From what is sounds like, you are blocking the air at the pipe. Pull the pipe and put a plug in the exhaust manifold. You can get one at the hardware store. I use the rubber plug with a wing nut on it. Put it in the manifold and tighten it up. A stethoscope is good or a length of tubing for checking. You can also spray simple green on critical areas and look for bubbles. Check around the powervalve plugs. Those o rings are easily damaged when sliding the plugs in. Crank it up a little higher and then log the drop per minute. You are definitely on the right track.