Fuel filter information

by Had Robinson
updated February 6, 2022

There are (3) fuel filters in most paramotors: a pickup tube filter, an inline filter, and the inlet fuel filter screen which is inside all diaphragm carburetors.  These three filters are replaceable.  The inline filter is the most important of the three.  If it is a cheap one or is missing, the inlet filter screen in the carburetor will likely clog and cause a lean condition (and overheating) or power fade/stall at or near full throttle.

A. Pickup tube filter (clunk)

Most clunks are the inexpensive variety like this one.  It does not have a steel ball to weigh it down but relies on the weight of the metal frame of the clunk, instead.  This clunk works on weed whackers but is not recommended for paramotors because of its low flow rate and how easy it tends to clog, especially if ethanol fuels are used.

cheap clunk

Below is a typical OEM clunk.  It contains a steel ball but it is only for adding weight to the clunk and does not provide any check valve function.  A plastic clunk with some sort of metal weight might will tend to float in the gasoline.

Miniplane clunk

Because most paramotors have priming systems, check valves in the clunk are largely unnecessary.  However, if the pump check valves inside the carburetor are not working well enough to keep the fuel from running back through the fuel lines, there are other serious problems that must be fixed.  Namely, the internal fuel pump will not be able to keep up with the fuel demand of the engine.  The only fix for this is a carburetor rebuild.  Pilots will eventually discover that when idle is erratic or difficult to adjust, it is time for a rebuild.

Generally, a clunk with a filter of fine, porous material (like those in the above photos) are only necessary if there is no inline fuel filter on the engine, which is rare.  Unfortunately, these types of filters become clogged more easily with dirt including the goo from using ethanol fuels (when the fuel comes in contact with water).  Cold weather can also choke fuel movement through this kind of filtering material due to the oil in the gasoline getting more viscous as the temperature drops.  Before there was gasoline with ethanol, clunks served a more important purpose by helping keep water out of the fuel lines.

INCREASING FLOW THROUGH A CLUNK   Pilots can modify clunks so that fuel will flow more easily through it.  1. Remove the filter from the tank.  2. Drill (2) 6mm (1/4") holes in the sides of the clunk.  If there is a steel ball, do not remove it.  If the holes are made too large, the ball may fall out.

Some OEM clunks have nipples with a narrow ID.  This can restrict fuel movement, especially at or near full throttle.  Use a 1/8" (3mm) drill bit to widen the ID of the nipple.  Remember that ease of fuel flow through the entire fuel system is a critical requirement for good engine operation and preventing overheating/stall/fade.

CLUNK FLOW CHECK   To determine if a clunk with a filter is in order is easy:  Disconnect the fuel line at the inlet side of the inline fuel filter.  Put a cup or container underneath the end of the disconnected fuel line so gasoline does not go everywhere.  Pressurize the fuel tank with the primer tube or squeeze the primer bulb.  Gasoline should pour out the end of the fuel line but not dribble.  If there is any doubt, replace the clunk or make holes in it.  If the clunk is like the one pictured below, it may be cleaned with compressed air.

QUALITY CLUNK   This quality clunk has a 50 mesh stainless steel screen with a body made of brass.  Cost: $8-$10 on eBay.  This clunk costs much more than the typical clunk shipped with paramotors, does not have to be modified, and is easy to clean.  However, these clunks usually have a check ball which should be removed, an easy task done by forcing the butt end of a drill bit down the nipple.  The bit will push the slight indentation in the brass fitting outwards and the small check valve ball will fall right out.  I recommend drilling out the check valve seat inside the clunk to at least 1/8" (3mm).

quality clunk

CHEAP CLUNK AND CHEAP FUEL LINE   Here is a photo of what typically happens to some clunks over time.  The fuel line will split at the clunk and will separate from the fuel pickup tube and then bounce around inside the tank.  Pilots should take a look in their fuel tanks periodically to be sure the clunk is not only attached but also does not have a split fuel line attached.  If possible replace the OEM with a high quality clunk, like the one pictured above.

clunk with split fuel line attached

B. Inlet valve filter screen

The inlet valve filter screen in all diaphragm carburetors can only be cleaned or replaced if the carburetor is disassembled.  It should always be checked if there are any fuel delivery problems in the engine i.e. fuel starvation.  NEVER OPERATE AN ENGINE WITHOUT THIS SCREEN.  Please see the carburetor rebuild page for photos and more info.

C. Inline fuel filter

(If desired, go to the filter installation steps directly and skip the discussion below)

Recommended inline fuel filters for paramotors which are available from various Internet sources.

Do NOT buy inline fuel filters sold in auto parts stores (other than the WIX 33001 which some stores stock).  They are of inferior quality and will not block the small particles that clog the inlet valve filter screen and other small passages in the carburetor.  Unfortunately, most inline filters are rated at 40 or more microns which is fine for 4 stroke engines but not enough for 2 stroke engines which have an inlet fuel filter screen that must be kept perfectly clear.

If you fly in cold weather <60F/15C, you should consider using a filter with a much larger filtering area than the OEM, such as the WIX 33001.  The OEM or Oregon filter may choke the flow of fuel because of the increased viscosity of the oil.

Why bother with a high quality filter?  In two stroke engines, the fuel filter is also the engine oil filter.  It must be able to trap all of the grit and other contaminants in the fuel and lubricating oil.

