Fuel tank pick up and antisiphon valve

[BRIELLY]

I took out the pickup and replaced the shut off valve but both looked fine. I ran it on a seperate tank and the bogging down problem remains. Is there any other fuel related issues that I'm overlooking? So far Ive rebuilt the carbs and fuel pump. Changed all filters and checked the tank. I'm beginning to think this may be an ignition problem which would suck fro me being that I'm not good with that kind of stuff.