About this guide: These adjustment instructions are provided by Dell Rice with Marvel carburetors he supplies for Model A Fords.
Dell notes that this carburetor was adjusted to run efficiently on his Model A, so it should already be adjusted reasonably close to what is needed. As all Model A's have their own personalities, some adjustment may be required.
Mounting the Carburetor
When mounting the carburetor with a new gasket, do not overtighten the two bolts. The top casting can warp or the threads can strip. Dell notes that the castings are pot metal and cautions against trying to torque the bolts down.
Initial Adjustments
- Idle air/gas mixture adjustment screw: Close the screw all the way in clockwise, then open it approximately 3/4 turn counterclockwise.
- Gas adjusting needle (GAV): From inside the car or truck, turn the GAV clockwise until it bottoms out. Do not force it. Then open it 1/2 to 3/4 turn counterclockwise.
After the engine warms up, the GAV can be turned clockwise to slow the engine speed or counterclockwise to speed it up. Once the engine is warmed up, the idle speed can be adjusted.
Set the Idle
Close the idle speed adjustment screw clockwise all the way and then open it 1/4 turn. Use the idle air adjustment screw to speed up or slow down the engine to the desired idle. Adjust the gas adjusting screw at the same time to balance the fuel/air mixture and achieve the best idle speed. Both exterior screws adjust the idle speed.
Starting Position
The gas adjusting needle (GAV) must be closed down as described above, and the throttle control on the steering column must be all the way up before the engine speed is slow enough for the idle circuit to work properly.
If the Float Needle Sticks Open
Occasionally during shipping or storage, the float valve needle can become stuck open, allowing fuel to leak through the vent hole or the rear of the carburetor while the engine is running. Dell recommends tapping firmly on the carburetor body at the top and front where the fuel line attaches. He specifically cautions against using a two-pound shop hammer. This will usually unstick the needle.
Fuel Filtration
Dell recommends installing an inline fuel filter between the sediment bowl and the carburetor. He notes that the filter in the gas tank and the sediment bowl catch larger particles, but smaller material can still pass through and a very small particle can jam the float valve and flood the carburetor. A clear inline filter also permits visual inspection and makes the filter element easy to clean or replace.
Air Filter Note
Dell also recommends that an air filter not be used with this setup. His handout states that an air filter restricts airflow, creating a fuel/air mixture that is too rich and fouls the spark plugs quickly. He notes that he does not use an air filter on his Model A and has experienced no problems.
This page is a transcription of Dell Rice's original January 28, 2025 setup and adjustment guide supplied with Marvel carburetors he provides for Model A Fords. Formatting and minor grammatical conventions have been cleaned up for online reading while retaining the procedures, settings, cautions, and recommendations in the source.