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Whether a carb is a float or diaphragm type, it will have a fuel inlet needle and seat. The seat in some float carbs and in most diaphragm carbs is not replaceable and if it recieves a nick or gets worn a little out of shape you may need to replace the carb. Here's a little trick that can save a carb if it isn't too far gone;
What you'll need. A Q-Tip type product, WD-40 or any brand of light oil, fine valve lapping compound and a pair of scissors.
Using the scissors, trim the sides of the Q-Tip to where it looks like it can be worked into the seat cavity. Wet the Q-Tip with the light oil and slowly insert into the seat cavity, turning as you go. Once to the bottom, turn a oouple additional times and slowly as you continue to turn, remove. You now have a tip shaped to the seat.
Dab a small amount of lapping compound on the tip of the Q-Tip and return to the seat cavity again while turning. Once at bottom continue to turn (lap) the seat with light pressure four or five times. Remove the Q-Tip and flush the seat cavity. Use the other end of the Q-Tip and repeat the shaping process to loosen any lapping compound that flushing didn't, and then flush again. Any compound left will grind up the new needle you're going to install so clean it out good.
That's about it. I've saved many carbs from the garbage this way and know it works.