If your Massey 30 with the Mercarb runs fine for a couple of minutes and then dies under load, even after multiple teardowns, you’re looking at a fuel delivery or fuel starvation issue that could be carb-related or upstream. Here’s a breakdown of likely causes:
1. Fuel Delivery Issues
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Restricted Fuel Flow: The tank outlet, fuel line, or inline filter could be partially blocked with rust, varnish, or debris. It will allow enough fuel to idle, but the bowl empties under load.
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Vent Problem: If the fuel tank vent is plugged, a vacuum will form in the tank and fuel flow will slow to a trickle. Opening the gas cap and retesting will confirm this quickly.
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Weak or Failing Fuel Pump: Even if it “seems” to work at idle, it might not maintain volume at higher demand.
2. Carburetor Internal Issues
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Float Level Too Low: If the float is set too low, there’s not enough fuel reserve in the bowl, so it runs out under load.
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Needle and Seat Restriction: Even if clean, the seat can have slight varnish or swelling of the Viton tip, restricting flow at higher demand.
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Main Jet Obstruction: Partial blockage in the main jet or emulsion tube will cause fuel starvation at mid–high throttle.
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Power Circuit Not Engaging: The Mercarb has a power enrichment system—if the passage or valve is plugged, the mixture will go too lean under load and stall.
3. Ignition & Other Engine Factors
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Weak Ignition Under Load: A failing coil or ignition condenser can mimic fuel starvation by cutting out when demand rises.
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Vacuum Leak: A leak at the intake manifold or carb base gasket will lean the mixture under load and cause a stall.
Quick Tests:
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Run the engine with the gas cap loose — if the problem disappears, the vent is blocked.
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While it’s running at load, pump the choke partially closed — if it picks up, it’s running lean (fuel starvation or power circuit issue).
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Use a clear temporary inline filter before the carb — watch if fuel flow stops when it starts to bog.
