do something and it is beneficial. I think carbs that are not tuned well, or tuned different, or perhaps at different sea level react to changes in this tube differently.
IMHO you are most likely fuel starved in that dead band of throttle. I used this simple trick to prove to myself that my engine was fuel starved in different throttle ranges. Warm up the bike, get rolling and pull the throttle into the range where it boggs. While bogging pull out the choke lever (which will allow extra gas through the carb rather than restricting air flow like a regular choke). If the engine jumps to life at the dead spot after you pull out the choke then you need more fuel in that throttle range. If it gets worse then you probably have too much fuel and need to do a float adjustment. You could also do a plug chop while the engine is bogging and then see if the plug is wet (flooded) or dry (fuel starved). In that lower throttle range it is possible that either condition could cause your bog....
Don't give up man, keep pushing and experimenting with it. I was at my wits end a couple times with the carb but after adjusting the float back and forth, fixing up air leaks and finally opening up my main jet I finally got what I wanted from this carb..... Good luck to you, I will be glad to help in any way I can....
