Walbro Carburetor Service

Hi, I’m Lois Alice, Program Manager for Walbro, and I’m Don Ross, Lawn and Garden Business Unit Manager. This video is intended to help you inspect in-service Walbro diaphragm carburetors, whether butterfly or rotary valve models.

As you know, a lot of changes in our industry have affected carburetor service. The introduction of ethanol into fuels affects carburetor operation, and tamper-resistant features limit the amount of adjustment users can make to carburetors. Today, we’re going to discuss these topics along with the inspection and service of diaphragm carburetors.

A basic understanding of carburetor function will be very handy for this video, so let’s start there.

The carburetor performs three important tasks. First, fuel is drawn from the gas tank into the carburetor. Second, the fuel is metered to match the engine’s RPM and load requirements. Finally, the metered amount of fuel is mixed with air and delivered to the crankcase at the proper time in the engine’s operating cycle.

To control the flow of fuel and air into the engine, the carburetor uses pressure created within the engine’s crankcase, known as an impulse. The movement of the piston creates two types of impulses: a negative impulse, or vacuum, created during the piston’s upward stroke, and a positive impulse generated during the piston’s downward motion.

These impulses are delivered to the carburetor through machined channels in a manifold or impulse fitting. The channels or fittings lead to the fuel pump. The carburetor throat contains the throttle valve and, in some models, the choke valve.

“That’s good, Louie, but if I think I’ve got a carburetor problem, where do I start?”

“Don, whenever you have a carburetor of any type that seems to be malfunctioning, you first want to eliminate any potential causes not directly related to the carburetor. Always use fresh fuel based on the engine manufacturer’s recommendations. Engine problems resulting from bad spark plugs, a clogged muffler, or low compression can also be misinterpreted as carburetor problems, so be sure to include these areas in your initial engine inspection procedures.”

Then check the fuel tank vent and the fuel filter for obstructions. Debris in either area could easily cause a fuel delivery problem. If the filter is dirty, clean or replace it with an original Walbro fuel filter.

If the tank and vent are clear, disassemble and inspect the air filter. Because carburetors depend on adequate airflow, a clogged filter could result in an overly rich condition. Next, check the fuel and primer lines. Be sure they aren’t pinched, plugged, or cut.

Your initial inspection should also include a pressure check of the fuel line and the carburetor. Pressure testing the carburetor reveals the operating condition of the inlet needle.

Connect your Walbro pressure tester, part number 57-11, to the fuel line. Apply air pressure by slowly depressing the plunger. The gauge will indicate an increase in pressure until the inlet needle valve unseats. This level is referred to as the pop-off pressure and may range from 15 to 50 PSI, depending on the carburetor design specifications.

Occasionally, the inlet needle valve will not unseat. In this case, you’ll need to disassemble the carburetor, which we’ll discuss shortly.

After popping off, the gauge will drop to indicate the pressure at which the inlet valve reseats. This is referred to as the reseat pressure. The inlet needle should reseat and hold a minimum of 5 PSI.

Complete the pressure check by performing the same test at the carburetor’s fuel inlet. If pressure tests show proper needle operation, it’s time to check the carburetor mounting area and the manifold and gasket connecting the carburetor to the engine’s crankcase for wear or damage.

Be sure to inspect the impulse passage leading to the fuel pump chamber. Crankcase impulses must be able to reach the carburetor for it to operate properly. To test, put a drop of oil over the passage, crank the engine a few times, and watch the oil. If the oil doesn’t move or only moves slightly, the passage is blocked and must be cleared.

If the engine you’re working on has a reed valve, check the valve. It should be free of debris and properly aligned with the intake port.

We should note that everything we’ve talked about so far applies to all types of diaphragm carburetors. Now we want to look specifically at servicing the two major types of Walbro diaphragm carburetors used in handheld equipment.

Walbro manufactures a variety of butterfly valve carburetor models, yet their components are all very similar. Let’s see how a butterfly valve carburetor functions.

The butterfly valve carburetor can be divided into three basic areas of operation: the fuel pump, the fuel metering area, and the throttle and choke valve area.

The fuel pump is made up of a cover, a gasket, a diaphragm, and the pump side of the carburetor body. The pump is activated by crankcase impulses delivered through a machined channel or an impulse fitting.

The metering side also includes a fuel inlet screen, inlet needle orifice, inlet needle, and fuel metering chamber. The metering chamber is made up of a cover, a metering diaphragm, a gasket, a metering lever pin and spring, a nozzle check valve, and the idle assembly located within the carburetor body, along with the idle adjustment screw.

The final area of operation includes the choke and throttle valve assemblies. The throttle valve assembly is made up of a shaft, lever assembly, spring, and valve. The choke assembly includes a shaft and lever assembly, choke friction ball, spring, and valve.

The amount of fuel delivered to the crankcase is determined by the position of the throttle valve. The more open the valve, the greater the airflow, and the greater the amount of fuel delivered to the crankcase.

“Well, Louie, that’s a lot of good information.”

