Take the air cleaner off, look down the carb throat, when you operate the throttle likage do you see a thin squirt of petrol in to the carb throat? Do this with the engine NOT running. There should be a nice positive squirt of petrol. If not the accelerator pump is sus, the pic looks ok but, is is nice and plyable? Cant really tell from the pic but it looks old ( but as I said cant realy tell) if this is ok, you need to clean the accel pump holes, normaly the little V shaped bit under the big screw in the carb throat. This should have one or two tiney little holes, one on each leg. (some have 2 some have 1, DGAV's seem to bo ok with either) Normaly blocked after sitting a while. But could be a blockadge eneywhere along the the curcuit from the diapham to the V jet and from the float bowl. The jet with the neadle sticking out in the bottom of the float bowl, is the power valve jet (econamizer) NOT the main jet. This is operated by the power valve diaphram in the lid of the carb (its the 3 screw chap with a spring on it poking down from the lid) These very often go hard, and dont work, this can also cause a flat spot if its buggered. Does your carb have the LOVE valve? looks same as accel pump cover but on the other end of the float bowl? When listening to your vid, the flat spot does sound like a lean spot, this normaly points to weak (lean mixture) at this point, this should be corected by the accel pump, power valve and some times the LOVE valve, and could be due to any of these playing up or an air leak. Another thing to check is the idle air bleed, these can get blocked easy too. Look at a parts diagram.