Rebuilding and tune up of the Walbro WG-8 & WB-37 carburetors part 2

by Had Robinson
updated August 9, 2020

Preliminaries

  1. Learn the parts of a Walbro carburetor – Study this diagram of the WG-8.  Go to the WB-37 page for specific instructions and a parts diagram for that carburetor.

    The diagrams are of generic carburetors.  All important parts are numbered and labeled.  Some parts may be slightly different in shape.  Some parts are unique to a particular series.  The metering lever diaphragm and the fuel pump cover designs vary.  NOTE: THE WG-8 DIAGRAM SHOWS A POWER NEEDLE (part# 40).  THIS DOES NOT EXIST ON ANY WG-8 CARBURETORS FOUND ON PARAMOTORS.  A FIXED MAIN JET IS ALWAYS USED INSTEAD.  The diagrams are from Walbro but and are 20-30 years old.  Walbro does *NOT* support ANY aviation use of their carburetors, especially these obsolete chain saw carburetors!!!

    It is helpful if you understand how a diaphragm carburetor works.  ZAMA's technical guide is the best there is because it is short, simple, and concise.  It is functionally identical to the Walbro.  It is decades newer and has more information than Walbro's technical guides on how diaphragm carburetors work (which are very technical and not useful to paramotor pilots).

  2. Ignition system check – Before attempting a carburetor tune-up or rebuild, check the ignition system first to see if there are any issues.  If your spark plug secondary wire is "open" (broken internally), you are wasting your time tuning up the carburetor.

  3. You may be wasting your time if the pop-off rest pressure is not checked to verify that the inlet needle valve seat is in good condition.  If you do not check it and your carburetor has less than a few hundred hours on it, the rebuild will likely work.  Note that the valve seat can corrode if gasoline containing ethanol is left in the carburetor.  Walbro has important information about ethanol fuels in their service video (see above).

  4. Coil gap – If the coil has never been replaced, this step may be skipped.  The coil gap is frequently adjusted incorrectly.  This page gives the instructions on how to check/replace the coil.  If a professional replaces the coil, print out the page on the coil and give it to him.  He will need the information.  Note:  it is a good idea (but not essential) to check the timing if the coil has been replaced.  This is because there may be significant variations in the mounting holes which could affect the timing, although slightly.

  5. Reed valve body leaks (Top 80 and others that feed the fuel pump pulses through the reed valve body) – If you are having fuel delivery problems (engine will not start of idle, stalls at full throttle), be sure that the reed valve block screws have been properly torqued down.  If the block is even slightly loose, the fuel pump will not function properly, if at all.  I have seen this in many brand new engines! 

  6. Fuel pump pulse passages (including tubing to the engine crankcase) must be carefully checked to be sure there are no obstructions.

    Top 80 ONLY
      The passageway through the reed valve body from the crankcase to the carburetor must not be blocked, even the slightest.  This must be carefully checked if problems are experienced.  Reaming the passageways from the crankcase, through the reed valve body, the carb body itself (tip from Vittorazi) to the fuel pump diaphragm helps pump performance, the *weakest point* in ALL Walbro carburetors.  This is because ALL of the Walbro carburetors used in paramotors were designed for chainsaws with fuel tanks at the same level as the carburetor.  I am working on a simple, reliable way to fix this inherent design flaw with the Walbro called the FSM.
  7. Check the pickup tube filter in the bottom of the fuel tank to be sure it is not clogged with gel/goo (due to ethanol fuels).  Replace it if in doubt.  If you can easily blow through it, all is OK.  The pickup tube filter (also called a "clunk") is weighted in order to keep it in the bottom of the tank.  You can use a clogged pickup tube filter by making a hole in it.  Eliminating the filter (but not the "clunk") on the end of the pickup tube is just one more way pilots can eliminate flow restrictions in the fuel system, our most common problem.
  8. If it has been more than a year replace the inline fuel filter.  Do *NOT* purchase fuel filters from an auto parts store because they are almost always of inferior quality.  Use a quality paper-type filter, like the OEM factory filter (available from Miniplane-USA).  Some auto parts stores may stock (or can order) the WIX #33001 inline fuel filter.  It is a superior quality inline fuel filter.

