But, before that, I wanted to test the fuel pump. The car appears to run the carbs dry when run at speeds above 60-70 kph, or up a big hill. If I crank it and idle for a minute to fill up the bowls, it's good to go again. Funny to see the perplexed drivers behind me for the first few times, but it's getting annoying.

Sorry, no pics for this one.

Vacuum gauge on the pump inlet shows a suction of 270 mmHg. Factory spec is 230-320 mmHg. Checks out.

Outlet shows a consistent pressure of 4 psi at cranking speed. The manual calls for 1.76 - 2.65. Hmm, this might be an issue, but I'm not sure what the Zeniths want to see.

At running speed, the fuel return valve is stuck open. Does not close regardless of RPM. At this point I'm not sure if either of these condition contribute to the problem. The plan is to have the Zeniths professionally rebuilt and see how it behaves. The real issue is likely a blockage in the line from the tank. All the hoses in the engine bay are new. I'm not sure if the pump inlet and carb return are connected to the correct hard line. One more thing to double check.

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1968 Mercedes 280S - The Demon Car