I would say if it didnt have the stumble before you installed the new rubber piece that you need to go over your work and see what you did. I am not sure what rubber piece you are talking about but if it has to do with the needle valve, it may not be allowing fuel into the bowl fast enough or you messed with the float and too much fuel is coming in and flooding. It will affect your drivablity either way. If you replaced the power valve, something is wrong there as that gives the engine the extra fuel needed to accelerate.
Like I said, check your work for proper operation.