Fuel system test for the WG-8 & WB-37

by Had Robinson

updated October 27, 2019

By far the most common problem is paramotors is fuel starvation (fade, stopping at the high end).  There are so many things that will cause it.  You must be patient and do these tests carefully and methodically.  If you do not have the patience or skill, find someone who does.  You will save a lot of $$$ and frustration.

Here are some tests for the WG-8 and WB-37 carburetors to see if your fuel system is in working order.  If you need to, you can refer to these diagrams to identify the parts of the carburetor that you have:  WG-8  WB-37.


For all engines that use the WG-8 carburetor with an internal pulse port (through the reed valve body) e.g. the Top 80:  If there are fuel delivery problems when the engine is running under load (in the air), check to be sure that the reed valve mounting screws are properly torqued down to specs (4-5 Nm).  If the reed valve is loose, the fuel pump will not work properly, if at all.

Another serious problem is that the carburetor metering lever value is excessive.  Sudden stoppage of the engine when it is at 1/2 or more throttle can be caused by this.  Go to this link for how to adjust it.  You will have to remove the metering lever diaphragm cover and diaphragm to do this.

1. Carburetor high speed system test

All engines except the Minari: Remove the air box.  Tilt the engine to the air box side.  Blow into the priming tube while pushing down on the fuel metering lever (ML) release button on the side of the carburetor.  (This is how you prime the carburetor.)  Hold the pressure until fuel runs out of the carburetor.  If it runs freely out in a small stream you will know that the filters and passageways right up to the main jet are likely clear and working correctly.  THIS WILL NOT TEST THE FUEL PUMP!  You could still have obstructions in the slow and intermediate fuel systems.  Problems there will cause the engine to idle poorly, if at all.  Once you release the priming lever and pressure in the fuel tank, fuel should remain in the fuel lines.  If the fuel runs back into the tank (sudden appearance of air in the lines), there is a problem with the check valves in the fuel pump.  Your engine will not idle properly and it will be hard to start.  You will need to rebuild your carburetor if this happens.  Note: For a thorough test of the fuel system, see fuel pump test on this page.

For the Minari: See the "Quick fuel system tests" on this page and run them.

2. Carburetor low speed systems test

To check whether the low speed system is OK (up to but not including the jets) just unscrew the idle adjustment screw and remove it.  (Be sure to first gently screw it in while counting the number of turns so that you can later return it to its original setting.)  Now, prime the carburetor as in #1 above.  Fuel should dribble out of the idle screw hole as well as from the main jet inside the carburetor.

For the Minari, the throttle must be tied or taped to the wide open position so that the excess fuel that dribbles into the engine can better evaporate and not flood it.  Prime the system in as short amount of time as possible!

3. Fuel pump and filters test

The fuel pump in the WG-8 & WB-37 carburetors is simple and reliable but not robust.  It was NOT designed to pump fuel from a tank located 60cm or more below the carburetor.  These carburetors (and the fuel pump) were designed for engines with a fuel tank at the same level as the carburetor.

Because of this design and the use of these carburetors outside their specifications, there are serious problems with fuel delivery to engines when used in marginal conditions.  These conditions include:

Any one or a combination of the above conditions can cause fuel starvation.

To help mitigate the problems caused by the original design of the WG-8 when used on the Top 80, pilots should ream the passageway in the reed valve.  Go to the Reed valve problem page for how to do this.

The fuel pump port on the WG-8 may also be widened to help increase the effectiveness of the pulses from the crankcase (the source of what operates the fuel pumps on all diaphragm carburetors, like the WG-8 and WB-37).  Here is how to modify the pulse port inside the WG-8 by increasing its diameter to 3.0mm (courtesy of Vittorazi).

In general, ensure that all passageways from the crankcase to the fuel pump are clear and have the greatest diameter as possible.  We are working on an auxiliary fuel pump which will overcome the design limitations of the WG-8 and the WB-37 carburetors.

