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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 are similar.  Some may have an adjustable main jet which must only be adjusted with great care because of the ease with which the engine can be leaned out – and burned up.

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.

Top 80 ONLY – When removing the carburetor, always check the torque on the reed valve body bolts.  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.

Pilots who are not experienced with working on carburetors should watch the official Walbro carburetor service video.  If you want to service your own carburetor properly, you need to 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 Info  If you want to understand how the Walbro works, the ZAMA carburetor is a Walbro knockoff.  ZAMA has this technical guide which is simpler to understand than anything from Walbro.  This guide contains instructions on how diaphragm carburetors work, a troubleshooting chart, and how to disassemble and service them.  They also have published some service tips which apply to most parts of the WG-8.  Contrary to ZAMA's advice about pop-off pressure, the Walbro must be checked.  Checking the pressure will not harm the Walbro (!) and the correct pressure must be maintained in order to avoid burning up the engine.  As ZAMA notes, it is the rest pressure that is more important.  The actual pop-off pressure is determined by the pop-off spring.  This spring can harden and increase the pressure enough to lean out the engine.  If the rest pressure cannot be maintained above specification, the metering lever valve is leaking.

If you do not want to do the repairs yourself but have a professional do it, contact us for details.

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

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 is has no choke.

Disassembly by Richard Cobb – TECHNICAL discussion (not useful for most pilots).  Additional notes by Had Robinson

Drips – All diaphragm carburetors drip fuel.  Do not attempt to fix this by putting RTV sealant everywhere.

Failure – 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.

High altitude use – There are number of things you can do to the WG-8 to greatly improve performance at high altitudes.

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

Jet sizes – Jet sizes and their dimensions.  Main jets: Top 80 #116, Thor 130 #130

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

Leaks – 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.

Metering lever adjustment – See step #17 on the link for how to adjust.  Set the ML height to 0.5mm – 0.7mm from the top of the metering
lever to the top edge of the carburetor body

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

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.

Performance issues, general – This 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 – This gives a thorough discussion on how the carburetor works.  Also check out Walbro's new video "Carburetor Service" above. 

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.

Tune-up – see "Rebuilding and tune up" above

Welch plug removal and installation