G3408 and G3412 Engines Caterpillar

When an engine is not developing sufficient power, the horsepower of the engine should be checked. Use the 1U-5470 Engine Pressure Group to check the pressure in the inlet manifold.

Illustration 1	g00293196
1U-5470 Engine Pressure Group

This tool group has a gauge that can read pressure in the inlet manifold. The Special Instruction, SEHS8524 is provided with this tool.

The engine must be adjusted correctly in order to obtain correct results from the tests.

Engine operating efficiency can be obtained by checking the inlet manifold pressure. Compare the pressure in the inlet manifold with the pressure that is given in the Technical Marketing Information. This test can be used if the engine is experiencing low horsepower. This test can not be used with any other problem.

Gas engines can burn a wide range of gaseous fuels. The BTU content of the fuel is a measure of the power content of the fuel. The fuels that have a high BTU content require less gas pressure to obtain a specific horsepower.

A complete fuel analysis must be conducted prior to putting the engine into service. Obtain a fuel analysis in order to determine the BTU content of the fuel. Calculate the methane number. The BTU content indicates the amount of energy that is available in the fuel. The methane number indicates the ability of the fuel to be ignited. The methane number is determined when you input the constituents of the fuel into the Methane Number Program, LEKQ6378. Use the methane number and the Engine Performance Guide, "Fuel Usage Guide" in order to determine the ignition timing.

Fuels with a low methane number burn so fast that the timing must be retarded. If the timing on an engine is too early, the fuel will burn too quickly. The fuel will burn before the piston passes the top center position. This results in knocking.

The air/fuel ratio adjustment is made by changing the gas pressure. Excess gas makes a rich mixture and not enough gas creates a lean mixture. Either one of these conditions will create poor engine performance. The air/fuel ratio at no load is controlled by the gas regulator. The load valve controls the air/fuel ratio. The load valve is located at the carburetor.

The high heat value (HHV) of a fuel is a unit of measurement. This is a measurement of the content of heat in the fuel. The low heat value (LHV) of the fuel is an important measurement. The process of combustion forms carbon dioxide and water. Heat that is needed for the conversion of water into vapor is lost to the engine. The heat that can be used in the fuel is the LHV of the fuel. The low heat value of a fuel is ten percent lower than the high heat value of natural gas. When the HHV is given, change the specification to the LHV. This will ensure that the correct results are obtained. Make reference to the Operation And Maintenance Manual.

Initial Adjustments

Carburetor Control Lever

Illustration 2	g00579993
Typical Example Of A Carburetor Lever And Linkage (1) Lever in the closed position. (2) Lever in the open position. (3) Carburetor lever. (4) Location of the throttle stop screw. (5) Pin. (6) Linkage rod between the carburetor and the governor, or the actuator.

1. Disconnect linkage rod (6) from carburetor lever (3) and governor lever (8) or actuator lever (11).

2. Turn the throttle stop screw (4) in the counterclockwise direction. Turn the throttle stop screw in order to close the throttle plate. Do not allow the screw to touch the stop.

3. Rotate the carburetor throttle plate to the closed position and hold. Carburetor lever (3) must be at the following closed position of angle (A) :

   N.A. engine with governor ... 40°

   T.A. engine with governor ... 15°

   T.A. engine with actuator ... 40°

4. Drill a 3.00 mm (0.12 inch) hole through lever (3) and the shaft. Install pin (5).
   Show/hide table

   Table 2

   Angle Of The Governor Lever
   Engine	G3408	G3412
   N.A.	10° ± 5°	26° ± 5°
   T.A.	26° ± 5°	26° ± 5°

Show/hide table



Illustration 3	g00580019
Governor Lever (7) Pin. (8) Lever. (9) Mounting holes for the linkage rod.

Show/hide table



Illustration 4	g00580020
Actuator Lever (11) Lever.

5. Rotate the governor control shaft to the Shutoff position. The governor lever (8) must be 26 degrees ± 15' from the horizontal. The actuator lever (11) must be at the following angle (B) from the horizontal:

   3408 T.A ... 20° + 10° - 0°

   3412 T.A. ... 12° ± 5°

   The governor shaft and the actuator shaft are splined.

