Fuel filter information
by Had Robinson
updated January 17, 2021
There are (3) fuel filters in most paramotors: a pickup tube filter, an inline filter, and the inlet fuel filter screen inside the carburetor (WG-8 and WB-37). All three filters are replaceable. However, the inline filter is, by far, the most important of the three. If it is a cheap one, the inlet filter screen in the carburetor will clog quickly and you may burn up your engine.
Pickup tube filter (clunk)
A clunk with a filter is only necessary if there is no inline fuel filter on the engine. Unfortunately, this filter will start to choke the flow of fuel in cold weather (50F/10C or below). The best thing to do is to replace it with a non-filtering clunk which is available on eBay. The clunk must fit 3/16" (5mm) fuel line. Note: Non-Miniplane engines may have a different fuel line size.
Alternatively, pilots can modify the clunk if it is the OEM clunk from Miniplane. Remove the filter from the tank. Use an ordinary 75 watt or greater electric soldering gun to
burn two 4mm (1/4") holes 12mm (1/2") down from the flat rim at the top of the filter and opposite sides. Drilling a hole will introduce *debris* into the fuel system whereas melting a hole will
not. The filter contains a steel ball and you want to have the hole remain open and above it when the filter is upright.
The ID of the barbed fitting hole on the filter is narrow enough to cause fuel flow restriction under some conditions (cold weather operations). Use a 1/8" (3mm) drill bit to widen the hole. Remember that ease of fuel flow is a critical component of engine operation and preventing overheating/stall/fade especially in very cold weather.
To determine if this filter is in order is easy: Disconnect the fuel line at the inlet side of the inline fuel filter (there is one there, right?) It helps to 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 and not dribble. If there are any doubts about the condition of the clunk, replace or modify it, per the above.
Inlet filter screen
The inlet fuel filter screen in all diaphragm carburetors can only be cleaned/replaced if the carburetor is disassembled. It must always be checked if there are any fuel delivery problems in the engine i.e. fuel starvation. NEVER OPERATE AN ENGINE WITHOUT THIS SCREEN.
Inline filter quality & capacity
(Go to installation steps directly and skip discussion below)
Do NOT buy inline fuel filters sold in auto parts stores other than a WIX. They are of *inferior* quality and will not block the small particles that clog the inlet filter screen in the carburetor.
The Italian OEM inline filter used in most paramotors is available from Miniplane-USA. 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. If flying in cold weather, the OEM 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. A clogged carburetor will lean out the fuel mixture and can burn up the engine.
The engine that had this carburetor had trouble with leaning out, stall 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.
A CLOGGED FUEL SYSTEM IN ANY HIGH PERFORMANCE ENGINE IS A POTENTIAL DISASTER. FUEL STARVATION IS THE #1 CAUSE OF ENGINE DAMAGE IN PARAMOTORS.
Aviation engine forums (ROTAX) note that paper filters are the best because they have the finest filtration media – around 10 microns vs. 40+ micros for sintered bronze or the pickup tube filters sitting at the bottom of fuel tanks. The non-paper filters do not trap the very small particles which will clog the pump inlet fuel filter screen and other tiny openings 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 5mm) due to a poor quality or non-existent inline fuel filter. (A new one is to the right.) Sintered bronze filters will allow contaminants to reach the carburetor. This engine experienced fuel starvation and overheated. *Some paramotors do not even come with a fuel filter!*
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.
Below is a photo of a sintered bronze filter. The dirt-bike guys derisively refer to these as "lawnmower filters" because of their inability to filter well. 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.
Fuel pump diaphragm showing signs of water contamination in the fuel, probably from using ethanol blends.
Below is a photo of the WIX #33001 inline fuel filter (L) and a similar filter (R) sold by the auto parts stores. The WIX can be used in all paramotors, if you can find it. 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 very large filtering area. Note that the size of the WIX filtering media is probably 20X that of the other. The filter media of the WIX is a nominal (and adequate) 12 microns. Who knows what it is of the filter on the right?
Most inline filters are rated at 40 or more microns which is not enough to keep the pump inlet fuel filter screen from clogging up. The absence of barbs on the hose connections guarantees that the filter on the right will leak or come loose at some point – which could be hazardous. Why put a cheap fuel filter on a $2,000 engine?
Depending on use, fuel filters should be replaced yearly. Ethanol fuel blends and fuel that is contaminated by water hasten the deterioration of paper fuel filters.
Ethanol fuel blends can form gelatinous blobs from contaminants which can quickly clog the fuel pickup tube filter in the tank. Be sure to check it often using the quick fuel system test.
- If existing tubing is hard to remove, heat the tubing
in the air of the fitting 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.
- When connecting the new filter, put a small amount of 100% silicone grease (Vaseline can used but it is not as good) on the fittings and inside the tubing. This will greatly ease the installation of the tubing. It will also
help keep it from splitting. Silicone grease is always the best for fuel fittings and tubing because it is 100% inert and is not soluble in any liquid. It also will not oxidize over
time. The downside of it is that it is expensive.
- Be certain to run fresh fuel through the system when replacing a fuel filter. Here's how: *Before* connecting the fuel line 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 carburetor will likely quickly clog up from tiny debris in the fuel. It takes just the tiniest amount of dust, grit, sand, dirt to clog the carburetor inlet valve screen.
- Complete the connection of the fuel line to the carburetor.
If the WIX filter is used, its large size has to be considered. The best location is just to the outside of the upper engine mount, between it and air box. It fits in here nicely. The downside is that the fuel line tubing will have to be longer. If the WIX is placed in the same location as the OEM filter, it may be tight and will rub on things. Keep an eye on it to sure the filter is not damaged.