3406E Miss

I found this problem.
I never would have guessed but it was so simple.
I actually found it because I had another problem which was the fuel levels in the tanks were not equal. I didn't know I had that problem. The passenger apparently wasn't using much fuel. The sending unit is in the passenger side so it appeared that I was getting 20 miles to the gallon until I ran out of fuel on the drivers side tank a few days ago. I went to the passenger side and low and behold there was big vacuum on that tank and it was full of fuel. I had some vice grips and a bottle of starter fluid so i blocked the driver side line and fired it back up and went ahead and delivered my load. The vent was blocked on the passenger tank.
Somehow this was effecting the fuel flow at high load and higher rpm and it caused a slight miss for about 2 seconds after I shifted gears at the higher rpm.