When I hold the diaphragm up to the light, it looks in good order, I can see no holes or perished areas.

Please check carefully the shape of the slider that it is the right one for you. Not all GT550 carbs are the same model.

I also have odds and ends for Keihin carbs such as spare tops and bowls, springs, floats, diaphragm with slide, idle adjust, rails email me for more details