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

by Had Robinson
updated August 5, 2019

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 widely.  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 the latest from Walbro but are still decades old.  Walbro does NOT support ANY aviation use of their 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.

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

  3. If the pop-off rest pressure is not checked to verify that the inlet needle valve seat is in good condition, you will also be wasting your time.  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).  This is why it is a good idea to purge your fuel system if you will not be flying for a few weeks.

  4. Coil gap – If the coil has not ever 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 is replaced with another coil.  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 (starvation, stall 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. 

  6. Fuel pump pulse passages (including tubing to the engine crankcase) must be carefully checked to be sure there are no obstructions.  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 always 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.  For Walbro, using their carburetors in paramotors is an illegal and hazardous use of their product.  We are on our own in this respect. 

Rebuilding the carburetor

Do not lose or damage the main jet (part #16), an expensive part.  We stock blank jets and can custom make any size for the WG series carburetors.  Contact us for pricing and to order.

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 carburetor as well as you can with air and carburetor spray cleaner.  (If you remove it before cleaning, it is easier to get sand and grit inside things.)  Remove the carburetor from the engine.  If you are particularly handy, this is not necessary.  When it is necessary to work on the fuel pump side of the carburetor, pull the carburetor off the mounting studs and rotate it 180 degrees and then put the top mounting hole of the carburetor on the bottom mounting stud.

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

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 air through it, it is OK.  The pickup tube filter is not needed if you have a quality fuel filter.  However, you must have a "clunk" which is a device that weighs down the end of the pickup tube so it stays in the bottom of the tank.  You can use the old pickup tube filter but it should have a hole made 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.

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

3. Inline fuel filter  If it has been more than a year or if ethanol fuels have been used, replace the fuel filter.  Do not purchase a fuel filter from an auto parts store because they are almost always of inferior quality.  Use a quality paper-type filter, like the OEM factory filter.

4. Metering lever spring  (15mm long when new)  It is a good idea to replace the metering lever (ML) spring when rebuilding the carburetor.  I do not recommend using "miracle springs" or larger main jets to fix fuel pump/fuel supply problems.  They are simply 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 ethanol fuels.  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 dramatically, 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.

5. Cover plate removal  Remove both cover plates, all diaphragms, and gaskets.

6. Remove ML  Remove the metering lever assembly.  On the WB-37, remove the circuit plate (part# 48).  This exposes the idle progression holes.

7. Inlet screen  Always replace the screen.  If the fuel inlet screen (#45) on the pump side of the carburetor has the tiniest bit of debris in it, it means that the fuel filter is bad or is not of the correct type.  Remove the screen with a fine pick-tool or needle.  Sintered bronze filters DO NOT FILTER the fine debris which will clog the fuel inlet screen.  Do not use this type of filter!

8. Examine very part of the carburetor   Remove the idle needle (#39).  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 fuel filters carefully.  Read the page on fuel/oil specifications and, 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?

9. Clean the inside of the carburetor  Spray carburetor cleaner through all passages of the carburetor and also spray the small parts.  Carburetor cleaner is available at auto parts stores.  Blow things out with compressed air.  Be certain that all passageways are clear.

10. 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 our 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, 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 should not be removed.

11. Clean the inlet needle valve seat  Take a Q-Tip, 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.  The downside is that it is a bit hard on the non-metal parts of the carburetor and the fuel lines.

12. Main nozzle check valve   It is very rare to have a problem with the main nozzle check valve (#15).  If it is defective or gummed up, the engine will not idle because air will flow from the main nozzle back into the metering chamber, effectively stopping fuel from entering the idle circuit.  If you have idle problems, it might be worth checking the valve.  The only way to check if the valve is leaking some is to take a piece of fuel tubing about a foot long and place one end of it tightly over the main jet (see arrow in photo below).  You should be able to blow but not suck through the valve.  The tough part is to be certain the tubing is tightly covering the main jet so that it does not leak.  A dab of Vaseline placed around the edges of the main jet can help seal things.  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 that covers the nozzle check valve well.

