This illustration shows the Marvel Schebler's idle circuit. When the throttle valve #6 is at the position between #7 & #8. This creates a vacuum that pulls fuel from the fuel bowl #1 up through the idle passage #10 and into the throttle bore. This is calibrated by going through the idle jet #11 and the idle adjusting needle #13. At the idle adjusting needle air in mixed with the fuel and air is also mixed with the fuel at the secondary idle orifice #8. The throttle valve does not completely close at idle.
- A vacuum leak at the intake or carburetor mounting would cause low vacuum at the idle circuit giving you a rough idle.
- The idle adjusting needle not adjusted correctly will cause a rough idle.
- A clogged, or partially clogged idle fuel passage #10 will cause a lean condition at idle.
- Either the secondary idle orifice, or primary idle orifice being clogged will cause a rich condition