Carb diaphragms are extremely expensive, about $60 each, as they are only sold integral with the slide. But they can be repaired easily and quickly.

With all else verified to be correctly set, one or more cylinders do not respond correctly to the throttle, and it is impossible to synchronize the carbs.

Remove the airboxes, this will allow you to see the carb intake and reach the slide. There is a small crescent shaped opening at the top of the carb intake, this is the inlet to the chamber below the diaphragm. With the enrichener knob ("choke") fully in on the later models, or the "choke" lever in the off position on the earlier models, do the following. Raise the slide fully with your fingers, and then cover the crescent shape hole with the thumb of your other hand. Release the slide, keeping the crescent shaped hole sealed with your thumb. If the diaphragm is OK, the slide will not descend until you remove your thumb and let the air escape. If the diaphragm has a pinhole, it will creep down. If the diaphragm has a tear, it will snap shut. The reason the enrichener circuit or "choke" must be off, is because the enrichener circuit opens a port to draw additional air from the chamber under the diaphragm, thus opening this chamber to atmosphere and allowing the slides to snap shut whether you have your thumb over the crescent shaped opening or not. Note; if the slide only creeps down, it is not essential it be repaired. A small single pinhole will not materially affect the operation of the carbs. But an ounce of prevention is worth a pound of cure; repair now will keep it from later tearing.

(1) A your NAPA store buy part # 765-2527 "Plasti Dip" spray-on heavy duty flexible rubber coating, which comes in a spray can and also in a ordinary can for brush application.  Under $10.  The part number given is for the spray in black, that's what I bought. Hereinafter called PD

(2) Remove the carburetors, remove the tops, and withdraw the diaphragm and slides. You might as well check them both while you have them out. But do not mix up the parts between the two carbs.

(3) Clean the diaphragms with denatured alcohol.

(4) Hold against the light to locate any pinholes or tears. 

(5) Spray a small portion of the PD onto a piece of shiny coated cardboard to form a puddle. Using a Q-tip take wet PD from the puddle, and lay it over any pinholes, over any small tear, and also paint it under the outside seating lip over all the tiny little holes which may be able to be seen against the light.  Put these repairs on the underside of the diaphragm only, so pressure holds them against the diaphragm, rather on the top where vacuum would work to pull them off. 

(6)Then give only the underside of the diaphragm one THIN coat of spray, including over the seating lip.  Too enthusiastic spraying of the diaphragm will make it too stiff. This layer will bond with the repairs, adhere to the diaphragm, and prevent the repairs be drawn through the tear by vacuum. Immediately wipe off any spray that went onto the slide body or the plastic clamping disc with a cotton ball wetted with rubbing alcohol.  No masking tape, it will cause ragged edges on the PD coat.

(7)Let it dry/cure for the recommended 4 hours.

(8) Assemble it into the carbs.

(9) Test again as described above.