Fuel System:
* Fuel Pressure: The most likely culprit. A faulty fuel pump can deliver inconsistent fuel pressure, causing erratic idle. Use a fuel pressure gauge to check pressure at idle and at higher RPMs. The pressure should be within the specifications for your carburetor.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms. Replace it. It's cheap and easy to do.
* Vacuum Leaks: Vacuum leaks can drastically affect idle. Inspect all vacuum hoses for cracks, holes, or loose connections. Pay close attention to the hoses connected to the carburetor and the brake booster. A propane torch test (carefully!) can help identify leaks. Spray propane around suspected areas; a rising idle indicates a leak.
* Fuel Pump Relay (if applicable): If your truck has an electric fuel pump, a failing relay might be interrupting the power supply, leading to inconsistent fuel delivery.
Engine & Idle Control:
* Choke: If the choke isn't operating correctly, it can cause high idle, especially when cold. Make sure the choke is working properly and opening as the engine warms up.
* Throttle Linkage: Verify that the throttle linkage is adjusted correctly and that it's not sticking or binding. A slightly misadjusted linkage can cause irregular idle.
* Idle Air Control (IAC) Valve (if equipped): Some 1979 models might have an IAC valve. A faulty IAC valve can cause erratic idling. Cleaning it or replacing it may resolve the issue.
* Distributor: The distributor's mechanical advance could be malfunctioning, causing timing issues affecting idle.
* PCV Valve: A clogged positive crankcase ventilation (PCV) valve can affect the engine's breathing and contribute to idle problems.
Carb Issues (Despite being new):
* Carb Adjustment: Even a new carburetor might need fine tuning. The idle mixture screws and idle speed screw need to be correctly adjusted. Consult your carburetor's instructions. Incorrect float level could also be a factor.
* Carb Installation: Make absolutely sure the carburetor is correctly installed and all gaskets are in place and not leaking.
Troubleshooting Steps:
1. Start with the easiest and cheapest: Fuel filter replacement and visual inspection of vacuum hoses.
2. Check fuel pressure. This is crucial.
3. Look for vacuum leaks.
4. Examine the choke operation.
5. Verify the throttle linkage adjustment.
6. If the above don't solve it, a professional mechanic will be needed. Diagnosing carburetor issues requires experience and specialized tools.
Don't jump to conclusions about the carburetor being the problem just because it's new. Many other factors can cause the symptoms you describe. Systematic troubleshooting is key.
