I have a 351w in a 70-f100. The motor has about 5000 on a rebuild. The vac is really low (I think around 8") and I was wondering if a restriction in the exhaust could cause this? I ask about this because I had to do some modifying on the exhaust. I can now hear something rattling inside that muffler. Timing and vac leaks are all in check. The performance has slid down hill after more miles have been added. The exhaust has got to be replaced anyway. But could it have this kind of effect with performance?

 

plain and simple, yup. no air out= no air in. no air in = low or no vacuum

 

You have the answer, one of the darndest things to find was the double wall header pipes which collapsed. That problem wasn't limited to Ford either, I have a friend who pulled his hair out with the same deal on a Honda.