Marvel carb Idle Adjusting Needle

[old31]

With a Marvel, when you turn the Idle Adjusting Needle in, (clockwise) are you increasing or decreasing the amount of fuel being used.

My plugs look a little dark, and I am not sure which way to go.be

[alexiskai]

See table

[old31]

So, based on the table, if I’m running a little rich, I would want to move the needle out is that correct?

[alexiskai]

No, sorry, the table is a little ambiguous. The headings correspond to the condition you want, not the condition you're trying to correct. Turning the Marvel idle screw out will richen the mixture. The way the Marvel carbs are constructed, the idle screw needle controls the entire fuel-air mixture coming from the idle circuit, which mixes with air coming through the throttle body to form the total FAM. So if you turn the needle in, you're decreasing the amount of fuel in the mix.

[TomInCologne]

Originally Posted by alexiskai

No, sorry, the table is a little ambiguous. The headings correspond to the condition you want, not the condition you're trying to correct. Turning the Marvel idle screw out will richen the mixture. The way the Marvel carbs are constructed, the idle screw needle controls the entire fuel-air mixture coming from the idle circuit, which mixes with air coming through the throttle body to form the total FAM. So if you turn the needle in, you're decreasing the amount of fuel in the mix.

Thanks! I was questioning myself...

[mcgarrett]

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It might be more accurate if we use the term "mixture" adjustment screw instead of idle adjustment needle. The idle (speed) adjustment is achieved by turning the screw on the throttle lever at the carburetor.

To recap; turn the mixture screw IN/clockwise to LEAN the idle mixture, and OUT/counter-clockwise to enrichen it.

[Y-Blockhead]

Originally Posted by alexiskai

See table

Originally Posted by old31

So, based on the table, if I’m running a little rich, I would want to move the needle out is that correct?

Wrong

IMO that table should be trashed. It can easily be construed to do exactly opposite what should be done (as old31 assumed in post #3).

The idle adjustment screw controls the fuel flow on a Marvel, so to increase the flow (enrichen) you turn it out (counter-clockwise).

On the Tilly and Zenith, the idle mixture screw controls the air flow.

[alexiskai]

Originally Posted by Y-Blockhead

The idle adjustment screw controls the fuel flow on a Marvel, so to increase the flow (enrichen) you turn it out (counter-clockwise).

On the Marvel, the idle mixture adjustment screw controls the fuel and air coming from the idle circuit. On both the Zenith and Marvel carbs, there are four components to the FAM at idle:

1. Air coming in through the main intake that diverts through a bypass leading to the idle jet
2. Fuel flow from the idle jet that mixes with the bypass air
3. Air entering the air bleed, which is a tiny hole on the outside of the carb, that mixes with 1 and 2
4. Any air that gets around the closed butterfly valve in the air horn that mixes with 1, 2, and 3 as it enters the manifold

The Zenith design puts the idle mixture adjustment needle in the path of the air bleed. Close it off and the FAM gets richer. The Marvel design puts it just before the air bleed, after the bypass air has mixed with fuel. Close it off and the FAM gets leaner.

[Y-Blockhead]

Originally Posted by alexiskai

On the Marvel, the idle mixture adjustment screw controls the fuel and air coming from the idle circuit. On both the Zenith and Marvel carbs, there are four components to the FAM at idle:

1. Air coming in through the main intake that diverts through a bypass leading to the idle jet
2. Fuel flow from the idle jet that mixes with the bypass air
3. Air entering the air bleed, which is a tiny hole on the outside of the carb, that mixes with 1 and 2
4. Any air that gets around the closed butterfly valve in the air horn that mixes with 1, 2, and 3 as it enters the manifold

The Zenith design puts the idle mixture adjustment needle in the path of the air bleed. Close it off and the FAM gets richer. The Marvel design puts it just before the air bleed, after the bypass air has mixed with fuel. Close it off and the FAM gets leaner.

Say it anyway you want to but turning the screw out will richen a Marvel.

Instructions for adjusting a Marvel/Allstate/Johnston