Important notes on Walbro carburetors

by Had Robinson
updated August 17, 2019

Diagrams of the WG-8 and the WB-37

The information here primarily for the WG and WB series carburetors.  The WB series and some others have an adjustable main jet which must be adjusted with great care because of the ease with which the engine can be leaned out – and burned up.  This is the primary reason why the WG series has a fixed main jet.  For additional info of the history of the WG carburetors, see the midrange tuning page.

For a QUICK lesson in how a diaphragm carburetor works, study this guide by ZAMA pages 2-4.  Note: the WG-8 and WG-37 do not have primer bulbs so you can ignore that section of the guide.  The WG-8 does not have an adjustable high speed jet but a fixed main jet.  I highly recommend that pilots actually learn a little about how these carburetors work before trying to adjust or repair them.

If you experiencing problems with your carburetor, see the performance problem page quick fuel system test for some simple things you can do to see if the carburetor is functioning correctly.

Before doing anything, rebuild the carburetor if it has not been done recently or when it was done is unknown.

The Walbro WG-8 and the WB-37 carburetors are obsolete chainsaw carburetors but, thankfully, are still manufactured and the repair kits are available.  That is why pilots often have headaches with their engines.  Federal EPA regulations on emissions from small engines have largely killed our ability to easily tune any diaphragm carburetor.  THE ONLY REASON THE U.S. AND THE E.U. PERMIT THE IMPORTATION OF PARAMOTORS WITH THESE OBSOLETE CARBURETORS IS BECAUSE AVIATION ENGINES ARE EXEMPT FROM EMISSION REGULATIONS.  Walbro used to make a WG that had an adjustable main jet but no longer.  It is what it is.

For liability reasons, Walbro will not help in any way whatsoever U.S. customers who use these carburetors in a prohibited application, that is, in aviation.  Even the paramotor manufacturers have very limited choices working with Walbro and why all paramotors are manufactured in other countries.  Nothing will change until the American legal system is radically changed.  The U.S. is the only country in the world where attorneys can enrich themselves from the misery of others.  It has almost completely destroyed industries that make high risk items like paramotors and paragliders.

1. Fuel pump design – the #1 problem

The fuel pump on the WG-8 and WB-37 was not intended or designed to be used with the base of the fuel tank located almost 2' (60cm) below the carburetor.  As a result, the fuel delivery system on these carburetors is very sensitive.  As fuel levels drop in the tank and/or fuel demand increases, the fuel pump cannot keep up.  If the entire fuel system is not in perfect order, the fuel mixture gradually leans out as the WOT (wide open throttle) is approached.  In order to compensate for this the pilot or the manufacturers will install a larger main jet in the WG-8 (with the WB37 the adjustable main jet is opened further) to allow more fuel to enter the engine at/near WOT.  The result is that in lesser throttle settings the fuel mixture is richer, sometimes too rich and the engine will "4 cycle" (run roughly).  I hope it is clear that increasing the main jet size by any means does NOT solve the basic problem.  The German Fresh Breeze company is the only one in the world that figured out this problem and put the fuel tank above the engine.

The pump in both the WG-8 and the WB-37 is powered by the pulses present in the crankcase of all 2 stroke engines.  These pulses are transferred through various passageways to the fuel pump diaphragm.  The pump operates just well enough to move fuel from the typical tank on a paramotor to the carburetor when conditions are perfect (72F/25C at sea level).  When any part of the fuel delivery system has problems, the engine can become starved of fuel, run lean, and sometimes overheat.  THIS IS THE NUMBER ONE PROBLEM WITH THE WALBRO CARBURETORS USED ON A PARAMOTOR.

The fuel is moved to the carburetor by a vacuum rather than by a positive pressure.  This distance (24"/60cm) is great enough to cause vaporization of the fuel in hot conditions, especially if MOGAS (motor vehicle fuel) is used.  Once this happens, the engine can become fuel starved and shutdown.  In very cold conditions, fuel viscosity increases with the result that the fuel pump's performance degrades (small surface area and short vertical movement of the pump diaphragm) and, once again, the engine can become fuel starved, especially at or near full throttle.  To help prevent fuel starvation, we have to be sure that the condition and adjustment of the carburetor and related parts are perfect.