Rebuilding the carburetor

Do not lose or damage the main jet (part #16).  It is not available in most countries.  However, we stock blank jets and can custom make any size for the WG series carburetors.  Contact us for pricing and to order.  The jets we supply are precision made and the jet opening measured to within 0.01mm.  Because jets are custom made to order, they are expensive compared to stock jets for most carburetors.

The part numbers given below are for the WG-8 and may differ from those of the WB-37.  The WB-37 has no removable Welch plugs but a circuit plate, instead.

Special tools needed

Clean the exterior of the carburetor as well as you can with air and cleaner BEFORE removing it from the engine.  Use compressed air to remove all traces of cleaner.  (If you remove it before cleaning, it is easier to get sand and grit inside things.)  Remove the carburetor from the engine.

1. Parts needed  The correct rebuild kits, inline fuel filters, and other parts are available from Miniplane-USA.

2. Video  Watch the Walbro carburetor service video, if you have not already.  It has important visual tips for a rebuild.

Parts under the metering diaphragm cover (the one with the hole in the center)

WG-8 carburetor - parts under the metering diaphragm cover

Parts under the pump cover.  The inlet filter screen (arrow) is clogged and needs to be replaced.  This motor could not achieve full power.  Pilot MUST install the correct type of inline fuel filter!

WG-8 carburetor showing clogged inlet filter screen

3. Metering lever spring  (15.0mm long when new)  It is a good idea to replace the metering lever (ML) spring when rebuilding the carburetor.  If the spring measures less than 14.9mm REPLACE IT.  If you have a pop-off gauge then you can easily test the pop-off pressure and immediately know if your existing spring is good.

I do not recommend using "miracle springs" or larger main jets to fix fuel pump/fuel supply problems.  They are Band-Aids that fail to address the root issue(s).  The life of the spring is dependent on what kind of fuel is used and what climate you fly in.  Remember that ethanol fuels attract water and the more humid it is, the more water will be in all gasoline containing ethanol.  It is so inexpensive to replace the spring and be assured that your pop-off pressure is correct (if you do not have a pop-off gauge).  Defective springs may look exactly the same as new springs but give a different pop-off and rest pressure.  A weak spring enriches the air/fuel mixture and engine performance will suffer, especially midrange.  A hardened spring (from age and corrosion) will lean the air/fuel mixture and may result in burning up the engine.  It is not worth saving a few dollars and then ruin your engine.  For more information on the ML spring, see the pop-off pressure page.

Note: the rebuilt kit will have many extra parts in it.  Hang on to them in case they might be needed in the future, e.g. if a Welch plug needs to be replaced.

4. Cover plates  Remove both cover plates, all diaphragms, and gaskets.  Each is connected to the carburetor body by (4) small screws.

5. Remove the ML  Remove the metering lever assembly by removing the single screw that holds the pivot.  Be careful to not lose any of these parts.

On the WB-37, remove the circuit plate #48.  This exposes the idle progression holes.

6. Inlet screen  Always replace the screen #45 (WG-8) or #43 (WB-37).  It is located below the pump cover and diaphragm.  If the fuel inlet screen has the tiniest bit of debris in it, it means that the inline fuel filter is bad or is not of the correct type.  Remove the screen with a fine pick-tool or needle.  Sintered bronze inline fuel filters DO NOT FILTER the fine debris which will clog the fuel inlet screen.  Do not use this type of filter!  Below are (2) filter screens.  They are small – about the width of a pencil.  The one on the left is almost completely clogged because the pilot had no inline filter installed.  A new one is on the right.

