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Onan B43G Carburetor

Onan B43G Carburetor Overhaul

Onan B43G carburetor problems that are not corrected by mixture adjustments are usually a result of gummed-up fuel passages or worn internal parts. The most effective solution is a carburetor overhaul. In general, overhauling a carburetor consists of disassembly, a thorough cleaning, and replacement of worn parts. Carburetor overhaul kits are available.

Nikki Carburetor

Removal (Nikki standard)

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.

Disassembly (Nikki 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.

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.

Reassembly and Installation (Nikki limited idle adjustment)

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 B43G - 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.

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.

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.

Marvel Schebler and Walbro Carburetor

Removal

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.

Disassembly

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). 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.

Reassembly and Installation

Install needle valve and seat, main jet and float assembly. Make sure that float pivot pin is properly placed and that float moves freely without binding. Gently push float tang down until needle just seats. Measure float level. Adjust float level if necessary. Release float tang and measure float drop. This is the distance from top of carburetor body to top of float. When checking float level and float drop, measure to float body, not seam. 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 B43G - 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.

Onan B43G - Pulsating-Diaphragm Fuel Pump

Pulsating-diaphragm fuel pumps, or pulse pumps, rely on changes in crankcase vacuum to create a pulsating movement of the pump diaphragm. As the engine’s pistons move outward, a vacuum is created. This vacuum is transmitted to the pump diaphragm, causing it to pull back and suck fuel into the pump. As the engine’s pistons move inward, crankcase vacuum is reduced and the diaphragm return spring pushes the pump diaphragm forward, forcing fuel through the pump outlet. Before testing make certain that fuel pump vacuum line connections are tight and free of leaks.

Fuel Pump Test Procedure

Operate engine at an idle for five minutes to ensure that carburetor is full of fuel. Shut engine off and remove fuel inlet line from fuel pump. Connect a vacuum gauge to fuel pump inlet using a piece of fuel hose with clamps. Start engine and allow to idle for at least five seconds. Record vacuum gauge reading. Move throttle control to high idle position. Wait at least five seconds and record vacuum gauge reading. Shut engine off and remove vacuum gauge hose from fuel pump inlet. Connect fuel inlet line to fuel pump. Remove fuel outlet line from fuel pump. Connect a pressure gauge to fuel pump outlet using a piece of fuel hose with clamps. Start engine and allow to idle for at least five seconds. While holding pressure gauge level with pump outlet record pressure gauge reading. Move throttle control to high idle position and allow engine to run for at least five seconds. While holding pressure gauge level with pump outlet record pressure gauge reading.

Onan B43G - Air Cleaner

Dirty air cleaner element can cause engine damage. Ensure air cleaner clean and free of excess debris. Engine is equipped with a paper element. If the engine is equipped with polyurethane precleaner, it must be removed, cleaned and oiled every 25 hours of operation; more often under extremely dusty conditions. To clean precleaner, wash in water and detergent. Remove excess water by squeezing like a sponge and allow to dry thoroughly. Distribute two tablespoons of SAE-30 engine oil evenly around the precleaner. Knead into the precleaner to dis¬tribute oil evenly. Wring out excess oil. Depending on conditions in which the engine is operating, the inner paper element should be replaced whenever it becomes excessively dirty or oily. Running engine without air cleaner element will result in engine damage. Do not run engine without air cleaner element installed.