Re: changing the accelerator pump diaphragm
The pump actuator bar attached to the throttle arm is hinged across a little black plunger on the pump. Under that is the diaphragm and under that is the spring. You remove the 4 screws and carefully hold the diaphragm and plunger assembly in place while you swing the cover out of the way. Remove the plunger / diaphragm and spring. Under that is a little rubber one-way valve and it needs to be replaced too - you have to pull it out being careful the tip doesn't fall into the float bowl.
The new rubber goes in by pulling it into the float bowl, then the spring / diaphragm / plunger and then the cover and screws.
You should really redo the whole carb. The kit is only about $20. Get a gallon can of carb soak and some spray carb cleaner and clean the whole thing up. The 2150 series 2bbl carbs are really easy to rebuild and well worth the time to get the venturis, vaccum holes and all cleaned out and that power valve replaced underneath the float bowl.