Walbro carburetor information, adjustment, & repair
by Had Robinson & others
The information below is primarily for the WG-8 and WB-37 carburetors. Other Walbro models are similar. The WB-37 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-8 has a fixed main jet.
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. 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, check the performance problem page, section "B" for some simple tests you can do.
The Walbro WG-8 and the WB-37 carburetors are obsolete in design but, thankfully, are still manufactured and the repair kits are available. For liability reasons, Walbro will not help in any way those who use these carburetors in a prohibited application, that is, in aviation. Even the paramotor manufacturers have very limited choices working with Walbro.
Fuel pump design problems
The fuel pump on the WG-8 and WB-37 was NEVER 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 is very sensitive. 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 barely operates 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.
The fuel is moved to the carburetor by a vacuum. 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. 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.
The only way to fix this design problem when operating in extreme conditions is to have an auxiliary fuel pump to assist the feeble fuel pump in the WG-8 and WB-37. I am working on this currently and will have a possible solution by early 2019. Meanwhile, 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 will help the transfer of pulses from the crankcase to the pump diaphragm.
Altitude, temperature, and humidity
All paramotors are sensitive to these properties of the atmosphere. Carburetors are manufactured to work at sea level. At high altitudes (about 4,000' MSL and above), the carburetor supplies too much fuel to the engine. 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" below for how to fix this. When temperatures drop, the viscosity of the fuel increases and moves more slowly in the weak and sensitive fuel systems of most 2 stroke paramotors. As a result, the main jet of the carburetor may have to be increased via replacement (WG-8) or adjustment (WB-37).
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 insure fuel reaches the carburetor especially under extreme conditions.
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. Correct torque is 2.5 Nm. If they are loose, remove them and apply blue threadlock and reinstall. If you have any suspicion that your reed valve is not properly sealed to the crankcase, see this page for help.
Pilots who are not experienced with working on carburetors should first watch the official Walbro carburetor service video. If you want to understand how the Walbro works, the ZAMA carburetor is a Walbro knockoff and has this technical guide (see pages 2-6). These pages contain instructions on how diaphragm carburetors work and a troubleshooting chart. Zama has also published some service tips which apply to most parts of the WG-8. The service manual for the WB-37 is also helpful because it is very close in design to the WG-8. The exploded view of the carburetor on page 2 is particularly good. There is no service manual for the WG-8 except what is in the pages on this website.
QUESTION: If you do not understand how a diaphragm carburetor works, how will you be able to fix it?
If you want to service your own carburetor 100% properly, you must invest in a pop-off gauge. It is impossible to check all of the functions of a diaphragm type carburetor without a pop-off gauge. If you do not have a pop-off gauge, you will have to trust that important parts of the carburetor are OK. Maybe they are working properly, maybe not.
ZAMA's advice about the hazards of checking the pop-off pressure are correct (over-pressurizing the fuel system). ZAMA is referring to the pop-off pressure, generally, on a new or rebuilt carburetor and not to the rest pressure nor if critical parts of the carburetor were changed e.g. the metering lever spring or metering lever dimension. They are concerned about using an air hose rather than the specialized pop-off pump and gauge which will not over-pressurize the fuel pump chamber and damage the diaphragm and seals. All carburetor manufacturers note that it is the rest pressure of the system that is the most important thing to check. If the rest pressure is less than 10 psi, the engine will not idle properly (or have a run-away idle) and no adjustment of the carburetor can fix it.
Metering lever spring change-out
I do not recommend changing out the OEM metering lever spring in the WG-8 or in the WB-37 because it is just too difficult to know how it will effect things. It is much easier to change out the main jet (WG-8) or correctly adjust the high speed needle (WB-37).
Special tools needed to service a carburetor – see the "Special tools needed" section of the Rebuilding and tune up of the Walbro carburetor page.
If your engine is not running right or not at all, first see Performance issues, general. It is a good place to start and has a checklist.
Adjustment, low speed – Study "Rebuilding and tune up" below first to be sure you understand the basics.
Adjustment, high speed – The low speed system must be perfectly adjusted before adjusting the high speed system.
Carburetor Rebuild – see "Rebuilding and tune up" below
Carburetor service video – see "Rebuilding and tune up part 1" below
Cold weather operations – Unfortunately, the WG-8 and the WB-37 are very sensitive to both high altitude and/or cold weather operations.
Cylinder head temperature gauge – Not having a CHT is like driving a car without any gauges or warning lights. Most of the time you don't need them....
Dimensions – by Gerry Farell – This information compares the dimensions of the various Walbro carburetors and is helpful if you are considering upgrading from the WG-8 to another Walbro model. Only the WB-37 would work on the Top 80 and the Thor 100 & 130 but the WB-37 has no choke. Zama does not make any carburetor that is compatible with paramotors.
Drips – All diaphragm carburetors drip fuel. Do not attempt to fix this by putting RTV sealant everywhere. 2 cycle engine carburetors are leaky by nature. There is no way to fix this due to the pulsing movement of air/fuel through them. However, there are leaks that will cause problems!
