Fuel system test for the WG-8 & WB-37
by Had Robinson
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.
NOTE: ALL CARBURETORS SHOULD BE REBUILT EVERY YEAR WHETHER THE ENGINE IS RUN OR NOT.
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
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
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
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:
- use of gasoline containing ethanol
- temperatures less than 15C (60F) or greater than 32C (90F)
- use of non-synthetic oils which do not retain their viscosity very well in low temperatures
- any leaks or restrictions in the fuel pump circuit
- operating at sea level in cold conditions
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 mis-diagnose 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!
Testing the filters
There are usually at least three filters: the fuel pickup tube filter, the main inline filter, and the filter inlet screen (in the carburetor).
The fuel pickup tube filter does not serve any purpose except to weigh down the pickup tube so it is always immersed in fuel. Go to this page to see how to bypass the actual filter but keep the necessary parts of it. 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.
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. Make sure there is fuel in the tank.
- Pressurize the fuel tank by blowing on the priming pipe and holding the pressure with the tip of the tongue.
- If fuel moves up and through the fuel lines and bubbles out of the inlet needle valve hole, skip to the "pump test" below and continue the test. (This test will not flood the engine, including the Minari.) If not, continue.
- Detach the fuel line that goes into the carburetor and pressurize the tank again.
- If fuel does not flow out of the open fuel line, you have either a clogged inline filter (the main filter) and/or a clogged fuel pickup tube filter. Stop the tests and fix the problem. If fuel dribbles freely out of the detached fuel line, the inlet filter screen in the carburetor is probably clogged. This will require a complete disassembly and repair of the carburetor.
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.
Before doing this test, be sure 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 test the fuel pump:
- Drain all but a few inches of fuel from the tank (this will thoroughly test the pump).
- Reattach the fuel line to the carburetor, if removed.
- Remove the spark plug then reattach it to the plug wire. Lay the spark plug on top of the engine so that it touches the aluminum cooling fins (this grounds the spark plug). If you do not do this, you may destroy your ignition coil.
- So that you do not wear out the starter, prime the carburetor in the usual way so that fuel dribbles out of the inlet needle valve hole.
- Pull on the starter. You should see fuel spurt out of the inlet needle valve hole just like a squirt-gun for a yard/meter or so. Here is a video of what it should look like. Every time the piston goes and up and down, a solid stream of fuel should come out of the hole. If no fuel spurts out, the fuel pump or the passageways from the crankcase to the pump are clogged or leaking.
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 there is no fuel flow or it is poor (a dribble and not a powerful spurt), the carburetor needs to be rebuilt, at the least.
Rarely, the cause of poor or little fuel flow can be due to no pulse reaching the fuel pump from the crankcase. Check to be sure the fuel pump port into the crankcase is clear and free of any gasket sealant.
For the Top 80 and all engines that use the WG-8 carburetor with an internal pulse port (through the reed valve body) : The carburetor will have to be removed to check this. Be certain that the gasket between the carburetor and the reed valve body has the pump port hole lined up with the holes in the reed valve and the carburetor. At the same time, check the torque on the reed body mounting screws. These screws should have BLUE threadlock applied to them.
If things look good to this point, you can check the fuel pump suction 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, you will have to rebuild your carburetor.
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
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
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 not working.