* Incorrect Ignition Timing: Even a slight misalignment of the distributor can cause rough idling and misfires. The timing needs to be precisely set using a timing light. This is the most likely culprit if everything else seems fine.
* Vacuum Leaks: A significant vacuum leak in the intake manifold or associated hoses can dramatically affect engine performance, causing rough idling and misfires. Carefully inspect all vacuum lines and the intake manifold for cracks or loose connections.
* Fuel Delivery Issues:
* Clogged Fuel Filter: A clogged filter restricts fuel flow, leading to lean conditions and misfires.
* Fuel Pump Issues: A weak or failing fuel pump may not supply sufficient fuel pressure.
* Fuel Injectors (if fuel injected): Clogged or faulty injectors can cause inconsistent fuel delivery. This is less likely if it was a carbureted engine.
* Incorrect Valve Adjustment (if applicable): Improperly adjusted valves can cause poor engine performance. This is more relevant to engines with mechanical valve lifters.
* Incorrect PCV System Setup: A faulty or improperly connected Positive Crankcase Ventilation (PCV) system can introduce crankcase gases into the intake, leading to running problems.
* Faulty Oxygen Sensor (if applicable): On fuel-injected engines, a bad oxygen sensor can cause the fuel mixture to be off, resulting in rough running.
* Compression Issues: While unlikely if the engine was running before the swap, low compression in one or more cylinders can cause misfires. A compression test will reveal this.
Troubleshooting Steps:
1. Verify Ignition Timing: This is the first thing to check. Use a timing light and adjust the distributor accordingly. Your vehicle's manual will specify the correct timing.
2. Check for Vacuum Leaks: Use a vacuum gauge or spray carb cleaner around suspected leak points while the engine is running. A rising vacuum gauge or a change in engine speed indicates a leak.
3. Inspect Fuel System: Check the fuel filter for clogging and ensure the fuel pump is delivering adequate pressure. If fuel-injected, consider inspecting the injectors (though this usually requires specialized tools).
4. Check Valve Adjustment (if applicable): If your engine has mechanical valve lifters, check and adjust them according to the manufacturer's specifications.
5. Examine PCV System: Make sure the PCV valve and its hoses are correctly connected and functioning.
6. Compression Test: Perform a compression test to rule out low compression in any cylinders.
7. Check for other electrical issues: faulty wiring, bad coil, etc.
Without more information on your specific engine (carburetor or fuel injection), the type of valve adjustment, and the steps taken during the engine swap, more precise troubleshooting is difficult. However, these steps should help you pinpoint the cause of the problem. If you're not comfortable performing these checks, it's best to seek help from a qualified mechanic.