“It is, Don, because a basic understanding of the butterfly valve carburetor’s function will come in handy as we inspect it for proper operation.”

So let’s get started with the proper procedure for inspecting the butterfly valve carburetor.

Before you begin inspecting the carburetor, be sure the metering and fuel pump areas are completely disassembled. Idle progression holes have been machined into the carburetor body. These passages are accessible through a Welsh plug.

To remove the Welsh plug, use tool number 500-16, available in both the 400-595 and 500-500 Walbro tool kits. Position the sharp end of the tool at a slight angle and tap until it penetrates the Welsh plug. Then apply light pressure to the handle until the plug pops out. Be careful not to contact or damage the carburetor body with the tool.

Once the carburetor is disassembled, clean the entire body and its covers. Use mineral spirits to avoid damage to carburetor parts, and blow dry with approximately 25 PSI of compressed air. Be sure all holes and passages are clear.

On models with a check valve that has a nozzle feed hole, you can verify the nozzle check valve’s operation with a small hose or fuel line. Position the hose or fuel line over the metering chamber side of the nozzle, then draw the nozzle check valve up against the retainer two or three times to ensure it’s operating properly. Do not use a metal tool to clear the nozzle.

Now we can reassemble the carburetor, beginning by installing the Welsh plug using tool number 500-15. Align the Welsh plug in position, place the tool on the dome part of the plug, then tap until the plug becomes flat. It’s important that the plug is flat so it doesn’t interfere with the function of the metering diaphragm assembly.

“In the past, I remember using nail polish to seal the Welsh plugs. Is that still true?”

“Because of ethanol fuels, we no longer use fingernail polish or any kind of sealant on Welsh plugs. We found that ethanol fuels can dissolve the sealant and create other problems.”

Now we can reassemble the inlet needle, metering lever pin, spring, and lever pin screw.

Next, check the lever height adjustment with Walbro’s metering gauge number 500-13. Position the gauge across the carburetor body. The gauge should just contact the metering lever without opening the needle valve.

If the lever is misaligned, use a small screwdriver to keep the head of the inlet needle stationary while using another screwdriver to lift or depress the metering lever arm until it is properly adjusted.

This would also be a good time to check the function of the inlet needle. With the metering cover and metering diaphragm assembly removed, apply pressure to the fuel inlet fitting and manually depress the metering lever to ensure the inlet needle moves freely.

If it doesn’t, disassemble the metering system completely and carefully clean the parts. Walbro small parts cleaner, part number 500-520, can help clean these components.

Carefully inspect the needle tip and the non-serviceable seat for debris or damage before reassembly.

If you’re working on an older carburetor with a circuit plate design, inspect the circuit plate and gasket for signs of damage or wear. The holes in the circuit plate should be free of obstructions.

Next, replace the metering diaphragm and gaskets using original Walbro parts.

“We understand that the likelihood of damage to soft parts in the carburetor is greater with ethanol fuels.”

“That’s right. While Walbro continues developing advanced engineering materials that are highly resistant to ethanol, ethanol fuels can still affect soft parts like diaphragms, fuel pumps, and gaskets on all carburetors. Ethanol also attracts moisture, causing corrosion of aluminum parts, and moisture in fuel reduces shelf life.”

“So how can we avoid problems?”

“First, use the fuel recommended by the engine manufacturer. Second, make sure it’s fresh. Finally, run the fuel out of the equipment if it’s going to sit unused for six weeks or more.”

“Great tips, Louie. Thanks.”

Let’s continue our butterfly valve carburetor inspection.

Reassemble the fuel pump beginning with a new fuel inlet screen. Fit the new screen into the screen pocket and press it into place using Walbro tool number 500-15. The screen should completely contact the casting wall.

Next, reassemble the pump diaphragm, gasket, pump cover, and cover screw. If any carburetor components are damaged, replace them with original Walbro parts.

A generic exploded view and a list of carburetor-specific parts can be found on Walbro’s website.

Before reinstalling, pressure test the carburetor as discussed earlier to ensure it has been properly reassembled. If the carburetor passes the pressure test, it can now be reinstalled on the engine and checked for proper adjustment.

“The way I understand it, carburetor adjustment procedures have changed to accommodate newer carburetors with tamper-resistant features.”

“That’s right. Due to environmental concerns, carburetors manufactured since the mid-1990s have included tamper-resistant features that limit the amount of adjustment available to consumers in order to maintain low emissions.”

The tools, techniques, and specifications needed to adjust these carburetors are available to authorized dealers directly from the engine manufacturer. However, resale of these tools to consumers can carry heavy penalties.

On all carburetors, tamper-resistant or not, you can still adjust the idle speed screw. Refer to the manufacturer’s recommendations for settings.

That covers the operation, troubleshooting, and adjustment of Walbro butterfly valve carburetors. Further servicing information and replacement parts can be obtained from your Walbro distributor.

Walbro also manufactures various versions of rotary valve carburetors. While the carburetors differ somewhat in design, the components can still be grouped into three basic areas of operation: the fuel pump, the metering area, and the throttle shaft assembly. Most of these carburetors also come equipped with their own primer system.

