missfire

can anybody think why my 2a has developed a missfire i swappped my manifold gasket i went from 2 single gaskets to 1 single and it only ran on 2 cylinders allmost like no petrol is getting to the 2 plugs nearest the bulkhead but i do have a spark from both plugs. i have since put the 2 single gaskets back on but it is still reacting the same only running on 2

you haven't got any old gasket down the ports have you?

did you slacken 4 bolts holding two manifolds together before fastening to head face,the 4 studs clamping two together have room to let them move so that both fit face square and equally ,i should think that you may have a gap