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How To Tune Holley Carburetors

  • How to tell what size carb you need:  You take the engine cubic inch and multiply by the maximum RPM and divide by 3456. If you have an automatic transmission you will need a vacuum sec. carb, a standard transmission can use a Double Pumper carb.  
  • How to adjust the mixture screws on your carburetor:  To set idle mixture screws perform the following steps:
    1. Get the engine up to temp
    2. Shut the engine off
    3. Install a vacuum gauge to a full manifold vacuum port
    4. Turn the screws all the way in until lightly seated
    5. Next, back them out 1 full turn and restart the engine making sure the choke is completely off
    6. While at idle, back them out 1/8 turn at a time, alternating between the 2 or 4, until highest vacuum is achieved. Be sure you keep all 2 or 4 screws even. You can also do this adjustment without a vacuum gauge, the adjustment are the same except you will adjust to highest engine RPM at idle.  
  • How to tell what size power valve you need: To properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve.  
  • How to adjust the fuel level on your carburetor: To adjust the fuel level you will need to take out the sight plug in the side of the fuel bowl and with it running Loosen the lock screw on top of the fuel bowl and turn the nut clockwise to lower the level or counter clockwise to bring up the level you want it to be at the bottom of the sight hole If you turned it too far down it might have bent the tang on the float. to get back to a starting place, you will need to take out the lock screw and turn the nut so the needle & seat is 1/2 way through the thickness of the nut then put the screw back in, then take the fuel bowl off and hold it upside down & level then bend the tang on the float so the float is level in the center of the fuel bowl. This will get you back to a starting place. 
  • How to cure stumble from a dead stop:  The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.  
  • How to adjust the pump shot on your carburetor:  Look down in the carb without it running move the throttle and see if the two sprays of fuel shoot as soon as the throttle moves if it is not instant back off the nut on the pump arm 1 full turn counter clockwise then check it again do this until it is instant.  
  • If your engine is running rich and my mixture screws do not have effect:  The first thing you need to check is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" power valve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance cam shafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle.

© Copyright Holley Performance Products, Inc.

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