Let’s take a closer look at the components that make up the rotary valve carburetor.

The first functional area includes the fuel pump found beneath the primer pump assembly. The fuel pump consists of a gasket, diaphragm, pump body assembly, and the pump side of the carburetor body.

Removing these components also reveals a fuel inlet screen and an inlet needle orifice. The carburetor body also contains a channel that allows crankcase impulses to enter the fuel pump.

The metering area is made up of the inlet needle seat, main nozzle check valve, metering diaphragm assembly, metering lever pin, spring, and inlet needle. The inlet needle is held in the closed position by the spring force against the lever.

Some rotary valve carburetors may contain a serviceable main jet.

The final area of operation is the throttle area. The rotary throttle barrel assembly includes the barrel valve, needle spring, throttle ramp, idle screw bracket, idle adjustment screw, swivel, and throttle lever.

Most Walbro rotary valve carburetors come equipped with a priming system. These systems are designed to purge air from the system and assist the carburetor during startup.

The primer is made up of the purge body assembly, cover, primer bulb, and a check valve that controls the flow of fuel and air during priming operation.

Depressing the primer bulb opens the outlet check valve, allowing air and fuel to exit the bulb. As the bulb returns to shape, a vacuum is created, opening the fuel inlet valve and causing fuel to circulate throughout the carburetor’s fuel circuit and into the bulb.

Under most circumstances, the bulb should only need to be depressed four or five times to properly prime the system.

To inspect rotary valve carburetors, begin by performing a general inspection of the areas surrounding the carburetor, including the air filter, muffler, spark plug, fuel filter, and fuel hoses.

If no problem is found, the carburetor must be disassembled. As you disassemble the carburetor, note the order and position of the components. This will help you correctly reassemble it after inspection.

Start the disassembly procedure at the primer pump assembly. Just below the primer assembly is the metering area, which consists of a metering diaphragm assembly, gasket, metering lever pin, spring, and inlet needle.

Leave the inlet needle, lever pin, and spring assembled within the pump body assembly unless they require replacement.

Removing the fuel pump body is also the first step in disassembling the fuel pump. The fuel pump consists of the body, diaphragm, pump gasket, and the fuel pump side of the carburetor.

The pump body assembly contains a fuel inlet screen and inlet needle orifice. The fuel pump side of the carburetor body also contains the main jet and O-ring.

Once the carburetor is completely disassembled, clean its metal components using mineral spirits.

Now that you’ve finished cleaning the components, it’s time to reassemble the carburetor. Begin by reinstalling the main jet, O-ring, and main jet into the pump side of the body.

Install the fuel inlet screen into the fuel pump cover using tool number 500-15. The screen should be pressed to the screen pocket step and fit flush with the pocket wall.

Reassemble the metering area using original Walbro parts.

Check the lever height adjustment with Walbro metering gauge number 500-13 and adjust as needed.

You also have the option of replacing the entire fuel pump body assembly as a replacement unit. This simplifies replacement and eliminates the need for metering lever height adjustment.

Now reassemble the fuel pump body assembly to the pump side of the carburetor body using a new gasket and diaphragm.

The final area to inspect and reassemble is the primer pump assembly. Reassemble using the gasket, diaphragm, purge body, bulb, and cover.

If the primer pump valve doesn’t seat properly or the primer bulb doesn’t return to shape, replace the entire primer pump assembly.

Once the carburetor has been completely reassembled, it must be pressure tested again. The procedure is the same as used in the initial inspection and for butterfly valve carburetors.

Using your Walbro pressure tester number 57-11, determine the valve’s pop-off and reseat pressure settings. Be sure the inlet needle valve is wet before pressure testing.

The gauge will indicate an increase in pressure until the inlet needle valve unseats. Depending on the carburetor’s design specifications, the inlet needle should unseat between 15 and 60 PSI. This level is referred to as the pop-off pressure.

The gauge will then drop to indicate the pressure where the inlet needle valve reseats. This is also referred to as the reseat pressure. The valve should reseat at a minimum of 5 PSI.

If the carburetor passes the pressure test, it can be reinstalled on the engine and checked for proper adjustment.

“As we discussed with butterfly valve carburetors, adjustment procedures are different now because newer carburetors have tamper-resistant features.”

“That’s right. Since the mid-1990s, carburetors have come equipped with tamper-resistant features designed to limit the amount of adjustment possible by consumers.”

If you’re an authorized dealer, the tools, techniques, and specifications necessary to adjust these carburetors are available from the engine manufacturer. However, these tools cannot be sold to consumers without the risk of heavy fines.

On all carburetors, tamper-resistant or not, you can still adjust the idle speed screw. Refer to the engine manufacturer’s recommendations for proper settings.

Congratulations. This concludes the operation, repair, and adjustment of Walbro rotary valve carburetors.

For more information on servicing Walbro diaphragm carburetors, the location of your nearest authorized Walbro distributor, and other information about Walbro and its products.