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Walbro carburetor information, adjustment, & repair

by Had Robinson & others

Introductory notes

The information below is primarily for the WG-8 carburetor.  Other models, including the WB-37, are similar.  Some may 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 Miniplane does not have an adjustable main jet on the Top 80.

If you experiencing problems with your carburetor, check the performance problem page, section "B" for some simple tests you can do.

Fuel pump  The fuel pump in the both WG-8 and the WB-37 is powered by the pulses present in the crankcase of all 2 stroke engines.  These carburetors were not designed for paramotors but for portable saws (Husqvarna) which have the fuel tank at almost the same level as the engine.

Consequently, these pumps barely operate well enough to pump fuel from the typical tank on a paramotor (which is affixed below the engine) in perfect conditions (72F,25C at sea level).

The minimum fuel level that must be moved to the carburetor by a vacuum can be 18" (46cm) or more below the fuel pump.  This distance 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 will become fuel starved, especially at or near full throttle.

The only way to fix this design problem 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 solution by early 2019.

Altitude  All paramotors are sensitive to altitude.  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.

Rebuild yearly   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 under all 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.

Service info   Pilots who are not experienced with working on carburetors should watch the official Walbro carburetor service video.  If you want to understand how the Walbro works, the ZAMA carburetor is a Walbro knockoff.  ZAMA has this technical guide which is vastly simpler to understand than anything from Walbro (see pages 2-6).  These (5) pages contain instructions on how diaphragm carburetors work and a troubleshooting chart.  If you do not understand how a diaphragm carburetor works, how will you be able to fix it?  They also have published some service tips which apply to most parts of the WG-8.

Pop-off gauge   If you want to service your own carburetor properly, you must invest in a pop-off gauge.  It is impossible to check the function 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.  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 i.e. the metering lever spring or metering lever height.  They are concerned, it may be assumed, about pilots 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, by far, 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.

Service Topics

Special tools needed to service a carburetor – you cannot service any diaphragm carburetor unless you have the necessary tools!

If your engine is not running right or not at all, see "Performance issues, general" below.  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

Cold weather operations – Unfortunately, the WG-8 and, to a lessor extent, 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.

Disassembly by Richard Cobb with notes by Had Robinson – TECHNICAL  For ordinary disassembly, see Rebuilding #1 and #2 below.

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 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 port.

High altitude use – Pilots can improve performance at high altitudes and/or cold conditions by modifying the WG-8 and WB-37 carburetors.

Installation – Be sure that the carburetor-reed valve body gasket is properly aligned with the pump port hole.

Jets for high altitude use – see "High altitude use" above

Jet sizes – Stock jet sizes (Main jets Top 80 #116, Thor 130 #130).  We stock main jets of all sizes.  Contact us to purchase.

Jet size modification – A non-permanent way to modify the stock jets in a Walbro carburetor.  It is always better and easier to change out the jet.

Leaks – see "Drips" above

Metering lever adjustment – See step #17 on the link for how to adjust.  Set the ML height to 0.5mm – 0.7mm.  If your metering lever diaphragm
has a tang instead of a button, set the height to 1.70mm.  Too low a height, the engine will not idle.  Too high, it will lean out at full throttle.

Metering lever function (from Walbro) - Also, see "Modifications" below for more information on this.

Midrange roughness and "four-cycling" – see "Performance tuning, midrange" below

Midrange stutter and roughness – see "Performance issues, 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

Parts Diagram  – For the Top 80.  The Thor's have a different sized main jet.  This diagram does not identify the priming lever which you must have.

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.

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 blended fuel.

Rebuilding and tune up part 1 – This new 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.  If your engine does not run, see "Performance issues, general" above.

Rebuilding and tune up part 2 of the Walbro WG-8 carburetor.  This includes the link to part 1 above.

Service manual (Walbro) – Also check out Walbro's new video "Carburetor Service" above.

Service manual (ZAMA) – This service manual is more 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!

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).

Throttle cable – Cleaning, modifications, and cruise control info.  For kill switch issues, see "kill switch problems".

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

Welch plug removal and installation