not enough backpressure?

[jkeesey]

Okay, so I recently put a glasspack muffler on my A with a short straight pipe after it. The entire exhaust is about 3ft total. After doing so it seems to have a miss under load. I went through the distributor and the entire fuel system and the only thing I can think at this point is there isn't enough backpressure. Does this sound right to anyone or am I just pissing in the wind.