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Onan B48G Carburetor

Onan B48G Nikki Carburetor

Removal and Disassembly

Remove air cleaner and hose. Disconnect governor and throttle linkage, choke control and fuel line from carburetor. Remove the four intake manifold cap screws and lift complete manifold assembly from engine. Remove carburetor from intake manifold.

Nikki carburetor limited idle adjustment - Remove main jet and idle adjustment needle. Remove attaching screws and separate upper and lower carburetor sections. Carefully note position of float assembly parts, then pull out retaining pin and float assembly. Remove needle valve.

Onan B48G Nikki Carburetor - Cleaning and Repair

Soak all metal components not replaced in carburetor cleaner. Do not soak non-metal floats or other non-metal parts. Follow the cleaning manufacturer’s recommendations. Clean all carbon from the carburetor bore, especially where the throttle and choke plates seat. Be careful not to plug the idle or main fuel ports. Dry out all passages with low pressure air (35 PSI).

Avoid using wire or other objects for cleaning which may increase the size of critical passages. Check the condition of the adjustment needle; replace if damaged. Replace float if loaded with fuel or damaged. Check the choke and throttle shafts for excessive play in their bore. This condition may necessitate replacement of the carburetor. Replace old components with new parts.

Nikki limited idle adjustment - Reassembly and Installation

- Install needle valve, main jet, and float assembly.
- Make sure float pivot pin is properly placed and float moves freely without binding.
- Turn carburetor on its side and measure float level. Adjust float level only if necessary.
- Measure float drop (the distance from the top of carburetor body to top of float). Adjust only if necessary.

- Position gasket on lower carburetor section and install upper carburetor section.
- Install idle adjustment screw, throttle stop screw, and fixed main jet plug.
- Mount carburetor on intake manifold and install assembly on engine.
- Mount air cleaner assembly. Connect air intake hose, breather hose, fuel line, vacuum line, and throttle linkage.
- Adjust carburetor and governor according to directions given in this section.

Onan B48G - Nikki Carburetor Adjustment

The carburetor idle and main mixture were set for maximum efficiency at the factory and should normally not be disturbed. If adjustments seem necessary, first be sure the ignition system is working properly and governor sensitivity is properly adjusted. If adjustment is needed, proceed as follows and refer to figures. Overtightening the mixture adjustment screw will cause carburetor damage. Turn mixture adjustment screw in only until light tension can be felt.

These initial adjustments will permit engine to start and warm up prior to final adjustment.

Onan B48G Nikki carburetor mixture screw settings

Turn idle mixture screw in until lightly seated, then back idle mixture screw out 3/4 turn. On engines equipped with main fuel adjustment, turn main mixture screw in until lightly seated then back the main mixture screw out 1-1/4 to 1-1/2 turns.

Onan B48G Nikki carburetor with semi-automatic choke and limited idle adjustment screw

No initial mixture adjustments are required. When replacing idle mixture screw, turn in until lightly seated, then turn screw back out 1-1/4 turns. Replace limiter cap with the plastic stop approximately centered.

Start the engine and allow it to warm up thoroughly (at least 10 minutes). Move the engine speed control to the slow position. Bend or turn the low speed stop on the governor so the throttle stop screw on the carburetor controls engine speed. Adjust throttle stop screw to obtain the following rpm: Nikki carburetor 1000 rpm.

Determine the best idle mixture setting by first turning the idle adjustment screw in until engine speed drops and then outward until engine speed drops again. Over a narrow range between these two settings, engine speed remains at its highest. Set the adjustment screw about 1/8 turn outward (rich) from the midpoint of this range.

Readjust throttle stop screw to obtain the RPM specified in step 3 and release the governor arm. Adjust governor low speed stop for 1100 ± 100 rpm idle. Move the engine speed control to the fast position. Bend the high speed stop on the governor so the engine runs at the equipment manufacturer’s recommended speed.

Check the main mixture adjustment (optional on some carburetors) by rapidly accelerating the engine from idle to full speed. The engine should accelerate evenly and without hesitation. If it does not, turn the main adjustment screw out in 1/8 turn increments until the engine accelerates smoothly, but do not turn it out more than 1/2 turn beyond the original setting.

Onan B48G Marvel Schebler and Walbro Carburetors

Removal and Disassembly

- Remove air cleaner and hose.
- Disconnect governor and throttle linkage, choke control and fuel line from carburetor.
- Remove the four intake manifold cap screws and lift complete manifold assembly from engine.
- Remove carburetor from intake manifold.

- Remove throttle and choke plate retaining screws and plates. Pull out throttle and choke shafts.
- Remove main adjustment and idle adjustment needles.
- Remove attaching screws and separate upper and lower carburetor sections.
- Carefully note position of float assembly parts, then pull out retaining pin and float assembly.
- Remove needle and unscrew needle valve seat.