Engine performance issues – see "Performance tuning" below
Failure points – There are (3) common points of failure. Also, see "Performance issues, general" and "Performance issues, midrange" below
Fuel filter – Info, types, problems
Fuel pump – see "Fuel system test" The fuel pump design of the WG-8 is marginal and will not work well except in perfect conditions.
Fuel system test – A quick and easy way to tell if the system is functioning properly.
Fuel line size – carburetor to inline filter ID 3.2mm (1/8") x OD 6.2mm (3/16"); inline filter to fuel tank ID 4.8mm (3/16") x OD 8mm (5/16") – use Excelon fuel line or equivalent. Do NOT use vinyl tubing. It will become stiff because of fuel additives and stress the connections on the tank and engine. The correct fuel line tubing is available from Miniplane-USA. Auto parts stores do NOT have the right type of tubing!
Gasket problems – The gasket between the carburetor and the reed valve can get misaligned and block the pump pulse port.
High altitude use – Greatly improve performance at high altitudes and/or in cold conditions.
Installation of the carburetor – Be ABSOLUTELY CERTAIN that the carburetor-reed valve body gasket is properly aligned with the pump port hole. If this gets misaligned, the engine may still run but poorly.
Jets for high altitude use – see "High altitude use" above
Jet modification – here is a way to decrease the size of a fixed main jet that it is not permanent
Leaks – see "Drips" above
Metering lever adjustment – Set the ML value to 0.5mm – 0.7mm. If your metering lever diaphragm has a tang instead of a button, set the value to 1.70mm. Too low a value, the engine will not idle. Too high, it will lean out at full throttle. There is now a video on how to do this.
Metering lever function – The ZAMA (a Walbro clone) has a much better service manual (see "Service manual (ZAMA)" below which explains how the diaphragm carburetor works. Also, see "Modifications" below for more information on the metering lever.
Midrange roughness, stutter, and four-cycling – see "Performance tuning, midrange" below
Modifications by Gerry Farell – TECHNICAL discussion (not useful for most pilots). Additional notes by Had Robinson.
Overhaul – see "Rebuilding and tune up" below
Overheating – This will destroy an engine. What causes it? How is it prevented?
Performance issues, general – Here is the info for troubleshooting a motor. It includes information on the ignition as well as the carburetor
Performance tuning, midrange – Here is how to modify your carburetor to increase performance and eliminate roughness in the midrange.
Performance tuning, full load – See "High altitude use" above.
Pick-up tube filter – see "Fuel filter" above
Pop-off pressure – The pressure must be within specs for maximum performance, to prevent engine damage, and for a steady idle.
Priming the fuel system on a paramotor – see "Starting your paramotor" below.
Purging the fuel system – It must be done if a paramotor is to be stored for more than a few weeks, especially if you are using ethanol fuels.
Rebuilding and tune up part 1 – START HERE! This BASIC video from Walbro is helpful for those who are not familiar with diaphragm type carburetors. It is a good place to start and includes testing of the pop-off pressure. Sections 1-5 are the most relevant for users of the WG and WB series carburetors. If your engine does not run, see "Performance issues, general" above.
Rebuilding and tune up part 2 for the WG-8 & WB-37 carburetors. Once you have studied part 1 above, continue here.
Reed valve – Note: A loose reed valve body or a clogged pulse port will affect the fuel pump. The mounting screws must be torqued to the correct value.
Service video for Walbro carburetors – see "Rebuilding and tune up part 1" above
Service manual – (not very helpful and out of date) This is for all Walbro diaphragm carburetors, including the WG series (the series used in obsolete chain saws and now paramotors).
Service manual WB-37 – This is very concise compared to the manual above that is for all Walbro carburetors. The WB is very close to the WG per operation. The main difference is that the WB has an adjustable high speed needle jet and a different metering lever adjustment value. The WB-37 also does not have a choke. If you wish to understand how the WG works, study this manual first. The ZAMA SM (below) is also a good SM to study.
Service manual (ZAMA) – This service manual is just a few pages, concise, and simpler to understand than the Walbro manual. Pages 2-6 are the relevant sections.
Service tips (from ZAMA) – ZAMA explains why measuring the pop-off pressure is rarely necessary. It's the REST pressure that is important! Also, using the wrong type of pump can easily over-pressurize the fuel pump chamber and damage the diaphragm.
Starting your paramotor – How to start your engine the first pull, every time. Note: this technique will not work on engines with vertical carburetors (Minari).
Storage – If you are storing your paramotor for more than a few weeks, PURGE THE FUEL SYSTEM!
Throttle return spring replacement – Improve throttle response, lessen hand fatigue with this modification/replacement.
Throttle shaft play – A worn out shaft leaks air and will cause the engine to idle poorly. A bad inlet needle seat will also cause this.
Troubleshooting chart – for those who are not familiar with how a diaphragm carburetor works, this chart from ZAMA can be a great help.
Tune-up – see "Rebuilding and tune up" above