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Fuel system test

by Had Robinson

Here are some tests 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.


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.  This is especially true for the Top 80.

1.  Carburetor high speed system

All engines except the Minari: Remove the air box but not the airbox junction.  Tilt the engine to the air box side.  Blow into the priming tube while pushing down on the fuel metering lever 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 and including the main jet are mostly clear and working.  THIS WILL NOT TEST THE FUEL PUMP!  (To test the fuel pump see #3 below.)  You could still have obstructions in the slow and intermediate speed 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 #3 below.

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 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.  Keep the time that you prime the system as SHORT as possible.

3. Fuel pump and filters

The fuel pump works by means of the rapidly changing pressure in the crankcase every time the piston goes up and down.  This pressure is transferred through a series of passageways to one side of the fuel pump diaphragm which, in turn, moves it up and down.  This movement creates a suction and pressure action which moves fuel into the carburetor and to the metering lever valve.  Leaks, clogs, or misalignment of the carburetor gasket can prevent the pulses from the engine reaching the fuel pump.

A problem with the fuel pump or the filters will cause things like high end fade after a few minutes (even up to 10 minutes or more), hard starting, and the presence of air in the fuel lines.  IF THIS IS NOT REPAIRED, THE ENGINE WILL OVERHEAT AND MAY BE DAMAGED.

Testing the filters

There are usually three filters: the fuel pickup tube filter, the main inline filter, and the filter inlet screen (in the carburetor).  To check the condition of all 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" and continue the test.  This test will not flood the engine, including the Minari.

If not, 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.  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 top grade inline filter is not used, it is common that the fuel inlet filter screen will become clogged with fine particles.

Pump test

Before doing this test, be sure to do the filter test above and leave the carburetor disassembled (the metering lever assembly 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.  Now, 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.

Warning for Top 80 engines : 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 specs.  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 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.  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.

Rarely, the cause of no 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: 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 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 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.