Walbro WG-8 & WB-37 carburetor info & others
by Had Robinson & others
updated March 11, 2021
Note: The WG-8 (no longer available) and the Husqvarna #503282001 (now available only through Husqvarna) are identical. The WB-37 and the MPE090 are tuned identically and have the same parts. The former has an external pulse port and the latter, an internal port.
Zama has this simple technical guide of how a diaphragm carburetor works. It is the place to begin if you have never worked on a paramotor engine.
Notes on Walbro carburetors (more technical) concerning design, inherent problems, fixes, and general information that is helpful to those rebuilding or troubleshooting.
ALL ENGINES WITH INTERNAL PULSE PORTS e.g. Top 80, newer Moster 185 – 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. If they are loose, remove them, clean the screws and holes with brake cleaner, blow dry, and apply blue threadlock and reinstall to the correct torque for your motor. If you have any suspicion that your reed valve is not properly sealed to the crankcase, see the reed valve page for help.
Special tools needed to service a carburetor – see the Special tools needed
If your engine is not running right or not at all, first see Performance issues, general. It is the right place to start and has a checklist.
Adjustment WG-8, low speed – Study "Rebuilding and tune up" below first to be sure you understand the basics.
Adjustment WG-8, high speed – The WG-8 high speed system has a fixed jet and cannot be adjusted. In certain circumstances e.g. high altitude, the main jet can be replaced with a different one.
Adjustment WB-37 – idle, low, and high speeds
Air box – the air box usually has an internal air filter. It must *NOT* be saturated with oil. It is unnecessary and can restrict air flow into the engine. Periodically wash the filter in hot soapy water, as needed.
Air filter – see "Air box" above
Black lever on the WG-8 throttle shaft – What is it for?
Buffering chambers – The inlet and outlet of the internal fuel pump have them.
Carburetor dimensions – by Gerry Farell and Had Robinson – This information compares the dimensions of the various Walbro carburetors and is helpful if you are considering changing from the WG-8 to another Walbro model. The WB-37 would work on the smaller engines (80cc to 130cc) but there might be more problems/roughness in the midrange. Zama does not make any carburetor that is compatible with paramotors.
Carburetor Rebuild – see "Rebuilding and tune up" below
Carburetor removal – It is usually held to the engine by (2) nuts which are just next to the air box flange. NEVER over-tighten these nuts! Some engines have long socket head screws which hold the carburetor to the engine. Remove the fuel tubing, throttle cable, choke wire, and throttle return spring.
Carburetor service video – see "Rebuilding and tune up part 1" below
Choke – What does it do? Why have it? Many models of the WB-37, sadly, do not have a choke.
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....
Diagram of the WB-37 – the diagrams are for most models, variations are minor.
Disassembly – see "Rebuilding and tune up part 1 and part 2" below
Drips – Two stroke engine carburetors drip/leak fuel by design. It cannot be helped.
Engine performance issues – see "Performance tuning" below
External pulse port installation – some models of the WG-8 only. It is needed for accessories including an auxiliary fuel pump
Failure points – There are (3) common points of failure in Walbro carburetors. Also, see "Performance issues, general" and "Performance issues, midrange" below
Fuel draining from the carburetor back into the tank when engine is off
Fuel filter – What kind/type to use? If you choose poorly, the engine will also run poorly, if at all. Removal and installation tips. The fuel system MUST be purged of old fuel and air when changing out the fuel filter!
Fuel pump – see Fuel system test The fuel pump design of the WG-8 and the WB-37 are marginal and will not work well except in perfect conditions. You must rebuild the carburetor at least once a year or more often if ethanol fuels are used so that the fuel pump diaphragm and check valves are in perfect order.
Fuel pump vacuum – 5" H20 (dry) 7" H20 (wet w/ fuel) It should be obvious why paramotors with fuel tanks way below the engine have fuel supply issues.
Fuel starvation – this is what it looks like. To fix this, see "Fuel system test" below.
Fuel system modification – the FSM dramatically fixes fuel starvation and most performance issues (still in the test phase).
Fuel system test – A quick and easy way to tell if the system is functioning properly.
Fuel tubing size – Use Tygon® LP1100 Low Permeation Fuel Tubing (ethanol resistant). It is available from Miniplane-USA. Do NOT use ordinary vinyl tubing as it will become stiff quickly and stress the connections on the tank and engine. Auto parts stores do NOT have the right type.
- carb 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")
Gasket problems – WG-8 only The gasket between the carburetor and the reed valve can get misaligned and block the pump pulse port.
High altitude use – WG-8 only 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 (fixed) for high altitude use – WG-8 only see "High altitude use" above
Jet modification – WG-8 only Here is a way to temporarily decrease the size of a fixed main jet that it is not permanent
Leaks – see "Drips" above
Metering lever adjustment – WG-8 0.7mm WB-37 1.19mm - 1.59mm. WG-8 only: If your metering lever diaphragm has a tang instead of a button, set the value to 1.7mm
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.
Metering lever height variation – WG-8 carburetors on new engines have one value and Miniplane's Italian service manual specifies another. Which is correct?
Metering lever spring – 15.0mm long when new. Replace if less than 14.9mm
Midrange roughness, stutter, and four-cycling – see "Performance tuning, midrange" below
Models in the WG series – The different WG models are a result of changes in the U.S. EPA air pollution regulations.
Modifications by Gerry Farell – WG-8 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?
Parts Diagram – see "Diagram" above for respective carburetor model
Performance issues, general – Here is the info for troubleshooting a motor. It includes information on the ignition as well as the carburetor
Performance tuning – Here is how to modify the WG or WB-37 carburetors 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 – WG-8 & WB-37 1.2-1.3 Bar (17.5-19.0 psi) Rest pressure > 0.68 Bar (10 psi.) The pressures must be within specs!
Priming the fuel system on a paramotor – see "Starting your paramotor" below.
Purging the system of fuel – 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 link is for the official Walbro BASIC video (21 min). It is helpful for those who are not familiar with diaphragm type carburetors. It is THE place to begin and includes testing of the pop-off pressure. Sections 1-5 are the most relevant for users of the WG and WB-37 carburetors. The video has examples of carburetors ruined by ethanol fuels.
Rebuilding and tune up part 2 for the WG-8 & WB-37 carburetors. Once you have studied part 1 above or already have a basic knowledge of diaphragm carburetors, continue here.
Replacement for the WG-8 – There are no currently available carburetors that are an exact replacement of the WG-8. However, a WB-37 may be used as a replacement but it has no choke.
Reed valve – 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 for all Walbro carburetors – (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). It has some good theory discussions but the ZAMA manual is better.
Service manual WB-37 – This is 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.
Stall – Going to full throttle, the engine stalls. Or, the pilot bends over and the engine stalls. What are the causes?
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 WG-8 – 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.
Tillotson repair kit info – cross reference for most of the Tillotson carburetors (now being used by Miniplane)
Troubleshooting chart – for those who are not familiar with how a diaphragm carburetor works, this chart from ZAMA can be a great help. None of the so-called troubleshooting charts are particularly useful. More often than not there are multiple problems with the carburetor.
Tune-up – see "Rebuilding and tune up" above
Warming up – when is a paramotor warmed up sufficiently and ready to fly? When the cylinder head temperature reaches 70-80ºC, the fuel/air mixture is sufficiently hot enough to ensure that it is 100% vaporized and that the engine will not be fuel starved and potentially overheat. Overheating can happen very quickly!