Carburetor Metering Diaphragm The Part That Breathes

The diaphragm is the only part inside the fuel pump that breathes. Every time it moves, it pushes the fuel forward once, and after millions of such cycles it hardens, tears or leaks, and the machine's fuel system starts to misbehave.

What does this diaphragm do? Inside the carburethere is a diaphragm fuel pump, and the diaphragm is its heart. As the engine runs, the diaphragm expands and contracts with the pressure, drawing fuel out of the tank and pushing it into the carburetor, while it also meters the amount. When the diaphragm fails, the fuel either stops or runs unevenly, and the machine loses power, idles rough, or will not start.

Why does it fail? The diaphragm is rubber, and it lives in fuel, close to the heat of the machine. Over time the rubber ages and hardens, the elasticity drops, and the edges thin and crack from the repeated flexing. It is a wear item, not something that breaks, it grows old.

What does it look like? The diaphragm is black rubber, with a square flange around the edge and screw holes at the four corners, one corner carrying a small lug for alignment. In the center is a recessed rubber dome, and on it a metal plate with a ring of round holes and a central metal pillar. The metal plate and the rubber work together as the metering assembly.

When to replace it? When the old diaphragm is torn, hardened and no longer springs back, or its edges have warped, it is time for a new one. When fitting it, get the direction right, metal plate forward, line the four corner holes up with the pump body, and press the diaphragm flat, with no wrinkles, because a wrinkle will tear the moment it flexes.