It's been a long time since I had a Moby w/ PHBG, but I remember a couple of tips I heard to improve fuel flow. Moby 70cc+ engines tend to be very 'buzzy" with vibration so the spring loaded needle was recommended to me and I think it did help my build.
Also the "metal disk" mentioned in the Wiki is another potential factor. Try removing it if you are using it, or if you are not using it maybe try opening up the holes and running it to see if there is a difference.
"It sits between the main jet and diffuser tube. It was designed to break up air bubbles in the fuel when high vibrations cause foaming within the carburetor Bowl. This help prevent air bubbles getting trapped within the fuel metering system which would cause an extreme lean condition under high throttle power demands. Many have removed this disk because the new fuel which has been developed has different properties of tension thus it tends to trap the air bubbles underneath the apron causing extremely lean conditions under high vibration modes at full power or close to it. By enlarging the holes in the disc will allow the air bubbles to flow through this mod must be done especially if you're running fuel with any alcohol in it and most new fuels due to the difference in the chemical blending from when this device was originally designed to work with the fuels properties of tension.
I have found that running carburetors that suffered from heavy foaming that by reinstalling the disk with larger holes assisted greatly with breaking up the bubbles and causing them to move away from the main jet pocket under the desk."