fuel problem i dont get it!!!?

hi this is james
fuel cannot get fuel throught to the combustion i have tryed 3 differnt carburetors and checked for blokeges and there is not any.i also have looked for blokeges in the fuel cap & the fuel line its all good.When i put a few drops of fuel in the combustion chamber it goes for a cupple of seconds. what is going on?
Any segestions much appreciated.