 inlet valve filter screen - one clogged, one new

7. Examine the carburetor   Remove the idle needle #39 (WG-8) or #40 (WB-37).  On the WB-37 remove the power needle #41.  With a magnifying headset or glass, carefully examine every part, including the carburetor body.  If there is any gunk or water drops anywhere, it is a symptom of using gasoline with ethanol, water in the fuel system, and/or old fuel (more than two weeks old).  This is warning that you need to check your fuel quality and replace all fuel filters.  Read the page on fuel/oil specifications.  If you use ethanol fuels, use the correct fuel additives and get an ethanol content tester to be sure your fuel does NOT contain excessive amounts of ethanol or is contaminated by water.  Why burn up a $2K engine because you did not check your fuel quality?  What is hard to understand about this?

8. Clean the inside of the carburetor  Spray carburetor cleaner through all passages of the *completely disassembled* carburetor and on the small parts.  Do NOT spray the cleaner on anything but metal.  Blow things out with compressed air.  Be certain that all passageways are clear.  DO NOT LEAVE ANY TRACES OF THE CLEANER INSIDE THE CARBURETOR!  It must be completely free of ANY solvent.  Most cleaners will destroy non-metal parts if exposed to the parts for any length of time.

9. Welch plugs  It is very rare that a Welch plug on a carburetor needs to be removed.  Special tools are needed to do this.  Go to the Welch plug page if your carburetor is particularly dirty and/or abused and you need to remove and replace the plug.  If the engine does not idle properly no matter what you do, it would be advisable to remove the plug and check things.  The brass cup plug for the main nozzle check valve is *NOT* available in North America and the EU and should not be removed.

10. Clean the inlet needle valve seat  Take a cotton swab, cut the cotton ball off of one end, and push it down into the inlet needle valve hole.  Firmly twist it back and forth.  This will clean the valve seat of light corrosion and gunk, if there is any.  Now, soak the other end of the Q-Tip with carburetor cleaner and clean the valve once more.  This will remove any fuel gum and residues, as the Walbro service manual notes.  Pilots might want to consider the regular addition of Chevron's Techron fuel additive to their fuel mix.  This stuff cleans the guts of carburetors and fuel system like nothing else.

11. Main nozzle check valve   It is extremely rare to have a defective main nozzle check valve #15 (WG-8) or #14 (WB-37) but it should always be checked to see if it is stuck or gummed up when doing a carburetor rebuild.  If it is gummed up, the engine will not idle properly because air will flow from the main nozzle back into the metering chamber.  This will stop or reduce fuel from entering the idle circuit, depending on how badly the check valve leaks.  It is easy to check whether the valve is leaking.  Take a piece of thin tubing about a foot long and place one end of it tightly over the main jet (see arrow in photo below).  Note the number of turns on the idle mixture screw and then screw it all the way in.  If this is not done air will leak through the idle circuit and you will not be able to adequately determine if the nozzle is working correctly.  You should be able to blow but not suck through the valve.  When the check valve closes it will make a noticeable "click" if it is working properly.  When finished with this test, restore idle mixture screw to its original setting.

Note: In the WG-8, the high speed circuit also feeds the low speed circuit through the low speed jet.  This is why the idle mixture screw must be screwed all the way in in order to test the check valve in the nozzle jet

If the check valve leaks, try spraying carburetor cleaner into the main jet so that it comes out the check valve in the throat of the carburetor.  Let things soak for a few minutes then blow high-pressure air through the main jet.  If it still leaks, you will have to replace the carburetor because the valve is not available as a separate item (in North America and the EU).  Furthermore, carburetor rebuild kits do not contain the special Welch plug (see photo below) that covers the nozzle check valve well.

Walbro WG-8 main nozzle check valve plug

12. Install the idle needle #39 (WG-8) or #40 (WB-37) to 1 1/4 turns out from all the way screwed in or what the engine manual specifies.  At high altitudes set it to 1 turn.  Be very careful not to force the needle against the seat or it will ruin the carburetor.  These are basic settings and will have to be tweaked later.

