1. Fuel System:
* Carburetion Issues: This is the most likely culprit. A poorly tuned carburetor will lead to stalling. Issues include:
* Incorrect jets: The jets need to be sized appropriately for the engine and application. A carburetor jetting kit for a Vortec 350 in a '91 Silverado (or a similar application) needs to be purchased.
* Fuel pump pressure: The fuel pump needs to supply sufficient pressure. Test fuel pressure at the carb inlet using a fuel pressure gauge. The pressure should match the carburetor's specification.
* Fuel filter: A clogged fuel filter restricts fuel flow. Replace it with a new one.
* Float level: An incorrect float level can lead to a lean or rich condition, causing stalling. This requires adjusting the float level according to the carburetor's instructions.
* Vacuum leaks: Leaks in the intake manifold, carb base gasket, or vacuum lines will affect the air/fuel mixture. Inspect all vacuum lines for cracks and the manifold gasket.
* Choke: The choke might be sticking or not functioning correctly. Observe its operation during startup.
2. Ignition System:
* Distributor: Make sure the distributor is correctly timed and the ignition curve is appropriate for the carburetor setup. A timing light is necessary to check the timing.
* Ignition coil: A weak or failing coil won't provide sufficient spark. Test the coil's output.
* Wires: Cracked or worn spark plug wires can cause misfires and stalling. Inspect for damage.
* Spark plugs: Worn or fouled spark plugs prevent proper ignition. Replace with new plugs appropriate for the engine.
* Pick up in the distributor: This could be failing to send a signal
3. Other Potential Issues:
* Vacuum Advance: This could be malfunctioning or incorrectly adjusted, affecting engine timing.
* Air Filter: A clogged air filter restricts airflow.
* Engine Temperature: Is the engine overheating? Overheating can cause various issues, including stalling. Check the coolant levels and temperature gauge.
* Wiring: Check for any loose or corroded wiring connections. This is especially important with the conversion to a carburetor.
Troubleshooting Steps:
1. Start with the basics: Check fuel level, air filter, and obvious wiring issues.
2. Fuel pressure test: This is crucial to rule out fuel delivery problems.
3. Vacuum leak test: Use a vacuum gauge or carb cleaner to detect vacuum leaks.
4. Ignition system check: Test the coil, wires, and plugs. Verify timing.
5. Carb inspection/adjustment: This will likely require adjustment or a rebuild, possibly with the help of a professional carburetor specialist.
6. Professional help: If you're not comfortable working on these systems, take it to a mechanic experienced with carburetor setups.
Remember, converting a fuel-injected engine to carburetor is not a simple swap. You've likely changed several key systems, and the problem could be anywhere within the engine's fuel and ignition systems. Careful and methodical troubleshooting is key.
