I am proud to admit I thought what dirkbalter wrote (he is an expert, I am an amateur).
This simple test shows overall flow efficiency of the fuel system, including return line (yes, if return line is clogged, you will have fuel starvation of the engine). Measure the flow of fuel at the end of the return line (at the tank). Put the end of return line into a canister - something you can see and measure fuel volume. Switch on the fuel pump for a couple of minutes to worm it up, then measure the flow. You should get, say 0.8 liter in 15 seconds, better if more. If it is 0.6 liter or less - there is a serious blockage in the fuel lines.
If you diagnose the block, go step by step: first filter is the screen in the fuel tank outlet, then there is a screen in the inlet of the fuel pump (if you do not have in-line filter before the fuel pump), then there is the main fuel filter, then there is return line that may be clogged (be careful if you decide to blow it through with the air, put some cloth around the opening).
Do not blow air to the tank. Be careful and take precaustions when working with gas.