Adjusting Holley & Demon Mixture Screws

The first step is to set the curb idle speed to between 750 and 850 RPM, then check ignition timing and adjust if necessary.

Now we are ready to set the mixture screws (Holley part # CS1540, Demon Part CS1540KH). Your carburetor may have two or four mixture screws. For those with two screws, turn the mixture screw in until you notice a drop in RPM, then back it out until you get the highest RPM you can. Stop and do the same to the other mixture screw. This is done to achieve the highest RPM for the motor at idle. It may be necessary to lower idle speed once you have completed the mixture process.

For those carburetors with four mixture screws, otherwise known as four corner idle, you will make smaller adjustments to all four screws. After timing and base idle is set, turn the first screw in to notice a drop in rpm. Then, in quarter turn increments, adjust each mixture screw until you no longer get a positive (meaning RPM rise) result from turning the screw. After this you may need to lower your base idle speed.

If the screws do not maintain adjustment position it is possible that the seal (Holley mixture seal part # G8-421 or Demon mixture seal part # OR274) needs to be replaced.