13. Install the new fuel inlet screen  Using the eraser end of an ordinary pencil, gently push the screen all the way in so that it completely contacts the casting wall below the inlet hole on the side.  If it is not pushed all the way in, fuel flow will be blocked.

14. Install the new inlet valve needle assembly i.e. the spring, the metering lever (ML), pin, etc.

15. Metering lever adjustment

(You may skip the following discussion here concerning the WG-8 diaphragm types and go directly to the adjustment section, if desired.)

Walbro cannot help us with any kind of support or information on these carburetors because they are not only obsolete (because of EPA rules) but are being used in an *unsupported* and *illegal* application according to Walbro i.e. aviation.  Many of the Walbro parts we use here have to be imported because of this, thanks to U.S. tort law.

Richard Cobb was the first person who noted the different types of ML diaphragms in the WG-8 carburetor: the button and the tang-type.  He is the one who gave us this important fact that helped unsnarl the ML height adjustment mystery.  It depends on the origin of the diaphragms – whether they come from Asia/EU (left photo) or from the U.S. (right photo).  The U.S. diaphragm has a button while the Asia/EU diaphragm has a tang.  However, in recent years both types of diaphragms have turned up in all regions.  Note in photo #1 below that the tang-type has a greater height (1mm) than the button-type above the body of the diaphragm.

Walbro WG8 metering lever diaphragms

Photo #1 (photos courtesy of Richard Cobb)

This is why the ML height must be adjusted differently for each type of diaphragm.

The obsolete Walbro documentation does not note these differences.

Photo #2 (below) is a closeup of photo #3 of the metering lever with a depth gauge set to the correct value (with the digital caliper) and is in place.  (The black double arrow is the actual height of the metering lever but it is impossible to measure this dimension.)  Instead, we measure the distance from the ML diaphragm gasket flange surface (see photo #3) to the top of the metering lever (the blue arrow in the photo).  The upper red arrow points to the straight edge of the depth gauge which rests on the diaphragm gasket flange surface.  The bottom red arrow points to the extended tip of the depth gauge which just "kisses" the top (and end) of the ML.

The distance between the (2) red arrows in photo #2 is what we set when we adjust the metering lever height.  It is a critical adjustment which must be set exactly.  Having the distance too great can result in the engine leaning out (and burning up).

When this distance is too great, the diaphragm will be unable to fully open the fuel inlet valve (on the other end of the ML) and the engine can be starved for fuel, especially at or near full throttle operation, if the ambient temperature is very cold, and/or if the fuel pump is weak (e.g. needs a rebuild).  If the distance is too little, the button/tang on the diaphragm will rest against the ML at idle and the ML valve will leak fuel making it hard to start, impossible to adjust the idle, and possibly flooding the engine.  You can immediately and easily test for a leaking ML valve by using a pop-off gauge.

Walbro's Service Manual correctly notes that ML height affects the fuel/air ratio entering the engine but the manual fails to note that this is true ONLY for gross maladjustments of the ML lever height i.e. 1.0mm or more off either way.  The only important consideration is that the ML diaphragm is able to fully open and close the ML inlet valve.

Walbro WG-8 metering lever closeup view

Photo #2 (above)

The red arrow in the photo below points to the close-up detail given in photo #2 above.  The black arrow points to the face of diaphragm gasket flange.  Note:  You should never clamp a carburetor directly in a vise.  Use wood blocks on either side to protect the faces of the carburetor.

Walbro metering lever adjustment

Photo #3 (above)

Why not use the official Walbro depth gauge for the WG-8 carburetor?

