Owner's Problem Description
The owner has a 1965 Chevy Impala with a 283 engine and a Rochester 2G carburetor. After a recent carb rebuild, the vehicle runs perfectly at idle and low throttle, but when the throttle is fully depressed, it shuts down momentarily before returning to rough idle. Previous repairs were limited to the carburetor rebuild, and the timing is set at 6 degrees advanced. The vehicle shows no other symptoms at idle.
Professional Technical Analysis
This 1965 Chevrolet Impala, equipped with a 283 V8 engine and Rochester 2G carburetor, is exhibiting a hesitation issue during acceleration. The vehicle has undergone a carburetor rebuild, and the timing is correctly set at 6 degrees advanced. The problem manifests when the accelerator is fully depressed, resulting in a momentary shutdown before the engine resumes rough idle, indicating potential fuel delivery issues. A critical analysis reveals that the accelerator pump may be the culprit if the fuel stream lacks sufficient force when the throttle is opened. The expected fuel pressure for this setup should be around 5-7 PSI. If the accelerator pump is malfunctioning, it may not deliver the required fuel volume quickly enough to maintain engine performance under heavy load conditions. The vehicle currently runs well under normal operating conditions but struggles under full throttle, suggesting a fuel starvation scenario. Environmental factors such as temperature and elevation could affect carburetor performance, especially as the vehicle ages. It is essential to verify the fuel bowl level during operation, as a low fuel level could exacerbate the hesitation issue. The observed behavior is typical in older carbureted engines where the fuel delivery system does not respond adequately to rapid throttle input.
Possible Causes
Most common causes (ordered by frequency):
- Accelerator Pump Failure: The accelerator pump in the Rochester 2G carburetor may not be functioning correctly, leading to insufficient fuel delivery during rapid throttle application. This can cause the engine to hesitate or stall temporarily. The pump should produce a strong stream of fuel when the throttle is opened quickly. - Weak fuel streams observed when throttle is opened; engine stutters under full throttle, indicating inadequate fuel delivery.
- Fuel Bowl Level Low: If the fuel bowl level is too low, it can restrict fuel flow to the engine under high demand, such as during full acceleration. This condition can cause hesitation and rough idle as the engine struggles to receive adequate fuel. - Checking the fuel bowl level reveals it is lower than the specifications, leading to sputtering when throttle is applied.
- Clogged Fuel Filter: A clogged fuel filter can restrict fuel flow to the carburetor, especially under higher demand scenarios. This can lead to hesitation when accelerating and rough idle conditions as the engine struggles to maintain adequate fuel supply. - Fuel filter inspection shows significant debris buildup; fuel pressure drops observed under load.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Visual Inspection of Accelerator Pump: With the engine off, visually inspect the accelerator pump operation by opening the throttle and observing the fuel streams. Ensure both streams are strong and consistent. If weak, check the placement and condition of the check balls in the accelerator pump assembly.
- STEP 2 - Check Fuel Bowl Level: Remove the top cover of the carburetor and check the fuel bowl level. It should be within the recommended range specified in the rebuild kit documentation. If low, this may indicate a problem with the float or a fuel delivery issue.
- STEP 3 - Fuel Pressure Testing: Using a fuel pressure gauge, measure the fuel pressure at the carburetor inlet. It should be between 5-7 PSI. Any pressure below this range could indicate a failing fuel pump or clogged filters.
- STEP 4 - Road Test for Performance: Conduct a controlled road test while monitoring the engine's performance under acceleration. Pay attention to any hesitations or stumbles. Document all observations for further analysis.