The engine that had this carburetor had trouble leaning out and stalled under load.  Is it any wonder?  Dirt in the fuel got inside the carburetor and clogged it.  The arrow points to the small cavity on the outlet side of the fuel pump, just before it goes through the inlet needle valve filter screen.  The pilot had an inline filter but it was one of the sintered bronze types (see below) commonly used on lawnmowers.

dirt in carburetor


Aviation engine forums (ROTAX) note that paper filters are the best because they have the finest filtration media – around 12 microns vs. 40+ micros for sintered bronze or the pickup tube filters (clunks) sitting at the bottom of fuel tanks.  The non-paper filters do not trap the small particles which will clog the pump inlet fuel filter screen in the carburetor.  A clear filter housing must be used because pilots need to be able to periodically check that the filter is not clogged or damaged.

Here is a clogged inlet filter screen (diameter about 6mm) with a new one to the right.  Sintered bronze filters will allow contaminants to reach the carburetor and why they should not be used.  This engine experienced fuel starvation and overheated.  Unbelievably, some paramotors do not even come with an inline fuel filter.

WG-8 clogged inlet filter screen

Here is another photo of a clogged inlet fuel filter screen.  The pilot of this engine had trouble at full throttle – when fuel demand outstripped the supply.  It is likely that this engine had no inline fuel filter or that it was the useless bronze sintered-type filter.  I continued to be amazed at how simple maintenance will ensure our engines run well.

Walbro clogged inlet filter screen

Below is a photo of a sintered bronze filter.  The media is just not fine enough to stop small particles from passing through.  These work satisfactorily in 4 cycle lawnmower engines, more or less, but in nothing else.

sintered bronze fuel filter

Fuel pump diaphragm showing signs of water contamination in the fuel, probably from using ethanol blends.

WG-8 fuel pump diaphragm with water contamination

Below is a photo of the WIX 33001 inline fuel filter (L) and a similar filter (R) sold by the auto parts stores.  The only problem with the WIX is that it is about twice the size of the OEM filters and may take a bit more thought as to where to put it.  If you fly in very cold weather, this filter has better flow characteristics because of its large filtering area.  The WIX filtering media is 12 microns which is excellent for all paramotors.  Who knows what it is of the filter on the right.  The fuel line nipples on the WIX have barbs which ensure that the connection will not leak.  The filter on the right will tend to leak fuel because it has no barbs on the nipples.  In addition, the fuel lines may come off if subjected to a pulling force.  Does it make sense to put a cheap fuel filter on a $2,000 engine?

WIX fuel filter and a poor quality inline fuel filter

If the WIX filter is used on the Miniplane Top 80, its large size has to be considered.  A possible location (shown here) is outside of the upper right engine mount, between it and the airbox.  The downside is that the top of the fuel line tubing will be even further above the level of fuel in the tank.  This will create a greater vacuum on the fuel and increase the chance of fuel vaporization at the most inconvenient times.  Placing the WIX in the same location as the OEM filter or lower is a tight fit and it may rub on things.  Keep an eye on it to sure the filter is not damaged.

WIX fuel filter location on the Top 80 paramator

This photo shows the WIX used in the FSM.  Note how it is installed by the use of a short piece of 1/4" (6mm) automotive fuel line and a zip tie.  This type of mounting parts on a paramotor is secure and reduces the effects of vibration.

Wix filter 33001

The Oregon 07-124 is a commercial grade chainsaw filter rated at 10 microns.  However, the filter has burrs (ridges) on the nipples from the injection moulding process.  As we have discovered, it is a source of small leaks, even when clamps are used.  The leaks are not a hazard but pilots will notice the buildup of oil at the filter nipples over time.  To fix this, remove the burrs with a razorblade.  Why does Oregon not clean them up?  It would involve another manufacturing step and increase the filter's cost.  Otherwise, these are excellent filters and superior to OEM's from Italy because they have no metal parts which can corrode and rust.  The Oregon's low fuel flow rate is fine for OEM paramotors but not for those that have the FSM installed.

Oregon 07-124 inline fuel filter

Depending on use, fuel filters should be replaced yearly at a minimum.  As the filter is used, its flow rate declines from the presence of invisible particles.  This is not much of an issue in automobiles but it is for paramotors because they sometimes run at full throttle for a relatively long time.  Fuel starvation is a huge problem for paramotors and must be avoided to prevent overheating and the destruction of these air cooled engines.

Note: ethanol fuel blends and fuel that is contaminated by water hasten the deterioration of paper fuel filters.

Installation instructions

Here are some tips on fuel line installation, the various types, and where to purchase.

  1. If existing tubing is hard to remove, heat the tubing with a hot air gun or a hair dryer until it is almost too hot to touch.  The tubing will easily slip right off of the fittings without damaging anything.  If the heated air blown on the skin is painful, decrease the heat setting.  It is possible to melt the fuel line so be careful.
  2. When connecting the new filter, heating the fuel line as above is helpful.  Also, putting a small amount of 100% silicone grease on the fittings and inside the tubing eases the installation.  It will also help keep it from splitting, a common problem with anything less than premium fuel line.
  3. Be certain to run fresh fuel through the system when replacing a fuel filter.  Here's how:  Before connecting the fuel line from the filter to the carburetor, prime the system as if for starting.  Put the end of the open fuel line in a container of some sort, and force fuel through the new filter into the container.  At least a 1/2 cup (125 ml) should be enough.  The fuel in the container can be put back in the fuel tank, if desired.  If the step is not done, the inlet valve filter screen in the carburetor may become clogged.
  4. Complete the connection of the fuel line to the carburetor.