Walbro metering lever adjustment gauge

Unfortunately, the official Walbro depth gauge can only be used to adjust the ML value of the tang-type diaphragm and not the button-type found on the WG-8.  The tang-type is 0.4mm higher than the button-type.  Why hasn't Walbro made a gauge for the button-type diaphragms?  We cannot know for sure.  Walbro *knows* that the WG-8 is used primarily for paramotors and not for the chainsaw for which it was originally designed.  Walbro will *hang up* the phone the moment anything concerning aviation comes out.  It is a tiny market for them and, I presume, they just do not want to waste money creating a special gauge that addresses the changes in the carburetor kits made over the decades.  If they did, it would be a tacit admission on their part that they are addressing the problems associated with the WG-8 being used on paramotors – an illegal application of their equipment, as far as they are concerned.

Adjust the metering lever height

Southwest Airports (SWA) has made a custom gauge (available exclusively from Miniplane-USA) that is similar to the official Walbro gauge but has the correct dimensions for the *button-type* diaphragm found in most WG-8 repair kits sold in NA and the EU.  The SWA gauge is clearly stamped on one side with "SouthwestAirsports.com" so as not to be confused with the official Walbro gauge.  Use of the custom gauge will save time, trouble, and expense for all pilots who service the Walbro WG-8 or WB-37 carburetor.  Fortunately, the WB-37 repair kits come with the height of the ML set correctly but, even so, the ML height should always be checked.  This is because the ML lever can be accidentally bent in handling.

  1. Use the custom SWA gauge to check the ML height of the button-type diaphragm ONLY.  The gauge also has a tab (visible in the upper right of the gauge) for measuring the ML height of the WB-37.  If it is the correct value, go to step "c" below, if not, go to step "b".  The tip of the gauge just kisses the end of the metering lever *without* opening the valve.  Here is a short video of how to use the gauge.  The Walbro video has more detail, if needed, on how to use a gauge of this type (the relevant section is about 9 minutes in).

    Southwest Airsports WG-8 metering lever adjustment gauge

    If you do not have the SWA gauge or have a tang-type diaphragm, follow these steps:

    Use a digital caliper and depth gauge to set the correct value

    WG-8
    : button-type = 0.7mm or tang-type = 1.7mm

    WB-37
    : 1.19mm-1.59mm.  The WB-37 rebuild kit has the ML lever set to the correct value by the factory so it does not need to be adjusted.  However, it is a good idea to check the ML height anyway in case the ML may have been bent during shipping and handling.

    Use of the special base with a digital caliper is faster, more accurate, and saves a step than using an ordinary depth gauge.  The gradations on most depth gauges are not fine enough to give an accurate measurement which is why we need the digital caliper. 

    If you do not have the special base, use the caliper to adjust the depth gauge so that the tip of the gauge rule protrudes the correct amount.  For more detailed information on the difference between the tang and button type diaphragms, you can take a look at this page.

  2. Change the ML value if necessary  There are two ways to do it.

    1.) Remove the ML lever. Hold the side going to the valve (the short side) with a pair of needle-nose pliers.  Bend the long side up or down a bit depending on whether the ML value it too great or too little, respectively.  Be careful doing this as you must not deform the pivot hole going through the center of the ML.

    2.) Alternately, use your fingers to adjust the ML.  The ML does not have to be removed.  It is highly recommended that pilots watch the Walbro video on how to do it (go about 9+ minutes in).  They recommend using screwdrivers but a fingernail works just as well.  Be very careful if you use a screwdriver.  The image below is from the ZAMA service manual.

    ZAMA's method to adjust metering levers

  3. Double check your work.  Connect a pop-off gauge to the carburetor, pressurize the system to just below the pop-off pressure value e.g. 15 psi.  The pressure should hold or very slowly drop.  Press down on the diaphragm side of the ML (the longer lever that has the spring below it).  The pressure should drop to zero *immediately*.

    It is always better to error on adjusting the height *less* than more.  If the ML value is way less (>0.2mm less than the specified value) the ML valve is forced open at rest and the engine will flood and not idle correctly.

