Have 66 327/350hp Holly 4 bbl on a cold start what would cause a back firer thru the cab?

 

1. Lean Cold-Start Mixture (Most Common)

A lean mixture (too much air, not enough gas) burns incredibly slowly. The mixture stays ignited all the way through the exhaust stroke and is still burning when the intake valve opens for the next cycle, igniting the incoming fuel in the intake manifold.

• Choke Misadjustment: The L79 uses a hot-air choke mechanism. If the choke plate isn't closing almost fully when cold, or if the fast-idle cam isn't stepping up the RPMs, the engine will run too lean.
  
• Accelerator Pump: When you pump the gas pedal before cranking, the Holley's accelerator pump should squirt a strong jet of fuel into the primaries. If the pump diaphragm is dried out, or the check needle is stuck, you won't get that necessary initial shot of fuel.
  
• Vacuum Leaks: A leak at the intake manifold gaskets or the carburetor base plate pulls in unmetered air, leaning out the mixture severely.
  

2. Ignition Timing and Spark Issues

If the spark happens at the wrong time, it can ignite the mixture while the intake valve is still unseated.

• Initial Timing Too Retarded: If the base timing is set too low (retarded), the burn cycle starts too late and carries over into the intake stroke.
  
• Moisture under the Distributor Cap: Cold, damp mornings can cause condensation inside the distributor cap. This moisture allows the high-voltage spark to jump (cross-fire) to the wrong spark plug wire, firing a cylinder whose intake valve is wide open.
  
• Crossed Plug Wires: Double-check the firing order (1-8-4-3-6-5-7-2). If two adjacent wires are swapped (common mix-ups are 5 and 7 at the back of the driver's side block), it will pop and backfire violently.
  

3. Valvetrain Issues (L79 Specifics)

The L79 is famous for its high-performance hydraulic lifter camshaft.

• Sticking Intake Valve: If the car has been sitting, a slightly gummy or sticking intake valve guide can cause the valve to hang open just long enough for the combustion flash to travel up into the intake manifold.
  

Where to Start Troubleshooting:

1. The Cold Visual Check: With the engine cold and off, remove the air cleaner. Manually flip the throttle lever. Look down the primary bores to ensure you see two solid streams of fuel from the accelerator pump nozzles, and verify that the choke blade has snapped shut.
   
2. Inspect the Cap: Pop the distributor cap off and look for moisture, carbon tracking, or hairline cracks.
   
   Good luck.
   

 

Thank you big help have a great day. Maury