* Fuel Pump: A weak or failing fuel pump is a prime suspect. Under hard acceleration, the engine demands more fuel, and a weak pump can't keep up. The symptoms you describe – power loss, almost stalling – are very characteristic of this. The pump might be getting old, the fuel filter might be clogged restricting flow, or there could be internal pump failure.
* Fuel Filter: A clogged fuel filter will restrict fuel flow, causing similar symptoms to a weak fuel pump. It's relatively inexpensive to replace and should be checked or changed regardless of the other issues.
* Carburetor Issues: With a carburetor-equipped 403, several things could be at play:
* Fuel level: The float in the carburetor might be sticking, preventing proper fuel level maintenance.
* Clogged jets: Dirt, debris, or varnish buildup in the carburetor jets can restrict fuel flow, particularly under high demand. A rebuild or thorough cleaning is likely necessary.
* Accelerator pump: The accelerator pump in the carburetor provides an extra shot of fuel when you accelerate. If it's weak or malfunctioning, you'll experience hesitation or power loss under acceleration.
* Vacuum leaks: Leaks in the vacuum system can disrupt the carburetor's operation and lead to poor fuel delivery.
* Ignition System: While less likely to cause the *exact* symptoms you describe, a weak ignition system (distributor cap, rotor, wires, coil) can contribute to poor performance under load. This would likely manifest as stumbling or misfiring rather than a complete power drop, but it's worth investigating if the fuel system checks out.
* Exhaust Restriction: A severely clogged exhaust system can back pressure up and cause a similar effect. While less common, a blocked catalytic converter or muffler could be a factor.
Troubleshooting Steps:
1. Check the fuel filter: This is the easiest and cheapest first step. Replace it with a new one.
2. Listen for the fuel pump: When you turn the ignition key to the "on" position (engine off), you should hear the fuel pump briefly prime. If you don't hear it, or it sounds weak, the pump might be failing.
3. Inspect the carburetor: If you're comfortable working on carburetors, inspect it for debris, check the float level, and test the accelerator pump. Otherwise, take it to a mechanic specializing in older vehicles.
4. Check for vacuum leaks: Listen for hissing sounds around the intake manifold and carburetor. You can also use a propane torch (carefully!) to check for leaks – if the engine speed changes when the propane is near a leak, you've found it.
5. Inspect the ignition system: Check the distributor cap, rotor, spark plug wires, and spark plugs for wear or damage.
Because you have a classic car, finding a mechanic specializing in older vehicles or someone familiar with the 403 engine is recommended. They'll have the experience to diagnose the problem more effectively and will possess the specialized tools often needed for carburetor work. Don't attempt extensive carburetor work unless you have the knowledge and experience. A poorly adjusted carburetor can cause significant damage to your engine.
