That's a tough one. There's a check valve at the fuel pump (in the back under thhe fuel tank) that might be leaking Could also be a leaky fuel pressure regulator (back of fuel meter) or maybe the inlet screen to the warm-up compensator (front of intake manifold). The screen is in the inlet fitting. You have to take the fuel line off and then unscrew the fitting to check/clean it. Mine had similar symptoms and it ended being the inlet screen.

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa