1. Vacuum Leaks: A significant vacuum leak can cause erratic idle, hesitation, and backfiring. The lean condition created by the leak can cause the backfire through the intake. The hanging RPMs could be due to the engine not properly transitioning between gears due to the inconsistent air/fuel mixture. Check all vacuum hoses for cracks, looseness, or disconnections. Pay close attention to the hoses connected to the intake manifold, carburetor, and brake booster.
2. Carburetor Issues: The 280ZX uses a complex SU or Hitachi carburetor (depending on the model). Several problems can arise:
* Dirty or Malfunctioning Carburetor: A dirty carburetor can lead to incorrect fuel/air mixture, causing backfires and hanging RPMs. Cleaning or rebuilding the carburetor is often necessary.
* Float Level Problems: An incorrect float level can cause either a rich or lean condition. A low float level will lead to a lean condition and backfiring.
* Stuck or Malfunctioning Choke: A stuck choke can also cause a rich condition, leading to backfiring. Ensure the choke is operating correctly.
* Vacuum Diaphragm Issues: The carburetor uses vacuum diaphragms to control various functions. If these diaphragms are torn or leaking, it can affect the fuel delivery.
3. Ignition System Problems:
* Distributor Issues: A worn distributor cap, rotor, or points (if it still uses points instead of electronic ignition) can cause misfires, leading to backfiring and rough running.
* Ignition Coil Problems: A weak or failing ignition coil can result in weak sparks, contributing to misfires.
* Worn Spark Plugs or Wires: Worn spark plugs or wires can also cause misfires. Inspect for fouling, wear, and proper gap.
* Timing Issues: Incorrect ignition timing can significantly affect engine performance and cause backfiring.
4. Exhaust System Restrictions: A clogged catalytic converter or exhaust system restriction can increase back pressure and cause backfiring, though less likely to cause hanging RPMs between gears.
5. Fuel Delivery Problems:
* Fuel Pump Issues: A weak or failing fuel pump might not provide sufficient fuel pressure, causing a lean condition.
* Clogged Fuel Filter: A clogged fuel filter restricts fuel flow, similar to a weak fuel pump.
Troubleshooting Steps:
1. Start with the simple things: Check all vacuum hoses, spark plugs, and wires first. These are the easiest and cheapest checks.
2. Listen Carefully: The location of the backfire can give clues. A backfire through the intake is usually a fuel or ignition problem.
3. Check the fuel pressure: If possible, check the fuel pressure to ensure it's within the specification.
4. Inspect the carburetor: Look for signs of dirt, leaks, or damage. Consider a rebuild or replacement if necessary.
5. Test the ignition system: Check the distributor, ignition coil, spark plugs, and wires. A professional mechanic can often test these components efficiently.
Important Note: Working on the fuel and ignition systems requires caution. Be aware of the potential for fire hazards. If you are not comfortable performing these repairs yourself, it's best to consult a qualified mechanic specializing in classic cars, especially Datsuns of this era. They have the experience and tools to properly diagnose and fix these kinds of issues.
