Rebuilding and tune up of the Walbro carburetor part 2

by Had Robinson

Preliminaries

  1. Learn the parts of a Walbro carburetor – study this diagram of the WG-8 carburetor or the WB-37 carburetor.  Note that the diagrams are of generic carburetors and that the actual shape of some parts of your carburetor may be slightly different, such as the metering diaphragm cover and the fuel pump cover.  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.
  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 probably 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 them, 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) as it is not easily available.  However, we stock plenty of them and can supply any sized main jet for the WG-8.  Contact us for pricing and to order.

Special tools needed

Clean the carburetor as well as you can with air and carburetor spray cleaner.  If you remove it first, it is easy to blow sand and grit inside the carburetor!  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  Purchase a WG-8 rebuild kit, a fuel filter, and a metering lever spring.  Check the pickup tube filter 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 on the end of the pickup tube is just one more way pilots can eliminate flow restrictions in the fuel system.  The rebuild kits and most parts are available from Miniplane-USA.

2. Video  Watch the Walbro carburetor service video.

3. Fuel filter  If it has been more than a year or if ethanol fuels have been used, it is a good idea to 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.  We have a better quality, heavy duty 10 micron filter that is used in commercial chainsaws that fits paramotors well.  Contact us to order.

4. Metering lever spring  It is a good idea to replace the metering lever (ML) spring when rebuilding the carburetor.  I do not recommend using so-called "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 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.  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) 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.

7. Clean inlet screen  If the fuel inlet screen (#45) on the pump side of the carburetor has the tiniest bit of debris in it, remove the screen with a fine pick-tool or needle.  The presence of debris means that the fuel filter is bad or is not the correct type.  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 the carburetor   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 filter carefully.  Read the page on fuel/oil specifications and, if you use ethanol fuels, get the recommended ethanol content tester to be sure your fuel does NOT contain excessive amounts of ethanol or is contaminated by water.  Why burn up a $5K engine because you did not check your fuel quality???

9. Clean the inside of the carburetor  If the carburetor had any gunk or water in it, also remove the idle needle (#39).  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 rare that the Welch plugs on the carburetor have 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 idle circuit welch plug.  The other welch plug for the main nozzle check valve is not available in North America and should not be removed.  Some pilots remove the welch plug just because it is there.  Why do this?  Unless you are skilled, it is easy to reinstall the new plug incorrectly.

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, 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 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 if it was removed.  If the screen is perfectly clean, it does not have to replaced and is the sign that you are using the correct fuel filter.  If in doubt, replace it.  Using the eraser end of an ordinary pencil, gently push the screen in just below the inlet hole which is on the side of the passage.  If the screen is not pushed to the proper depth, the fuel pump will not be able to deliver an adequate amount of fuel at full load and the engine will lean out and likely overheat.

15. Install the new inlet needle and related parts.

16. Adjust the metering lever (ML)

Here is some important theory on what is going with the metering lever of this diaphragm carburetor.  Be sure to reference the (3) photos below in order to better understand this important step.  Use the inexpensive depth gauge and a digital caliper to do this 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 must be used to measure the depth gauge.  Use of the base with a digital caliper is faster, more accurate, and saves a step.

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 to the world 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 that the tang type has a greater height (1.0mm) than the button type above the body of the diaphragm.  This is why the ML height must be adjusted differently for each type of diaphragm.  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 which is no longer in production.  Walbro will not even talk to anyone who uses this carburetor for aviation purposes.

Walbro WG8 metering lever diaphragms

Photo #1 (photo courtesy of Richard Cobb)

The official Walbro ML gauge (#500-13) is for the tang type diaphragm ONLY and cannot be used with the button type diaphragm.

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 ML has a U shaped slot in it that is used to connect the ML to the tang style diaphragm (see photo #1).  The slot is not used with the button style diaphragm which is now the most common type of diaphragm.  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 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 1.0mm higher than the button type and requires a different value.

Video of adjusting the metering lever.  It will help with understanding the following instructions.

  1. Set the digital caliper to the correct value for either the button (0.5mm - 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 exactly the correct amount.  If the diaphragm is the button style and the cover has a built-in priming button (instead of a lever) the ML value should be set to 0.7mm (the maximum value).  This is because the built-in priming button may contact the top of the ML diaphragm and cause the valve to leak if the smaller value is used.  For more detailed information on the difference between the tang and button type diaphragms, you can take a look at this article.

  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. To change the ML value, remove the ML and 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.
  4.  Double check your work.  You must be certain to adjust the ML to the correct value.  It is always better to error on making the height less than more.

At this time, do not install the new gasket, diaphragm, and cover on the metering lever side of the carburetor – these will be installed after the pop-off pressure is checked.

17. Install the fuel pump diaphragm, gasket, and cover plate.  Be CERTAIN that the diaphragm 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.

18. Check the pop-off and rest pressures  Connect the pop-off gauge to the fuel inlet fitting and pressurize the gauge.  Pop-off should be 1.2-1.3 Bar (17.5-19.0 psi) and the rest pressure must be 0.69 Bar (10 psi) or greater.  If the rest pressure will not hold, the valve seat is damaged/worn out and the carburetor must be replaced.  It is also possible that you set the ML value too low.  If the rest pressure is below specification but holds, it means that the pop-off spring is faulty (weak) and must be replaced.  However, it should be replaced every time the carburetor is rebuilt, anyway.  Low pop-off pressure should rarely be a problem unless the inlet valve seat or spring is defective.

19. Install the carburetor on the engine, replace the fuel filter, but do not connect the fuel line to the carburetor.  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.  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.  It is a useless filter in the system and I have no idea why it is used, except to hold down the end of the fuel pick-up tube.

21. Pressurize the tank  Connect the fuel line and pressurize the tank (or use the priming bulb).  No fuel should leak out of the carburetor, including past the inlet needle valve.

22. Install the rest of the ML assy  Install the new ML gasket, diaphragm, and plate cover.  Be CERTAIN that the gasket goes on first and then the diaphragm (this is opposite of how the fuel pump diaphragm is installed).   Install the air box.

23. Prime the carburetor and start the engine using this method.  If done properly, the choke will never 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.

24. 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.  Why not install an EFI system?

25. Test fly the engine.  Properly tuned engines should be able to run at full throttle for at least 5 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 a incorrect size of the main jet.  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!  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