No fuel to engine

you can check if it's venting by simply removing the gas cap and watching the flow. if it flows, put the cap back on. if it stops then, it's not venting properly.

another thing to check is a clogged petcock, fuel line, or jets. the stock tank is notorious for having dirt, rust, or a mysterious white powder in them, sometimes even silver paint.

your float level may be too low, also.

or, do you have the carb with a secondary petcock? or, are they turned the correct way? the lever points in the direction of fuel flow. sideways is off.

I am assuming that we are talking about a Chinese in frame engine kit. My first guess is the fuel valve is not flowing enough fuel.
Have you checked it for good flow?
Turn the gas off and remove the fuel hose from the carburetor and put the end in a coffee can or some kind of disposable container you can see down into. (Do not use a styrofoam cup!) Turn the fuel valve on for a few seconds and watch the rate of flow out of it. It should be a steady stream. If it is a trickle then there is a problem.
There is a very fine mesh screen on the intake tube of the stock fuel valve. Also the stock fuel tanks commonly have alot of rust and junk inside them that can quickly clog this screen. If the screen is indeed clogged, clean it out, do not remove it. It is there for good reason.
What type of carburetor do you have? There are 2 commonly sold with the kits, one with a valve on the side of it and one without.