Why Doesn’t Adjusting Idle Mixture Screws Make any Difference?
The idle mixture screws adjust the amount of air/fuel mixture that goes up to the venturi at idle. When adjusting the screws doesn’t change the RPM then it means there is a problem with either the air, or the amount of fuel.
- The mixture screw may be bent, or damaged. Look for any scoring on the tip of the screw. This is most often caused by turning the screw in too hard.
- The air and/or fuel passage is plugged. Squirt carburetor cleaner into the screws hole, blocking the transition hole in the bore. Watch the venturi to see if the carb cleaner makes it through.
- There could be a vacuum leak. Check the mounting gasket and any other vacuum sources.
- Make sure the float bowl gasket is correct and not allowing air through the gasket.
- Check for a bent metering rod.
- A warped carburetor could be allowing air to sneak in through the float bowl gasket.
- Make sure the metering rods are adjusted.
Surges at cruising, or high speeds
- Clogged main jets.
- Low fuel pump pressure, or volume. Test for 4 lbs.
- Dirty air bleeds.
- Dirty filter, or filter screens.
- Float out of adjustment.