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Rebuilding and tune up of the Walbro carburetor

by Had Robinson

These instructions are for the WG-8 but can be used for other similar Walbro models, like the WB-37.

Remember that a cooler running engine will last longer.  Heat kills small engines.  Keep this in mind when tuning your engine.  Install a cylinder head temperature gauge before you wish you had.

Note: If your clutched engine engages at idle (< 2,500 RPM), it is difficult or impossible to adjust the idle properly.  Fix this first.

Two things wear out on this carburetor after 100's of hours of use: the throttle shaft bushings and the inlet needle valve seat.  These are not repairable nor replaceable and the carburetor must be replaced.  As the carburetor ages, it gets more leaky.


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, pilots must first check the ignition system to see if there are any issues.  I.e. if your spark plug secondary wire is "open", you are wasting your time tuning up the carburetor.

3. Pop-off pressure – If the pop-off rest pressure is not checked to verify that the inlet needle valve seat is in good condition, you could be wasting your time.  If it is not checked 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 ethanol fuel mixes are left in the carburetor.  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, be certain to print out our 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 because there may be significant variations in the mounting holes which could affect the timing although slightly.

5. Reed valve block leaks – 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.  Also, 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 there are problems.


WARNING: Do not lose or damage the main jet (#16) as it is difficult to find a replacement.  If you need one, we have a few in stock.  Contact us.

Special tools needed  depth gauge; digital caliper to measure the depth gauge; Fine Hook & Pick Set Harbor Freight #93514; pop-off gauge; welch plug tool (if replacing the welch plug)

Clean the carburetor as well as you can with air and carburetor spray cleaner.  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  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 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 heavy duty 10 micron filter that is used in commercial chain saws 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.  The life of the spring is dependent on what kind of fuel is used and what climate you fly in.  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.

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. Clean outside  Clean the outside of the carburetor with brake cleaner, blow dry with compressed air, and remove the carburetor from the engine.

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

7. Remove ML  Remove the metering lever assembly.

8. 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!

9. 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.

10. Clean 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.

11. 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.

12. 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.

13. Main nozzle check valve   It is 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 into the metering chamber, effectively stopping fuel from entering the idle circuit.  The only way to check if the valve is working 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).  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

14. 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 may have to be tweaked later.

15. 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.

16. Install the new inlet needle valve and related parts.

17. Adjust the metering lever (ML) height  Use the depth gauge and a digital caliper to do this.  (The depth gauge gradations are not fine enough to give an accurate measurement which is why the digital caliper must be used to measure the height of the depth gauge.)

ML diaphragms can come in two different types, depending 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.  Note that the tang type has a greater height (1.0mm) than the button type which is why the ML height must be adjusted differently for each type of diaphragm.  The official Walbro ML adjustment gauge cannot be used with the diaphragm that has the button because it will give the incorrect ML height causing the carburetor to lean out the fuel mixture when the main jet is active.

Walbro WG8 metering lever diaphragms
photo courtesy of

Button type  set the ML height to 0.5mm – 0.7mm from the top of the metering lever to the top edge of the carburetor body.  If the diaphragm cover has a built-in priming button (instead of a lever) adjust the ML height to 0.7mm.  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.  A leaking ML valve will make it difficult or impossible to adjust the idle.

Tang type  set the ML height to 1.7mm, the preformed height of all new ML's included in the rebuild kits.  The official Walbro ML adjustment gauge is set give this height (1.7mm).

For more information on the difference between the tang and button type diaphragms, please take a look at this article.

To change ML height, gently bend the side going to the valve (the short side).  Use a pair of needle nose pliers to hold the short side of the ML while you bend the long side up a bit.  Be extremely careful doing this as you must not deform the pivot hole going through the center of the ML.

Depth gauge for measuring the ML height  The red arrow points to the actual height of the ML lever.  Adjust the depth gauge so that the tip of the rule just touches the ML but does not move it.  Remove the depth gauge and measure the height with a digital caliper.  This is because the gradations on the gauge are not fine enough to give an accurate measurement.

depth gauge used to measure metering lever height

Note: the Walbro depth gauge should not be used to adjust the ML height unless you have the tang type diaphragm.  The tang is about 1.0mm higher than the button type which requires a greater ML height.  If your diaphragm has the tang, then adjust the ML height to 1.7mm, the height of the Walbro gauge.

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.

18. 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) should be used.  The downside of this gasket is that it is not as supple as the rubber diaphragm and will not pump quite as well.

19. Check the pop-off pressure  It should be 1.2-1.3 Bar (17.5-19.0 psi).  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 (but you should have checked this earlier).  It is also possible that you set the ML height too low which will hold the valve open.  If the rest pressure is below specification but holds, it means that the pop-off spring is bad 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.

20. 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 0.9 Nm (8 in. lb.).  Over-tightening will deform and ruin the gaskets.

21. Check the fuel system  Place a small amount of fuel in the tank and pressurize the tank via the priming tube.  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.  But if AVGAS or ethanol-free gasoline is being used, this will not be a problem.

22. Pressurize the tank  Connect the fuel line and pressurize the tank.  No fuel should leak out of the carburetor, including past the inlet needle valve.

23. Install 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.

24. 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 on engines where the pilot cannot easily position the inlet side of the carburetor lower than the outlet side e.g. the Minari.

25. Adjust carburetor  After the engine has warmed up to operating temperature, adjust the carburetor.  Remember that changes in altitude, temperature, and humidity will affect the low speed adjustments on the carburetor.

26. Test fly the engine.  Properly tuned engines should be able to run at full throttle for at least 5 minutes.  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.  Rarely, the cause can be a loose or missing main jet.  If things do not work right, see our performance issues page.

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.