The strangest thing is going on with my Echo Back Pack Blower. It will not start with the tube attached. If I take the tube off, it starts right up and runs fine. Put the tube back on while its's running, and it runs fine. I have rebuilt the carb, new filters, new plug, still at a loss. Do not know what else to do. Please help, it's driving me crazy. Thank you.

The tube will add increased air friction so the velocity of the air is restricted when pulling the starter.

What might cause that?

The reeds or a small hole in the fuel system (e.g. the priming bulb). Increased piston friction. That's always a problem with the aluminum block of the Echo stuff. The engines overheat if the cooling fins get clogged by leaves.

Yes the tube is increasing the "back pressure", check the tube for any blockages, also, check what kiss told you to check.