Show/hide table



Illustration 5	g00594531

6. While the levers (3) and (8) or lever (11) are in the closed position, adjust the rod (6). Install the rod to the bottom side of the lever (3). Install the rod to the following locations on lever (8) or lever (11) :

   3412 with actuator ... side "C"

   All other locations ... side "D"

   3408 with governor ... outer hole (9)

   3412 with governor ... outer hole (9)

   Note: A spacer is required between rod (6) and lever (8). This is not required on 3412 Engines with electric governors. Another spacer is required between rod (6) and lever (3) on 3408 Engines with electric governors.

   Linkage adjustment should now be complete. The governor is in the shutoff position. The carburetor throttle plate is on the bottom in the throttle body.

7. Turn the throttle stop screw (4) in the clockwise direction. Turn the screw so that contact is made with the throttle plate and the bottom of the throttle body.

8. Check linkage operation. The linkage must not bind at any position.

Adjusting The Load Valve For High Pressure Fuel Systems

Illustration 6	g00580024
Carburetor Adjustment (1) Load valve.

Turn the load valve (1) of the carburetor. Turn this valve to the center position. The center position is between "L" and "R". "L" represents a lean fuel mixture. "R" represents a rich fuel mixture. This is a starting point. Further adjustment may be required in order to meet the correct air/fuel ratio.

On a dual fuel engine, the load valve (1) is used for the fuel system with a low BTU content only. For a fuel system with a high BTU content, the load valve (2) is set to the center position. This center position is between "L" and "R". "L" represents a lean fuel mixture. "R" represents a rich fuel mixture. Further adjustment may be required.

Illustration 7	g00580085
Dual Fuel Adjustment (View A-A) (2) Load valve.

Illustration 8	g00595175
Dual Fuel Adjustment (2) Load valve. (3) Pressure regulator. (4) Regulator for vacuum.

Low Pressure Fuel System

Illustration 9	g00637475
(1) Load Valve. (2) Carburetor.

Turn the power adjustment screw (1) on the carburetor in the clockwise direction until the screw bottoms out. Turn the screw counterclockwise for four full turns. This should be done for all low pressure gas engines.

Note: Turn the power adjustment screw clockwise for a leaner fuel mixture. Turn the power adjustment screw counterclockwise for a richer fuel mixture.

Final Adjustments

Final adjustments are required for proper engine performance at both no load and normal load. The instructions for adjusting the following items also pertain to a dual fuel engine.

• gas pressure regulators

• engine timing

• adjusting load valves

On dual fuel engines, the two fuels utilize separate gas pressure regulators and load valves. Complete all of the final adjustments for the fuel system with a low BTU content. Proceed to the fuel system with a high BTU content.

Gas Pressure Regulators

The gas pressure regulator requires adjustment during the following conditions:

• Engine installation

• Reworking of the regulator

• Change in the BTU content of the fuel

In order to adjust the gas pressure regulator correctly, the 1U-5470 Engine Pressure Group must be used. This pressure group will measure the air/fuel differential pressure.

Perform the adjustments on the gas pressure regulator as the adjustments are outlined in this section. The difference in pressure for G3408 Engines and G3412 Engines with high pressure systems and with low pressure systems is equal. The difference in pressure should be equal to the following amount.

Difference in pressure ... 1 to 1.2 kPa (4 to 5 inch of H₂O)

Engines that use negative pressure regulators should be set to -0.25 kPa (-1.00 inch of H₂O).

Adjustment For Gas Regulators With Known BTU Content Of The Fuel

The following tools will be needed in order to perform these adjustments:

• 8T-5160 Gas Regulator Setting Tool

• 1U-5470 Engine Pressure Group

Illustration 10	g00580080
Pressure Measurement (1) Carburetor. (2) Plug. (3) Location for the tee for the pressure tap. (4) Cap over the adjustment screw.

Illustration 11	g00580088
Pressure Measurement On Dual Fuel Engines (1) Carburetor. (2) Plug. (3) Plug. (4) Cap over the adjustment screw. (5) Cap over the adjustment screw. (6) Plug on the balance line. (7) Location for the tee for the pressure tap.

1. Make sure that the main valve is shut off.

   Note: If the BTU content is not known in a pressure regulator, disregard steps 2, 3, 4, 10, 11, and 12.

2. Remove the plug from the gas line at location (2) .

3. For pressure regulators, remove the elbow at point (2) and replace the elbow with a tee. Remove plug (6) on regulators that measure vacuum.