Vittorazi suggests that users modify the pulse port in the WG-8 by increasing its diameter to 3.0mm.  This may help the transfer of pulses from the crankcase to the pump diaphragm.  As with all the "miracle springs", larger jets, and what-not, this fix does not fix the inherit problem with the Walbro fuel delivery system.

In order to analyze the inherent design issues of the Walbro chainsaw carburetors, I had to install external test ports on the carburetor so that pressure measurements can be accurately made while running the engine in the air.  It is impossible to know what is going on inside a carburetor without the use of test ports.  The test port on the right will enable measurement of the fuel pressure right at the inlet needle valve.  The left test port can be used to operate the fuel pump (Polini Thor 130) or be used to evaluate the crankcase pulses (Top 80).  As of June 2019, I am working on a PERMANENT, RELIABLE, and INEXPENSIVE solution to the Walbro fuel pump issue that will greatly help the larger engines that use the WG-8 and WB-37 carburetors.

WG-8 carburetor with external test ports installed

2. Altitude, temperature, and humidity

All paramotors are also sensitive to the properties of the atmosphere.  Carburetors are manufactured to work at sea level.  At high altitudes (about 4,000' MSL and above), the carburetor runs rich, especially in the midrange.   To run smoothly and efficiently, the fuel/air mixture must be leaned out.  The higher you are, the more pronounced is the problem.  See High altitude use for how to fix this.  When temperatures drop, the viscosity of the fuel increases and moves more slowly through the fuel system.  As a result, fuel pressure drops and the main jet may have to be increased via replacement (WG-8) or adjustment (WB-37).  Again, this does not solve the root problem of these carburetors.

3. Yearly rebuild required

All carburetors should be rebuilt every year if they have had any contact with fuel, especially fuel containing ethanol.  The flexible parts of the carburetor begin to age when they contact gasoline and it is much worse if it contains ethanol.  Do not waste your time tuning your engine without first rebuilding the carburetor if it has not been done recently.  The fuel pump diaphragm must be supple and in perfect condition to ensure adequate fuel pump pressure, especially under extreme conditions.  The valves and buffering chambers all function correctly ONLY if the diaphragm material is supple i.e. new.

4. Reed valve body leaks

Top 80 ONLY – CAUTION  When removing the carburetor, always check the torque on the reed valve body screws.  If they are loose, the valve body will leak and the fuel pump will not work properly, lowering the fuel pressure and leaning out the engine.  Correct torque is 2.5 Nm (22 in lb).  If they are loose, remove them and apply blue threadlock and reinstall.  (Brake cleaner and air must be used to remove traces of oil from the screws and internal threads.)  If you have any suspicion that your reed valve is not properly sealed to the crankcase, see the reed valve page for help.

5. Additional jet inside the WG-8

The WG-8 has an additional "side jet" that is active from the midrange to full throttle.  In this photo of the idle progression hole well, it is the brass jet to the right with the tiny hole.  It allows fuel in the ML diaphragm cavity to flow directly into the throat.  On the other hand, fuel for the nozzle jet and the idle circuit all goes through the fixed main jet.  The location of the hole in the throat is closer to the throttle shaft than the other idle progression holes which makes the fuel flow less sensitive to the throttle position.  Through testing, it was discovered that it is for smooth acceleration from idle/low midrange to 1/2 throttle and more.  If this jet is plugged, the engine will likely stall when accelerating, regardless of the idle jet setting.  The jet adds just enough fuel into the engine during this early stage of operation.  It is possible to get through the midrange to 1/2 throttle and more if the throttle is pumped with this jet plugged.  Do not modify this jet!

6. Black lever on the throttle shaft

The WG-8 (and some other WG models like the WG-7,9,10) have a black lever on the throttle shaft which, over the years, has puzzled pilots.  It is part of the throttle system on the Husqvarna engines and is not used or needed on paramotors.  It can safely be removed or cut off.  Remember:  Walbro will not do anything to upgrade/change these chainsaw carburetors for aviation use.

