bought this to fix fuel drain back problem

Think about it for a minute.....the fuel the float bowl in your carb cannot flow all on its own up, through the needle and seat, and drain back into the gas tank. It is physically impossible as the needle and seat is above the fuel level in the carb. It would be kind of like the water in your kitchen sink all on its own forming a water spout and going back into the faucet. The only plausible explanation - and every thermoquad that I have owned has done this (I haven't had many AFBs or their variants, but they may have the same issue) - is that the fuel evaporates, at least to a point where the accelerator pump is dry. An electric fuel pump causes this problem to go away as it runs before you try to start the engine, re filling the float bowl. When my carter equipped cars sat for an extended period of time I always cranked the starter first for a few seconds so the mechanical fuel pump re primed the float bowl, then stopped, pumped the gas pedal twice (accelerator pump) and they fire right up. Also realize that the accelerator pump pulls gas from the float bowl, not the gas tank, so pumping it 15 or 20 times to try to prime the engine is pointless unless the float bowl first has fuel in it.

I would bet that you will find your problem is still there.

Just my 2 cents