Cleaning and Repair

Soak all metal components not replaced in carburetor cleaner. Do not soak non-metal floats or other non-metal parts. Follow the cleaning manufacturer’s recommendations. Clean all carbon from the carburetor bore, especially where the throttle and choke plates seat. Be careful not to plug the idle or main fuel ports. Dry out all passages with low pressure air (35 PSI).

Position gasket on carburetor and attach carburetor sections together with screws. Slide in throttle shaft and install plate using new screws. Before tightening the screws, the plate must be centered in the bore. To do so, back off the throttle stop screw as necessary and completely close the throttle lever. Seat the plate by tapping with a small screwdriver, then tighten screws. Install the choke shaft and plate in the same manner.

Install idle adjustment screw, throttle stop screw, and fixed main jet plug or optional main fuel adjustment needle. Mount carburetor on intake manifold and install assembly on engine. Mount air cleaner assembly. Connect air intake hose, breather hose, fuel line, vacuum line, and throttle linkage. Adjust carburetor and governor according to directions given in this section.

Onan B48G - Marvel Schebler and Walbro Carburetor Adjustment

These initial adjustments will permit engine to start and warm up prior to final adjustment. Marvel Schebler carburetor mixture screw settings: Turn both mixture screws in until lightly seated, then back the idle mixture screw out 1 turn and the main mixture screw out 1-1/4 turns. Walbro carburetor mixture screw settings: Turn idle mixture screw in until lightly seated, then back idle mixture screw out 1-1/8 turns. On engines equipped with main fuel adjustment, turn main mixture screw in until lightly seated then back the main mixture screw out 1-1/2 turns.

Start the engine and allow it to warm up thoroughly (at least 10 minutes). Move the engine speed control to the slow position. Bend or turn the low speed stop on the governor so the throttle stop screw on the carburetor controls engine speed. Adjust throttle stop screw to obtain the following rpm: Marvel Schebler carburetor without governor low speed adjustment screw 1200 rpm, with governor low speed adjustment screw 1100 rpm.

Walbro carburetor 1000 rpm. Determine the best idle mixture setting by first turning the idle adjustment screw in until engine speed drops and then outward until engine speed drops again. Over a narrow range between these two settings, engine speed remains at its highest. Set the adjustment screw about 1/8 turn outward (rich) from the midpoint of this range.

Readjust throttle stop screw to obtain the RPM specified in step 3 and release the governor arm. Adjust governor low speed stop for 1100 ± 100 rpm idle. Move the engine speed control to the fast position. Bend the high speed stop on the governor so the engine runs at the equipment manufacturer’s recommended speed. Check the main mixture adjustment (optional on some carburetors) by rapidly accelerating the engine from idle to full speed. The engine should accelerate evenly and without hesitation. If it does not, turn the main adjustment screw out in 1/8 turn increments until the engine accelerates smoothly, but do not turn it out more than 1/2 turn beyond the original setting.

Governor adjustment

Onan B48G engines equipped with variable speed governors are adapted for use where a wide range of speed settings are desired. Engine speed is controlled at any given point between minimum and maximum by simply shifting the throttle lever on the control panel until the desired speed is reached. Engines equipped with fixed speed governors are generally used where a single constant speed is required.

Engine speed is controlled by adjusting governor spring tension. The fixed and variable speed governor give an automatic decrease in sensitivity when the speed is increased. The result is good stability at all speeds. A reliable instrument for checking engine speed is required for accurate governor adjustment. Engine speed can be checked with a tachometer.

Check the governor arm, linkage, throttle shaft, and lever for binding condition or excessive slack and wear at connecting points. A binding condition at any point will cause the governor to act slowly and regulation will be poor. Excessive looseness may cause a hunting condition and regulation could be erratic. Work the arm back and forth several times by hand while the engine is idling to check for above conditions.

If the variable speed governor is hunting or not operating properly, adjust as follows. Disconnect linkage from one of holes. Push linkage and governor arm as far back (toward carburetor) as they will go. Holding linkage and governor arm toward direction of carburetor, insert end of linkage into whichever hole in governor arm lines up the closest. If between two holes, insert in next hole out.

To adjust fixed speed governor: Adjust governor link length so that with the engine stopped and tension on the governor spring, the stop on the throttle shaft assembly almost touches the stop on the side of the carburetor. On side pull governors the governor spring is set by the factory in the third hole of the governor arm (Third hole from pivot).

On front pull governors the governor spring is set by the factory in the second hole of the governor arm adapter (Second hole from pivot). To increase sensitivity, move spring loop into a hole closer to the pivot. To decrease sensitivity, move spring loop into a hole farther away from the pivot. After sensitivity has been set, recheck the low speed rpm setting. Adjust if necessary.