4. Connect the 1U-5470 Engine Pressure Group and the shutoff valve. This can display the pressure differential on the pressure regulators. This connection should be made between location (2) and location (4). Use locations (3) and (6) for pressure regulators to measure a vacuum.

5. Remove the cap (5) for the pressure regulators. Remove the cap (7) for the pressure regulators that measure vacuum. If a locknut is present, loosen the locknut. Install the correct tools from the 8T-5160 Gas Regulator Setting Tool. On regulators that measure vacuum, the screw should be adjusted to a different position. This screw should be adjusted to the halfway point.

6. Open the main gas valve. Start the engine and run the engine at high idle.

7. Adjust the regulator. Turn the screw for the positive regulator in the clockwise direction. This will cause the amount of output on the fuel pressure regulator to increase. On negative regulators, turning the nut inward will decrease the amount of output. This will reduce the vacuum.

8. Shut off the engine.

9. Remove the 8T-5160 Gas Regulator Setting Tool from the regulator. Install the cap (5) or the cap (7) .

10. Remove the 1U-5470 Engine Pressure Group from the engine.

11. Install plug (2) and install plug (3).

12. For pressure regulators, install a cap or a plug at location (4) and (5). The elbow that was removed earlier at this location will not be used.

Engine Timing

1. Start the engine after the adjustment for the regulator is complete. Allow the engine to warm up to normal operating temperatures at minimum load.

2. Set the engine at rated speed with minimum load and adjust engine timing to the correct value. See MAGNETO TIMING in the IGNITION SYSTEM section for the correct specifications.

3. On a dual fuel engine, the timing must be reset if the fuel is changed.

Adjusting The Load Valve

Refer to Special Instruction, REHS0371.

The following factors are influenced by the air/fuel ratio.

• Fuel consumption

• Engine power output

• Engine stability

• Engine component life

• Deterioration of lube oil

The air/fuel ratio which produces the lowest fuel consumption generally produces the maximum oxides of nitrogen NO. This level of NO can cause rapid deterioration of the lube oil through nitration. Richer air/fuel ratios reduce NO levels. A rich air/fuel ratio will have the following effects on the engine.

• Increase in fuel consumption

• Increased exhaust temperature

• Increase in levels of carbon monoxide

A lean air/fuel mixture will have the following effects on the engine.

• Reduction in fuel consumption

• Reduced exhaust temperature

• Increase in levels of NO

• Reduction in power output

• Increase in the tendency for misfiring of the engine

The most accurate method of adjusting the air/fuel ratio involves the use of an oxygen meter. This method consumes the least amount of time as well. The measurement of free oxygen O₂ in the exhaust gas is an accurate indicator of air/fuel ratio. If optimum fuel consumption is of prime importance, an O₂ level of 1% to 2% will be required. This setting requires close monitoring of oil performance. Oil change intervals may be shortened. Refer to Lubrication Section of the Application and Installation Guide, LEBH6154 at your Caterpillar dealer for information on oil change intervals.

An air/fuel ratio which reduces oxygen in the exhaust to zero, or an air/fuel ratio that is rich is preferred in certain circumstances. One of these circumstances includes a necessity for the maximum life of the oil that is possible. This includes a necessity for minimum maintenance. Air/fuel ratios that do not contain excess fuel or excess oxygen have low levels of NO. Minimum sensitivity of the oil is created if this type of air/fuel ratio is used. This type of air/fuel ratio is used with wellhead gas.

The following instructions provide the correct procedure for adjusting the load valve. Adjust the load valve so that the desired O₂ level in the exhaust is reached. The load valve should not have an effect on the air/fuel ratio if there is no load on the engine.

Note: A dual fuel engine has a load valve for each different kind of fuel. Refer to Initial Adjustments for the location of the load valves.

Adjustment Procedure With An Oxygen Meter

1. Start the engine and run the engine at normal load.

Show/hide table



Illustration 12	g00580851
Location For Oxygen Meter (1) Plug

2. Remove plug (1) at the exhaust elbow and use an oxygen meter to check O₂ level. If the O₂ level is not at the desired setting, slowly adjust the load valve. Turn the valve to the "L" position or the "R" position. Continue to turn the valve in order to reach the desired O₂ level.