Small Engines (Lawn Mowers, etc.): Craftsman backpack blower, fuel tank level, fuel cap

Question
Hi, I have a craftsman backpack blower model #316.79499 2-stroke. My problem is that if the tank if full it runs fine, as soon as it gets between 3/4 and 1/2 full it starts to bog out at high rpm's, do you think this is a carb or an air problem, if I fill it up when it begins to bog it runs fine again till it hits that fuel tank level again. Any Ideas? I can get a carb for it through sears for about 46 bucks but I thought I'd ask around before spending the money.
Thanks,
Karl

Answer
First,  make sure the fuel cap vent is working properly, and not making a vacuum in the fuel tank. Secondly, make sure the fuel filter is laying in the fuel tank correctly, and able to reach the fuel when the level drops (might have shifted positions), Third, check the entire fuel line for cracks, or brakes ( very common with these smaller tools, the lines dry out, and get hard, and brittle) The crack or brake could be sometimes covered in fuel, then at such a level out of the fuel and sucking air. .......Tom