7. Buffering chambers

If you carefully examine the fuel pump side of these carburetors, you will see that the pump inlet and outlet both have small buffering chambers that are created by the material in the pump diaphragm and the fuel pump cover.  THESE CHAMBERS ARE CRITICAL TO THE OPERATION OF THE FUEL PUMP!  One side buffers the inlet to the pump and other buffers the outlet.  The exterior of the buffering chambers are exposed to atmospheric pressure (the two tiny holes in the fuel pump cover).  If the diaphragm is old and stiff, the buffering chambers cannot do their job and the metering lever valve will not work correctly, either greatly leaning or enriching the fuel mixture.  I discovered this in tests when I disabled the chambers.  Fuel pump pressure to the ML valve would increase by 300% sometimes.

8. Metering lever

WG-8 carburetors of new Miniplane Top 80 engines have a metering lever (ML) value set to 1.7mm with the button-type diaphragm (BTD).

In our shop, we have many engines that have been burned up.  Engines that have come in here with problems at or near full throttle (lack of power, not reaching full speed) had ML heights of 1.7mm.  Tests have demonstrated that this ML value is too high and why engines set to this value tend to overheat and have less power output (or stall) at or near full throttle.

What happened?  It is due to the two different types of ML diaphragms supplied in rebuild kits and installed in the WG-8 carburetors.  The diaphragms differ in height and, therefore, require a different setting for the ML.  The diaphragm (not the valve) has a total independent travel of about 3.25mm which would mean that the ML valve would have a range of movement of about ½ that value (1.62mm) because the distance from the pivot to the diaphragm center is about double from the pivot to the ML.  How much does the ML valve have to be raised in order for it to deliver the maximum amount of fuel at wide open throttle (WOT)?  I am guessing that the valve value is 0.5mm because that is the value when a tang-type diaphragm (TTD) is used and the ML height is set to 1.7mm, the height of the official Walbro gauge.

If a BTD is supplied in a kit that has a pre-set ML designed for a TTD, there will be a problem: too lean a mixture.  Pilots will discover that most kits supplied in the U.S. have a BTD.  Digging through Miniplane documentation in Italian I discovered that, at some point, they were aware of the difference between the BTD and TTD because their service manual gives an ML height of 0.5mm – 0.7mm.  Carburetors with the BTD and the ML set to this range do not overheat and achieve full power.

The net result is that the two types of diaphragms require two different ML settings.  The majority of rebuild kits have the BTD but the ML has a preset height of 1.7mm (the correct value for the TTD).  No wonder we have problems and is why pilots must check the ML height on their carburetors and correct it as necessary.

The BTD is 0.38mm thinner than the TTD.  If the ML is set to 1.7mm on a carburetor with a BTD, the ML has an operating range of about 0.85mm and a valve range of 0.42mm.  This is about 20% less of the valve travel required to allow full flow through the valve and explains why overheating and poor WOT performance occurs.

As the diaphragm moves in towards the carburetor, the more the ML valve opens to release fuel into the engine.  More force is required to move the diaphragm in due to the ML spring.  Therefore, an increase in the ML height results in a greater force needed to move the diaphragm.  It follows that a greater ML height results in a leaner air fuel mixture.  Changing the tension of the ML spring will change the air fuel mixture.  The change in spring tension can be measured with a pop-off gauge. 

Overheating in the Top 80 is the most common problem so having the ML height set to 0.5mm - 0.7mm is the best option.  If it is set to the higher value, the engine will run a bit leaner which may be of benefit when operating at higher altitudes.

New engines need to have the ML height checked to be sure the ML value is set to the correct value.  That is 0.7mm for the button-type diaphragm and 1.7mm for the tang type diaphragm.  While the Top 80 will run at the 1.7mm value with the button-type diaphragm, it will run better if the range is opened and adjusted to the Miniplane specification (0.7mm).

If you get your rebuild kits from Miniplane, the diaphragm may have the tang on it.  Note that if the metering lever value is set way too low, the engine will not start easily because it will always flood.  If too high, the engine will go lean at high loads and possibly overheat.  It is best to error on the side of too low as it will not hurt anything.  If you have a kit with a tanged button, be sure to engage the tang in the metering lever when reassembling the carburetor.)  As Gerry Farell notes, the pop-off pressure is one of the two most important factors that changes the air fuel mixture ratio (the other value is the size of the main jet).  To change the ML height, bend the short end, not the long end of the lever.  If you bend the long end, you may affect the lever’s smooth functioning on the pivot.  Remember that the ML moves up and down at up 9,800 times/minute at full load.

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