9
IDLE MIXTURE NEEDLES:
Idle mixture needles control the air/fuel mixture at idle. These have been preset at the factory and SHOULD NOT need any
adjustments. However, if you feel that adjustment is necessary, you can use the following procedure to do so. When tuning the
idle mixture, you’re actually tuning for the best manifold vacuum. Idle mixture needles are found on the primary metering blocks.
If you change one idle mixture needle, you must change the other idle mixture needle by the same amount. Here are the proper
steps for setting the idle mixture needles.
1. Attach the vacuum gauge to a manifold vacuum port.
2.
Adjust each idle mixture screw (Figures 9 & 10) 1/8 turn at a time, alternating between each screw. Turn them equally, until
you achieve the highest possible vacuum reading without adjusting the curb idle speed screw. Turn screws in to lean the
mixture. Turn them out to richen the mixture.
Figure 9
Figure 10
Figure 11
3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the curb idle speed screw,
as shown in Figure 11.
4. If a vacuum gauge is not available, use a tachometer to obtain the highest RPM.
ROUGH IDLE AND VACUUM LEAKS:
If a rough idle persists after the engine has been started and the mixture screws
adjusted, check for manifold vacuum leaks. These could result from unplugged vacuum fittings or a carburetor flange gasket
that was torn during installation. Recheck for proper attachment of all vacuum lines and check the lines for cracks.
If the manifold was changed, a manifold vacuum leak could occur at the cylinder head/manifold surface due to damaged gaskets
or improper torquing. Frequently, manifold vacuum leaks occur from the valley side of the manifold. These are very difficult to
detect, unless a discernible whistle can be heard.
NOTE:
In most cases, when rough idle occurs after a carburetor/manifold change, they result from manifold vacuum leaks
similar to those described above. Assuring a proper manifold installation rather than assuming the carburetor is not
functioning properly will ultimately save time.
