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. The idle jet is not removable. 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. As the throttle valve #6 is slowly opened from the slow idle position it gradually subjects the secondary idle orifice #8 to intake manifold vacuum, and the secondary idle orifice #8 no longer bleeds air to the idle fuel passage #10 but feeds an additional quantity of fuel into the engine. As the throttle valve is opened further the idle fuel begins to fade out allowing the power system to take over.
Spray carburetor cleaner through each of these passages to make sure they are clean and clean.
- 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