WG-8 carburetor with meter lever diaphragm removed

13. Install the idle needle (part #39) to 1 1/4 turns out from all the way screwed in.  Be very careful not to force the needle against the seat or it will ruin the carburetor.  Those who live at high altitude should set the idle needle to 1 1/8 turns.  These are basic settings and will have to be tweaked later.

14. 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.

15. Install the new inlet needle and related parts.

16. Metering lever (ML) adjustment

(You may want to skip the explanation below and go directly to the adjustment section.)  Remember that the ML adjustment can a bit low and it will not affect anything.

Here is some important information on the metering lever used in all diaphragm carburetors.  Just remember that Walbro cannot help 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* (according to Walbro) application -> aviation.  Many of the Walbro parts we use here have to be imported because of this, thanks to U.S. tort law, the only kind of its type in the world.

Richard Cobb was the first person that I know of who noticed the different types of ML diaphragms: the tab and the tang-type.  He is the one who gave us this important fact that helped unsnarl the ML height adjustment issue.  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 (0.4mm) 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 WB-37 does not have this issue, thankfully.

The obsolete Walbro documentation does not discuss these differences because the design of the carburetor was never intended for aviation use but for a chainsaw.  Walbro will not even talk to anyone who uses this carburetor for aviation purposes.

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 notes that ML height affects how fuel air ratio entering the engine but this does not apply to the WG or WB series carburetors.  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 photo #3 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)

Remember that the Walbro depth gauge cannot be used to adjust the ML value unless you have the tang-type diaphragm.  The tang-type is 0.4mm higher than the button-type and requires a different ML height value.

Video of adjusting the metering lever.  It will help with understanding the following instructions.  Use the inexpensive depth gauge and a digital caliper to adjust the ML if that is all you have.  The gradations on most depth gauges are not fine enough to give an accurate measurement which is why the digital caliper should be used to measure the depth gauge.  Use of the special base with a digital caliper is faster, more accurate, and saves a step.

  1. Set the digital caliper to the correct value for either the button (0.7mm) or the tang (1.7mm) type diaphragm.  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.  In testing, the maximum ML height for the button type diaphragm can be 1.3mm.  When the value exceeds this, the ML valve is not able to fully open and the engine will lean out (burn up) at or near full throttle.

  2. Place the calibrated depth gauge on the carburetor and move it into the position shown in photo #3.  If the ML is adjusted correctly, the tip of the depth gauge will just kiss the ML (see photo #2).  The tip of the gauge must not push down on the ML.

  3. Change the ML value  There are two ways to do it.  The safest is to remove the ML.  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.  Alternately, it saves a lot of time to to change the ML value per this diagram from ZAMA, but care must be exercised.  Use only your fingers to adjust the ML this way.  The Walbro video shows how to do it, if in doubt.  Be VERY careful if you use a screwdriver to hold the ML.

    ZAMA's method to adjust metering levers

  4.  Double check your work.  It is always better to error on making the height less than more.

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

17. 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).

IF YOU HAVE THE 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.

18. Check the pop-off and rest pressures  Please 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.

19. 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.

20. 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.  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, better, have a hole drilled in it so it will never clog again.  This filter is an obsolete addition when leaded motor fuel was used decades ago.

21. 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.

22. 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 Q-Tip 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.

23. Adjust carburetor  After the engine has warmed up to operating temperature, 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.

24. 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, fade or lack of power output is probably due to ignition system problems such as a broken secondary wire or a worn out or incorrectly gapped spark plug.  Clogged fuel filters can also cause this problem.  Rarely, the cause can be an incorrect sized main jet (WG-8 only).  If things do not work right, see our performance issues page.  Remember that testing the engine on the ground is not accurate because of the difference in loading and temperature than when flying.  Always remember that the fuel pump on these carburetors was NEVER designed to pump fuel from a tank 18" below the carburetor!  We are working on a solution to this.

Engine output will drop about 100 RPM for every 1,000' of altitude above sea level.  Propeller condition and type will also influence power output dramatically.

Turkey Vulture

USHPA
USPPA