Most of the time the diaphragms are still good. Doing a 7psi pressure test on the fuel inlet before disassembly will help you assess things. If it holds pressure, you can probably reuse existing components. If it leaks down, hold the carb by the tubing to your pressure source and dunk it in a bowl of water. This wont hurt anything since the carb is under pressure. Bubbles along the mating edges show you gasket leaks. Bubbles coming from the venturi are usually from the metering valve. Regardless of the test results, examine the metering diaphragm for distortion and suppleness. Hold it up to a light and look for pinholes. Check the fuel pump diaphragm for deep bowing into unsupported areas - it may have stiffened and stretched. Check the tip of the inlet needle under strong magnification - you don't want to see a prominent indented ring. The valve seat should be smooth and shiny.
Cleaning: I clean the diaphragms and gaskets with WD-40. (The metering diaphragm is almost always stuck to its gasket and I let it be.) If the valve seat needs cleaning, cut a Q-tip in half, moisten the stick end, and dip it in baking soda. This fits nicely in the valve seat. Twirl it with slight pressure to remove crud. The carb block and small parts go into the ultrasound cleaner with water + Dawn. This may be overkill, but it's quick and easy and just might dislodge any gunk under the welch plug. Rinse well and blow out gently. Never put the air hose right up to any orifice (may damage a check valve). Remember to check that the metering lever is at the correct level for your carb. If you replace the diaphragm, measure the plunger length to make sure it matches the old one. On any part, if in doubt, replace. OEM kits are the way to go. Metering side: gasket next to block; fuel pump side: diaphragm next to block. Reassemble and do pressure test. This time it must pass before using. Once you get the hang of it, carb rebuilding is one of the easiest jobs in saw restoration.
