the accelerator pump on a honda keihin carburettor is on the right side back corner if you are looking in the engine room from the front, its a 3 screw setup, In original condition there is no gasket in there, the rubber diaphragm itself makes a lip seal on the outer edge and there are two tiny O rings that seal the shoot and intake ports. You need to replace all three parts and make sure the base is tightened very evenly - it WILL warp if you go nuts on the screws. (I have seen mechanics use a paper to try to seal its external leak - it creates a clearance in the pump and the plunger does not work to full volume.
The lever that pushes the pump rod is also finely setup, it should contact (not push) the plunger rod at idle position. If there is a delay in its pressing you will feel a jerk.
The secondary throttle diaphragm would also be shot (they usually are in 5 years on Honda) and they are not serviceable - they are replaced on Keihin carburettor.
Replace the carburettor mount gasket with a genuine part only or with a proper branded replacement part like DNJ or Victor Reinz.
btw - if your accelerator pump is not flat (check on a plate of glass or engineer straight edge) and is only slightly out of shape you can face it with some fine emery paste and a piece of glass - only 3 or 4 passes - Dont grind away otherwise the tiny ports wont hold the O rings.