Manufacturer Reference

Zenith Series 68 Carburetor

Installation & Adjustment

Zenith Fuel Systems LLC — Series 68 Universal Type Carburetors

View Original Zenith Instructions (PDF)

Before Installation

Compare the old carburetor with the new one to check the choke and throttle lever length, position, and other connections.

Zenith Series 68 universal carburetor
Figure 1 — Zenith Series 68 universal carburetor.

Choke & Throttle Setup

Choke Lever and Choke Tube Holder

  1. Use the old carburetor as a guide for the position of the choke lever and choke tube holder on the new carburetor.
  2. To reposition the choke lever, loosen its nut, rotate the lever on the shaft to the desired location, and tighten securely.
  3. To reposition the tube holder, remove its two screws, relocate the holder, then reinstall and tighten the screws.
  4. The choke lever and tube holder can be reversed to the opposite side. Reassemble the parts in reverse order and position them as on the old carburetor.
Zenith choke lever and choke tube holder parts
Figure 2 — Choke lever and choke tube holder.

Throttle Lever and Stop

  1. Use the original carburetor as a guide to determine the correct throttle lever length and the position of the lever and stop on the new carburetor.
  2. The throttle lever can be reversed and installed on the opposite side of the shaft as shown in Figure 3.
  3. For Bowden-wire operation, Zenith specifies attaching the swivel to the 3/16-inch hole in the lever with the appropriate washer and retainer.
  4. A Universal Spring Loaded Lever (C24-181) was offered for engines equipped with a governor to return the throttle to the minimum hand-throttle setting.
Zenith throttle lever parts
Figure 3 — Throttle lever arrangement.

Air Cleaner, Fuel Inlet & Mounting

Air Cleaner Adapter

For installations with a 1½-inch air-cleaner clamp band, install the air-intake adapter in the air intake and tap gently to seat it.

Zenith air cleaner adapter
Figure 4 — Air cleaner adapter.

Fuel Inlet

Three fuel inlets are provided. Choose inlet 1, 2, or 3 for the fuel-line connection and plug the other two.

Three Zenith carburetor fuel inlet positions
Figure 6 — Three available fuel-inlet positions.

Flange Studs and Gaskets

Install the short threaded end of the studs in the carburetor flange and tighten securely. Place the gasket over the studs, then attach the carburetor to the manifold flange with nuts, flat washers, and lockwashers.

Zenith note: On engines with a 7/8-inch or 1-inch manifold flange opening, use three furnished gaskets to assure throttle-plate clearance.
Zenith carburetor flange studs and gaskets
Figure 5 — Flange studs and gaskets.

Preliminary Adjustments — Before Starting Engine

  1. Hold the throttle closed and set the throttle-stop screw 1½ full turns from the completely closed throttle position. The throttle plate must be slightly open to permit the engine to run.
  2. Set the idle-adjusting needle at one full turn open.
  3. Set the main-jet adjusting needle at 1½ to 2 turns open. Some models do not have a main-jet adjustment.

After installing the carburetor, start the engine and let it run a few minutes until it is warm enough for the final adjustments.

Final Adjustments — With Engine Running

  1. Set the throttle-stop screw for the desired idling speed with the choke open.
  2. Set the idle-adjusting needle for smooth idling. Turning the needle in (clockwise) reduces idle air and enriches the mixture; turning it out (counterclockwise) admits more air and leans the mixture. Avoid using a mixture that is too lean.
  3. For models with a main-jet adjustment, hold the throttle at about two-thirds of top engine speed. Turn the needle in until the engine begins to run irregularly, then turn it out while counting turns until engine speed begins to drop again. Set the needle about halfway between these lean and rich points, then hold the adjustment and tighten the packing nut.

Important Zenith Notes

Updraft carburetor: Zenith notes that gasoline can accumulate in the intake manifold from condensation and drain back into the carburetor. The fuel bowl has a drain for this purpose. Do not automatically interpret fuel dripping from the air horn/intake as flooding or leaking.

Zenith also instructs the installer to check all fuel-line connections for leakage, inspect distributor points for pitting and proper gap, and make sure spark plugs are clean and properly gapped.

Source: Zenith Fuel Systems LLC, Zenith Carburetor Installation Instructions — Series 68 Universal Type Carburetors. This page reorganizes and lightly condenses the original two-page manufacturer instruction sheet for online reference. Measurements, adjustment settings, cautions, and technical meaning are retained from the source. Consult the original scanned instructions using the PDF link above.