When a Carter BBD carburetor (or any carburetor) only runs with the choke closed and stalls or runs poorly when the choke opens, it’s almost always due to the engine running too lean. This means there isn’t enough fuel reaching the engine when the choke is off (open). Here’s a breakdown of common causes and how to diagnose them:
1. Clogged Idle or Main Circuits
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What Happens: The choke “enriches” the mixture by blocking air, so a blockage in the carburetor’s idle or main fuel circuits can be temporarily masked by the choke being closed.
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Solution: Clean the carburetor thoroughly, paying extra attention to the idle tubes, passages, jets, and air bleeds. Even a tiny obstruction can cause this issue.
2. Vacuum Leaks
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What Happens: Unmetered air entering from a leaking gasket, throttle shaft, intake manifold, or vacuum hose will make the mixture too lean for the engine to run without the choke.
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Solution: Inspect all gaskets, hoses, and the carburetor base for leaks. Spray carb cleaner or propane around suspected areas—if the idle changes, you’ve found a leak.
3. Low Float Level or Fuel Delivery Issues
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What Happens: If the float is set too low or fuel isn’t reaching the carburetor fast enough, not enough fuel enters the mixing circuits.
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Solution: Check and adjust the float height. Inspect the fuel filter and fuel pump, and ensure there’s no blockage in the tank or fuel lines.
4. Improperly Installed or Missing Parts
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What Happens: Missing or incorrectly installed gaskets, jets, or internal plugs can cause fuel to bypass or fail to reach certain circuits.
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Solution: Rebuild the carburetor with a quality kit, following the manual and double-checking every part and gasket.
5. Incorrect Idle Mixture Adjustment
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What Happens: If idle mixture screws are turned in too far, or passages are blocked, the carburetor won’t supply enough fuel at idle.
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Solution: Back out the mixture screws to the recommended setting and fine-tune for the smoothest idle with the choke off.
6. Engine Problems (Less Common)
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What Happens: Low compression, poor ignition, or valve issues can also cause similar symptoms, but usually the root is in the carburetor or fuel supply.
Summary:
If your Carter BBD only runs with the choke closed, the engine is starving for fuel due to carburetor blockage, vacuum leaks, or fuel delivery problems. Cleaning the carburetor, checking for leaks, and setting float/mixture properly will almost always solve it.