For the WB-37, install the circuit plate with the diaphragm and gasket.

16. Install the fuel pump and ML diaphragms, gaskets, and cover plates.  For the fuel pump diaphragm, it goes on first and then the gasket.  Most kits contain an ethanol resistant fuel pump diaphragm.  If ethanol blends are used, this special diaphragm (clear in color) can be used.  The downside of this diaphragm is that it is not as supple as the nitrile rubber diaphragm and will not pump quite as well, especially in cold environments.  For the ML side of the carburetor, install the gasket FIRST and THEN the diaphragm (it is the opposite of the fuel pump side).  Tighten screws incrementally in a "X" pattern.  If the screws are not tightened properly the carburetor will not work.

IF YOU HAVE A TANG-TYPE DIAPHRAGM, BE CERTAIN THAT THE SLOT IN THE ML IS ENGAGED WITH THE TANG OF THE DIAPHRAGM!  If it is not engaged, the ML lever valve will NOT function correctly.

17. Check the pop-off and rest pressures  Go to the link and follow the steps.  If you do not have a pop-off gauge, this step can be skipped but you will not be sure of what is wrong if you have problems with the carburetor later on.  If you do not have a pop-off gauge you can check whether the ML valve is leaking by connecting a piece of tubing the fuel inlet fitting and attempting to blow through the tube.  It should be impossible to blow any air into the carburetor.  While keeping air pressure in the tube, push down on the priming lever.  It should be easy to blow air into the carburetor.

18. Install the carburetor on the engine, install the air box (if there is one), replace the fuel filter, but do not connect the fuel line to the carburetor.  Place the end in a jar for testing in the next step.  Note: Do NOT over-tighten the nuts which hold the carburetor on the engine.  They should be just tight enough to squeeze the rubber air box gasket and make a seal.  If you have a quality torque wrench, the proper value is about 0.9 Nm (8 in. lb.).  Over-tightening will deform and ruin the gaskets.

19. Purge/check the fuel system  Place a small amount of fuel in the tank and pressurize the tank via the priming tube (or squeeze the priming bulb, if present).  Fuel should dribble freely out of the line that connects to the carburetor inlet fitting.  This is an important step that ensures that the fuel system up to the carburetor is working properly and debris in the system has been flushed out.  If ethanol fuels have been used, it is common for the filter on the end of the pick-up tube in the tank to become clogged with gel/goo.  If this is the case, it must be replaced or have a hole drilled in it so it will never clog again.  This filter is an obsolete holdover from when leaded motor fuel was used decades ago.

20. Pressurize the tank  Connect the fuel line to the carburetor and pressurize the tank (or use the priming bulb).  No fuel should leak out of the carburetor, including past the inlet needle valve.  THIS IS A CRITICAL TEST AND MUST BE PERFORMED!

21. Prime the carburetor and start the engine using this method.  If done properly, the choke will rarely be needed except to stop the engine in case of a kill switch failure or to start the engine in the air after a long period.  If the carburetor does not have a priming lever, use the end of a cotton swab that has had the cotton cut off or a tooth pick.  It is advisable to purchase a priming lever (or "primer spring") if it is missing.  These are available from Miniplane-USA.  Note that this technique for easily starting the engine will not work as well on engines where the pilot cannot easily position the inlet side of the carburetor lower than the outlet side e.g. the Minari and other engines with a vertically mounted carburetor.

22. Adjust carburetor  After the engine has warmed up to operating temperature (70C), adjust the carburetor.  Remember that changes in altitude, ambient temperature, fuel and oil type, and/or humidity will affect the low and high speed adjustments on the carburetor.  Installing an EFI system is not a good idea.

23. Test fly the engine.  Properly tuned engines should be able to run at full throttle for at least 3 minutes.  If you have rebuilt the carburetor properly and the fuel systems is 100% in order go to our troubleshooting page for answers.

Turkey Vulture