The fuel pump works by means of the rapidly changing pressure (pulses) in the crankcase every time the piston goes up and down.  These pulses are transferred through a series of passageways to the fuel pump diaphragm which, in turn, moves it up and down.  By the use of one-way valves, the pump sucks fuel from the storage tank and forces it to the metering lever valve in the carburetor.  Leaks, clogs, or misalignment of the pump cover gasket can prevent the pulses from the engine reaching the fuel pump.

A problem with the fuel pump and/or the filters will cause high-end fade when operating at or near full throttle.  Problems of fuel starvation can occur immediately, after a few minutes, or even up to 10 minutes or more.  During marginal conditions fuel starvation can occur gradually.  (Pilots often misdiagnose this problem thinking that fuel starvation can occur immediately.)  IF THESE PARTS OF THE FUEL SYSTEM ARE NOT REPAIRED, THE ENGINE MAY OVERHEAT AND BE DAMAGED!

Filter tests

There are usually at least three filters: the fuel pickup tube filter, the main inline filter, and the fuel inlet filter screen (in the carburetor).

The fuel pickup tube filter removes large particles and goo from getting to the inline fuel filter.  It also weighs down the pickup tube so it is always immersed in fuel.  This filter is small and can easily get clogged or restricted by the goo that ethanol fuels tend to create in the tank.  The inline filter is the most important filter of all and must be of the correct quality and size.  If it is not of the correct filtering media (10-12 microns), the fuel inlet filter screen will get clogged.

To check the condition of the fuel filters, remove the metering diaphragm cover, the diaphragm, and the metering lever assembly.  It is easy to drop/lose the interior parts of the carburetor so be careful.

Other things that could stop fuel movement are: a collapsed fuel line, a faulty check valve at the quick disconnect on the tank, or debris in the fuel line somewhere.

If a quality inline filter is not used, the fuel inlet filter screen in the carburetor will become clogged with fine particles.

Pump test

Before doing this test, it is a good idea to do the filter test above and leave the carburetor disassembled (the ML diaphragm cover, the ML diaphragm, the ML, and the inlet valve must be removed).  If the filters or fuel lines are clogged, the fuel pump cannot do its job.  For the Top 80: The airbox junction must be attached to the carburetor.  If the carburetor is loose, the pump will not receive any pressure pulses from the crankcase and it will NOT pump fuel.

To do a thorough test of the fuel pump:

For the Top 80 and all engines that use the WG-8 carburetor with an internal pulse port (through the reed valve body): if the reed valve body is even slightly loose, the fuel pump may pass this test but when the engine warms up and is under load (flying), the pump may fail.  Check the mounting screws for the reed valve body that they are torqued to specifications.  Blue threadlock MUST be used on these screws!  The carburetor will have to be removed to check the reed valve.

After the test, be certain that no bubbles can be seen moving slowly back down through the fuel line towards the fuel filter and tank.  Movement of air bubbles (not fuel vapor bubbles) indicates a failure of the check valves in the fuel pump and you will have to rebuild the carburetor.  Ethanol in the gasoline is very hard on the fuel pump diaphragm material.  If all is well, put everything back carefully.

Note that it is normal to have bubbles in the fuel line, especially in hot weather.  It is fuel vapor, not air (unless you have a leak in your fuel filter).

If you are curious to know what the fuel pump suction vacuum is, you can check it with a vacuum gauge (sold at auto parts stores).  The value should be what is given on our specifications page.  If it is below the specified value, a rebuild will help.

General info on the tests

If the engine starts and then stalls after a minute or two, you probably have a problem with the fuel pump diaphragm or check valves in the pump.  Ethanol blends will quickly stiffen soft carburetor parts that are not specifically manufactured to withstand powerful solvents like ethanol.  Generally, carburetors should be rebuilt every year because of problems with the check valves that are a part of the pump diaphragm.  Age and warping from additives and solvents in gasoline are big factors in causing the valves to leak.  Rebuild kits are available from Miniplane-USA.

After you perform these tests, remember to be sure to see if any air or fuel vapor bubbles appear in the fuel line right where it is attached to the carburetor.  Look at it for about 15 seconds.  If you see any bubbles moving up and out of the carburetor, through the fuel line, and then back down into the tank, you will most likely have to rebuild the carburetor because the pump check valves are leaking.

Turkey Vulture