Join Date: May 2014
Location: Maple Ridge, BC Canada
Model: 97 gl1500 SE
Like DriverRider said, with one vacuum piston not moving, it's not going to operate correctly, which might cause the fuel spitting issue. Here's a quick test - throttle up like you were, but then using your finger, carefully move the left vacuum piston to about the same position as the right one and see if the fuel spitting stops. If it does, you know it's not really a 'problem' - once the left piston/diaphragm is fixed, that problem should go away.
Re the left vacuum piston/diaphragm - as others have said, since it was just rebuild, the diaphragm may not have seated fully - it's less likely that it's torn (my first reply I didn't know the history, but it sounds like you recently had the carb rebuilt.) You might find that it's just not seated right and getting it re-seated might fix that problem. Some people soak the diaphragm in hot water for a minute and say that makes it easier to get it to seat properly. If you've got it this far, and your mechanic is away, you might want to try pulling the carb and looking at the left diaphragm.
Re the fuel leak - you'll likely need to get the carbs off to figure that out considering where it's coming from - if you pull the carbs to check the left diaphragm, you can inspect the fuel hoses under there while you're at it.
Bikes I've owned since June, 2013:
Since sold:11 CBR250R, 89 ZG1000, 92 Ninja 500, 85 V45 Magna, 84 DT200, 82 Virago 750, 92 Virago 1100, 04 Kawasaki ZZR250, 07 Vstrom 650 ABS, 88 GL1500, 14 Ninja 300, 86 V65 Magna, 96 VLX600, 08 WR250X, 09 KLX250S, 87 FZR1000, 14 Ninja 650, plus some more...
2010 Husqvarna TE250
2015 Kawasaki Ninja 650
2008 GL1800 with bagger kit