It would help if you told us the make and model of carburetor. Essentially you usually start with both screws open about one and a quarter turns, then adjust them both with the engine running. Walbro recommend opening them one and a half turns IIRC, so you will be leaning them off throughout the adjustment process. Are you sure you have a strong, consistent spark? If you don't, you are wasting your time with the carburetor, it isn't to blame.
If you have ignition and a good spark plug, you should be able to start the engine - but there is a ritual that has to be followed with the choke. In most cases the ritual is, with the engine cold and clear of fuel (that is, not when you have already fouled everything up by pulling it over repeatedly with the choke closed) you fully close the choke and pull the starter until the engine burps - that is, runs for no more than half a second then stops. This usually takes about three pulls, but on a few engines it is more difficult. You then half-open the choke and pull again. It nearly always starts on the first or second pull on half choke. However if it has a belly full of fuel due to being pulled on full choke after it has burped, you will have little chance of starting it. You must commence the ritual with the engine clear of fuel.
