How to Tune a Holley 650 Carburetor


Holley carburetors are extremely popular for their ease of adjustability and lack of moving parts. They contain two float bowls--one for the primaries and one for the secondaries. The float bowls have observation holes for outside adjustment of the fuel level. There are two replaceable jets on either side and a vacuum-operated power valve located in the primary side. On a 650cfm carburetor such as this, there are two idle mixture screws--one on either side of the carburetor, and an idle adjustment on the linkage. The accelerator pump is also adjustable and replaceable with different sizes to accommodate different cams. The vacuum-operated secondaries are adjustable with different strength springs.

Things You'll Need

  • Set of ¼-inch drive sockets
  • ¼-inch Drive ratchet
  • Flat head screwdriver
  • Phillips screwdriver
  • 5/8-inch Wrench
  • Tachometer
  • Remove the air cleaner. Step on the accelerator pedal slightly. Observe the choke on the carburetor. The choke should be closed after stepping on the accelerator of a cold engine.

  • Start the vehicle and return to observe the operation of the choke and fast idle cam. The engine rpm should be around 1,500 rpm. The choke should start to open within two minutes as the engine warms up. With a tap on the accelerator pedal or on the throttle linkage of the carburetor, the rpm should drop to idle.

  • If the choke fails to close all the way or open all the way, use the Phillips screwdriver to loosen the screws on the black choke spring housing and turn the housing to adjust as needed.

  • Shut the engine off and remove the large plugs on the passenger side of the carburetor. If fuel is level with the bottom of the hole, it is correct. If the level is other than normal, adjustment can be made at the top of each float bowl. There is a common lock screw on top of a 5/8-inch nut.

  • Loosen the screw with a common screwdriver and turn the nut with a 5/8-inch wrench until the desired level is achieved. Lock the nut down with the screw. If the fuel level was too high, turn the nut clockwise, and conversely for too low. If the fuel level is too high, lower the fuel level, install the sight plug, start the engine and shut it down and recheck the level. Do not attempt to set the level with the engine running because the fuel pressure will spray fuel from the adjusting nut.

  • Hook up the tachometer and start the engine. Adjust the mixture screw on the driver's side first. Turn the screw to the right until the engine rpm begins to drop. Turn the screw to the left until the highest rpm is achieved and the rpm remains stable, then turn it to the right one quarter turn. Repeat the process on the opposite side and then repeat the process on both sides one more time.

  • Adjust the accelerator pump to begin operation when the throttle linkage has moved one-quarter inch. Do this by adjusting the nut on the bottom of the spring that touches the pump handle on the driver's side front of the carburetor.

Tips & Warnings

  • Adjusting the primary or secondary jets for high performance reasons is a simple matter of removing the four screws in each float bowl and removing the bowl to expose the jets. The jets have a size inscribed on them for identification. Unscrew the old jets and screw the new jets in place, then replace the float bowls. Replacing the accelerator pump for performance reasons is simply a matter of removing them with the use of a Phillips screwdriver externally. Many sizes are available from Holley.

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