i put a crate motor in a few months ago and thought the bugs were out.the car runs great plenty of power no stumble etc.after about 300 miles or so it will start to hesitate and it gets worse the more you drive it.put in new plugs and it works great.i called barry grant and they said it is probably my power valve.is that something on my carb?they said to check my vacuum but they wouldnt tell me the cure.it is a 355 blue print engine,msd vacuum advance edelbrock 110 gph fuel pump speeddemon vs carb and a comp cam xe 268.this cam is suppose to make good vacuum but if this is my problem how do i fix it because everyone says i have a good setup.

 

i checked my mixure screws the ones towards the firewall were open three quarters the way out and the ones in the front were open one and a quarter.does that help anymore

 

Who tuned it? I assume someone adjusted it to make it run right. If not maybe you should do it yourself or get someone else to do it.