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Rebuilding and tune up of the Walbro WG-8 carburetor

by Had Robinson

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

Remember that a cooler running engine will last longer.  Heat kills small engines.  Keep this in mind when tuning the Top 80.  Install a cylinder head temperature gauge before you wish you had.

Note: If the clutch 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 300 hours or so: the throttle shaft bushings and the inlet needle valve seat.  These are not repairable nor replaceable and the carburetor must be replaced.  You will have no idea if the inlet needle valve seat is bad unless you have a pop-off gauge.

Preliminaries

1. Learn the parts of the carburetor, if needed.  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.  The only difference is that most Walbro's do not have a priming system.

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 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.  The 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.

5. Reed valve block leaks -- If you are having fuel delivery problems (starvation), 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.

Rebuilding

WARNING: Do not lose or damage the main jet (#16).  This jet is not available in North America (as of the time this article was written).  If you do lose or damage it, we can custom make a new one of the correct size.  Contact us.

The rebuild kits and most parts are available from Miniplane-USA.

Clean the carburetor as well as you can with air or 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.

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 factory filter.

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 live and fly in.  It is so inexpensive to replace the spring and be assured that your pop-off pressure is correct.  A weak spring enriches the air/fuel mixture and engine performance will suffer dramatically, especially midrange.

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 in it means that you are not using the correct fuel filter.  Sintered bronze filters DO NOT filter the fine debris which will clog the fuel inlet screen. 

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.

It is rare that the Welch plugs on the carburetor will have to be removed.  Special tools are needed to do this.

11. Clean the inlet needle valve seat  Take a Q-Tip, cut the cotton ball off of one end, and make a fine point with a pencil sharpener or razor blade.  Take the pointed end, push it down into the inlet needle valve hole, and firmly twist the Q-Tip 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 tubing.

12. 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 at all because air will flow from the main nozzle into the metering chamber, effectively stopping fuel from entering the idle circuit.  Usually, copious amounts of carburetor cleaner blown into the hole of the main jet will clean things up.  The ONLY way to check this is 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) and then try to suck on the other end of the tubing.  You should be able to blow easily but not suck.  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, remove the Welch plug (to the lower left of the main jet in the photo below) and see why the valve is not functioning.  You will need a Welch plug service tool to do this (available from Walbro distributors or from us).  Thankfully, a replacement plug is included in the Walbro service kits, if you need it.  Remember to put a thin ring of nail polish around the edge of the Welch plug and then wipe it clean once driven in.  The Walbro manual gives some tips of how to remove/install the plug.  Hopefully, the valve is only gunked up and is fixable.  However, if anything else is wrong, the carburetor be replaced as this part is not available in North America.

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.

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.  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 you do not do this correctly, 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 valve and related parts.  At this point, the metering lever height must be checked and adjusted.  The Italian Top 80 service manual specifies a distance of 0.5mm – 0.7mm (from the top of the metering lever to the top edge of the carburetor body).  The preformed height of new ML's is different (1.7mm) than specifications and must be changed to the correct value.  For the curious, I discuss this problem in this article.

If a depth gauge and a caliper to measure it with are not available, the old ML can be used as it is about 0.7mm thick.  Bend the OLD metering lever to 90 degrees.  In the depression where the new ML sits, put the bent part of the old ML right next to the forked end of the new ML.  They should be the same height.  The ML adjustment can be off a tenth of a millimeter and is not critical, like the pop-off pressure -- which is critical!

If necessary, remove the ML and 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.  Be extremely careful doing this as you must not deform the pivot hole through the ML.  If the distance is too great, the carburetor will lean out at high loads.  If it is too little, the valve will leak and the idle will be impossible to adjust.

Note: the Walbro depth gauge should not be used because it does not give the correct value for the WG-8 used in the Top 80.  The WG-8 was not designed for the Top 80 but for some chainsaw engine and is why their gauge should not be used.  The gauge gives an ML height that is too great for the Top 80.

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.

16. 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, but this should not be an issue.

17. Check the pop-off pressure  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)!  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.

18. Install the carburetor on the engine, replace the fuel filter, but do not connect the fuel line to the carburetor.

19. 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 you are using AVGAS or ethanol free gasoline, hopefully, so this will not be a problem.

20. 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.

21. 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.

22. 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 (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.

23. 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.

24. 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.  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.

vulture