Holley 847 Troubleshooting

Is Your 847 Flooding on Start Up?

The Holley 847 choke plate has a poppet valve. The purpose of this valve is to open on start up so that the air isn't choked completely causing a too much fuel condition. Make sure the poppet valve is not sticking. Remember that only vacuum is used to open this valve.

What if the Poppet Valve is Missing From the Choke Valve?

Just the opposite from above. With the valve missing, there may be too much air entering the carburetor in a cold engine condition thus starving the engine of necessary fuel. If you have a "hard to keep running when cold condition" check out the choke valve and poppet valve. When cold the poppet valve will be closed.

Main Discharge Check Weight is Missing
No check weight available, but you can use most anything in it's place. A 2nd check ball, or a piece of welding rod. Just be sure that there is room for the check ball & weight to move up when it need to. 1/8" or so would be fine.


Common Troubleshooting Steps

Fuel Delivery & Flooding

  • Check the Needle and Seat: Dirt or debris trapped here is the most common cause of flooding.

  • Inspect the Float: Check for a "heavy" or "loaded" float that has absorbed fuel or has a leak, which will prevent it from rising to shut off the fuel.

  • Verify Fuel Pressure: Ensure your fuel pump is not putting out excessive pressure; vintage carburetors like the 847 typically require low pressure (around 2.5 to 3.5 PSI).

Idle & Low-Speed Issues

  • Clear the Idle Circuit: Small passages in the idle circuit can easily become clogged with varnish or sediment.

  • Adjust the Idle Mixture Screw: Start by turning the screw in until lightly seated, then back it out about 1-1/2 turns as a baseline for adjustment.

  • Check for Vacuum Leaks: Inspect the mounting gasket and the throttle shaft for wear, as air leaks here will cause a lean condition and rough idle.

Acceleration & Performance

  • Test the Accelerator Pump: Ensure the pump diaphragm is not torn and that the check balls are seating correctly to provide a strong "squirt" of fuel.

  • Inspect the Main Jet: A clogged main jet will cause the engine to stumble or die when transitioning from idle to higher speeds.

  • Check the Power Valve: If the power valve (economizer) is ruptured, it will cause an extremely rich condition, often resulting in black smoke from the exhaust.

Maintenance Tips

  • Clean Thoroughly: Use a dedicated carburetor cleaner and compressed air to ensure every internal passage is clear.

  • Replace Gaskets: Always use fresh gaskets during a rebuild to prevent external fuel leaks or internal vacuum leaks.

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