A bubble in the gas line isn't a huge deal, you can fix that pretty easy. Just bleed the gas line.
Ok carb work: If you go to www.themopedjunkyard.com and check out the "Sachs" page, you'll see a diagram of the Bing Carb. The jet are numbers 22 and 23. Make sure air flows through those. That black button pushes the float down, simulating that the gas level in the bowl is low, thus gas will start to flow from the tank to the bowl. When the level is high enough, a small cone shaped needle that is attached to the float, fills the jet and stops the flow of gas.
That jet, which I can't remember if it comes out or not, needs to be clear of gunk. The needle needs to be in good shape too, otherwise gas can leak if it's not coherent and smooth.
Just examine the carb. Anywhere you see small holes where it looks like gas should flow, make sure you can blow air through them. An air hose works great.
There is carb cleaner out there you can soak the carb in, but you can't have any gaskets, (which is the cork part between the two metal parts, #'s 7, 21 ) plastic or rubber sitting in the solution. After you soak it wash it with hot soapy water, rinse thoroughly and air dry with the air hose. NO WATER ANYWHERE should be present when you put it back together.
Just have a nice clean, free-flowing carb, and you should be getting the gas.
Did you try to turn the engine over while having the spark plug (wire attached) grounded to the cooling fins of the head? If you get spark, you should be in business then.
Yeah, so to get more top end out of my moped, I enlarged the plastic, cylinder shaped part of the air box with a hot, over sized rod. I made the shape of it like a trombone bell, so that the air flows faster over the curve. That allows more air to flow in. I also put some air scoops out of the intake silencer (gray part which holds the air filter). I am 170lbs, and it reaches 30-32 with me on it. Now I need to get my Tomos shifting into second.
Best of Luck,