by Had Robinson
updated August 5, 2019
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 modification
The pickup tube filter might have been useful (before the environmentalists forced us to use gasoline with ethanol) because Miniplane claims that the pickup tube filter is able to separate any water in the tank (!) from the gasoline. If there is water in your fuel tank these days, you will have all sorts of other problems than can damage your engine.
In other words, you can get rid of the filtering part of the pickup tube filter to eliminate any chance that it is restricting fuel flow in the system. However, you do need something to weigh down the end of the pickup tube. Here is what to do if you have the Miniplane pickup tube filter (or similar):
- Remove the filter from the tank.
- Use an ordinary 75 watt or greater electric soldering gun to burn a 4mm (1/4") hole exactly 12mm (1/2") down from the flat rim at the top of the filter. Drilling a hole, on the other hand, will introduce debris into the filter whereas melting a hole will not. The filter contains a steel ball and you want to have the hole above it when the filter is upright.
- Use a tooth pick or a steel pick and air to remove any debris inside the filter, if present.
- Reinstall the filter in the tank.
Inlet fuel filter
The inlet fuel filter screen in the carburetor 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.
Filter quality & capacity
The original OEM filter is available from Miniplane-USA. If you fly in very cold weather, you should consider using a filter with a much larger filtering area than the OEM size such as the WIX #33001 which rarely can be found in auto parts stores or can be ordered online.
Do NOT use the inline fuel filters (other than the WIX) sold in auto parts stores. They are of inferior quality and will not block the small particles that clog the inlet filter screen in the carburetor.
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. This will prevent the carburetor from getting clogged. A clogged carburetor will lean out the fuel mixture and can burn up the engine. The lubricating oil also needs to be filtered in order to prevent premature wear of engine parts.
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 pump inlet fuel filter screen from a Walbro carburetor. Instead of using a quality filter, the pilot used a sintered bronze filter which allowed contaminants to reach the carburetor. This engine experienced fuel starvation and overheated. Fortunately, it was not damaged because the lack of fuel was so severe the engine shut down before major damage could occur. Some paramotors do not even come with a fuel filter.
Here is 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 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.
The in-tank filter on the end of the fuel pickup hose should always be checked regularly to be sure it is not clogged. This can be easily done by removing the fuel line to the carburetor and pressurizing the fuel tank using the priming tube with a hefty breath or the primer bulb, if your engine has one. Fuel should come easily out of the fuel line. There should be no restrictions on the fuel movement. This quick test will also assure you that the fuel system from the tank is in good order.
Ethanol fuel blends can form gelatinous globs from contaminants which will quickly clog the fuel pickup tube filter in the tank. Be sure to check it often using the quick fuel system test or the test above.
- To easily remove the existing fuel filter from the tubing, 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.
- When connecting the filter, put a small amount of 100% silicone grease (Vaseline can used but it is not as good) on the outlets 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: Before connecting the the fuel line to the carburetor, prime the system as if for starting. Put the 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 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 enough to leak air.