* Fuel Pump: The fuel pump might be weak. It can easily supply enough fuel for idling but struggles to keep up with the increased demand when accelerating. A weak pump will often show symptoms like this - good idle, poor acceleration.
* Fuel Filter: A clogged fuel filter will restrict fuel flow, especially under higher demand. Even a recently rebuilt carb can't overcome a severely restricted filter.
* Ignition System: While less likely given the good idle, a weak ignition system (coil, wires, distributor cap, rotor) can cause a lack of power under load. Poor spark under high engine demand is a possible cause of sputtering. Check for spark strength at the plugs while someone cranks the engine.
* Clogged Exhaust: A heavily restricted exhaust system will create backpressure that starves the engine of power.
* Vacuum Leaks: While less likely given the good idle, significant vacuum leaks after the throttle plates can still negatively impact performance under load.
Troubleshooting Steps:
1. Check the fuel pressure: This is the most important step. You need a fuel pressure gauge to check the pressure at the carburetor inlet. Compare the reading to the specifications for your fuel pump. Low pressure is a strong indicator of a failing pump or clogged filter.
2. Replace the fuel filter: This is cheap and easy. It's always a good idea to replace this regardless of the age.
3. Inspect the fuel lines: Look for any kinks, cracks, or restrictions in the fuel lines.
4. Check the ignition system: Inspect the spark plug wires, distributor cap, rotor, and coil for wear or damage. Consider replacing the cap and rotor as a preventative measure, they are inexpensive. A weak coil can manifest in this way.
5. Check the exhaust system: Inspect the exhaust pipes and muffler for any blockages.
6. (Less Likely) Carburetor issue (again): Although you've had it rebuilt, there's a slim chance of a problem with the accelerator pump circuit or a metering issue that only shows under load. This is less likely given your description, but if all else fails, a careful re-examination of the carb may be needed.
Start with the fuel pressure and fuel filter, as those are the most probable causes given your description. If the fuel pressure is low, then you've likely found your problem.
