First thing is to make sure that the tank vents are not obstructed. Secondly, could your L-OFF-R tanks selector have an air leak, most likely to be around the shaft seal? I fought this for quite some time. Since the line is most usually under suction from the pumps a failed o-ring will generally not leak fuel. But it will allow air to be drawn in with the fuel supply to the carbs. If you have a return line to the tank from the distribution block back to the tank (and you should), it takes quite a large air leak for the engine to genuinely starve for fuel. However, as you get to that point, there will be occasional roughness. Plus, with any air mixed in fuel doesn't flow smoothly into the bowls, it "spurts" erratically causing both lean "miss" and periodic "flooding". Diaphragm pumps pump air as easily as fuel, so this WON'T generally show up as a fluctuation of fuel pressure!
My problem got impossible to deny when it the engine would quit running on the ground when the Right tank was selected - it seemed okay with the Left. Yet, it makes sense: since the volume pumped by the engine-driven pump is dependent on rpm, though the engine would start and idle okay since little fuel is needed, it would die at a fast idle of 2500-3000 rpm when more fuel was needed but the mechanical pump had too little rpm to keep up. Later, with the selector out of the airplane, the leak was confirmed to be worse set to "L" than to "R".
It's kind of messy to diagnose with the selector in the plane, but I did so and wrote a few pages about it and the repair. If you want to send me a private note, I can make arrangements for you to retrieve what I have on the subject via Dropbox.
