Cold weather operations for paramotors
by Had Robinson
updated February 15, 2020
Cold weather operations (<15C/60F) require a perfectly operating fuel system and ignition (including possible modifications to the carburetor). The correct fuel-air ratio must be maintained and enough fuel must be delivered to the engine. The correct type of inline fuel filter should also be used and the pickup tube filter in the tank (if there is one) may need to be modified.
Pilots operating in very cold conditions (<0C/32F) can add a carburetor pre-heat system. This greatly helps fuel vaporization as well as lowering the density of the air and viscosity of the fuel entering the engine.
Before modifying anything in the fuel/air delivery systems or checking the items below, perform a thorough fuel system test on the engine. It is a waste of time if this test not done.
Cold weather increases the density of a gas and the viscosity of a fluid. This means that air, gasoline, and oil move through the same sized pipe/tube, filter, or screen more slowly as the temperature decreases. Thankfully, restrictions in air flow through the engine is not a problem. However, adequate fuel and oil delivery is a serious issue as the temperature drops. This is mostly due to the inherent problems with the fuel pump design in the WG and WB series carburetors and the presence of a very fine inlet filter screen in these carburetors. Cold air has a higher density and, accordingly, requires more fuel to burn to prevent the mixture from being too lean.
Here are things to check when operating when it is cold (<15C/60F). The colder it is, the more pronounced the problems are and the more likely a lean condition will be experienced. It is good news that a lean condition is less likely to cause overheating of the engine.
- Fuel pump (vacuum side of the pump diaphragm, not the fuel side) – Be certain that the all tubing and passageways to this side of the pump are clear. You may even have to ream the ports
in the engine and/or increase the diameter of all tubing and fittings that go to the fuel pump. With the Top 80 it is common to have restrictions in these parts or, rarely, discover that
gaskets have been installed incorrectly. The pump may not work but there is enough vacuum in the metering lever chamber to suck fuel through the system, especially if the tank is
filled to the top.
- Diaphragms – Stiff/old/perforated fuel pump diaphragms cannot pump well. If in doubt, rebuild the carburetor. Old diaphragms not only do not pump well but the valves
that are a part of them may also leak.
- Inlet filter screen (inside the carburetor) – If it is clogged, the fuel will not flow well. You will have to take the pump cover off to check it. While
you are at it, rebuild the carburetor if in any doubt as to the last time this was done. The screen is very fine and significantly restricts the movement of the oil contained in the
gasoline. Using a premium 10 micron inline fuel filter (OEM or better) would require the inlet filter screen. I will be testing an engine without the screen and post what I find
- Inline fuel filter – If your filter is not of premium quality and in perfect condition, fuel flow will be restricted.
The larger the filter, the easier the fuel-oil mix will move through it. The WIX #33001 premium inline fuel filter has the greatest filter area of any filter that will easily fit on a paramotor.
- pickup tube filter in the fuel tank (a particular problem when ethanol fuels are used). If you have a top quality inline
fuel filter you do not need this filter. However, you do need something to weigh down the end of the pickup tube a.k.a. a "clunk". Clunks without a filter can be purchased from
any karting supply firm. If you have the Miniplane pickup tube filter (or similar), it can be modified to allow a greater movement of fuel. 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 remain open and above it when the filter is upright.
It is true that this filter can help separate water from the fuel mix. However, if you have free water in your fuel tank and you are using gasoline with ethanol, the fuel is 100% saturated
with water and you will have other problems. All gasoline that contains ethanol should be test to be sure that the ethanol is not saturated with water. Our
fuel-oil specifications page has instructions for those who only have ethanol/gasoline mixes available and need to test for the presence of water.
- Metering lever value – if it is excessive it will cause a lean fuel/air condition at/near wide open throttle. If operating in cold weather is
common, set the ML to the minimum value given in the WG-8 specifications or the WB-37 specifications.
- Brittle (stiff) metering lever spring – It will take additional force to open and, as a result, lean out the fuel/air mixture. If in doubt, replace it. You should
measure the pop-off and rest pressure of the ML valve. For how to do this see the carburetor tune-up page. For some reason,
Walbro does not include this spring in their rebuild kits. It can be ordered from Miniplane-USA.
- main jet – (WG-8 only) Very cold conditions (near or below 0C/32F) changes the fuel viscosity (and the suppleness of the pump and metering lever diaphragms) dramatically, leaning everything
out. A larger main jet will be required. Increase the size of the jet by "2" (.02mm) from the stock value one step at a time, as needed. Some shops increase the jet to 160,
for example, which only masks defects in other parts of the system, if that. This is not the correct way to adjust for cold operating temperatures and, as things warm up, will cause poor full
throttle operation. We can supply any sized main jet for the WG series carburetors. Contact us for more information and to order.
All jets are custom made to very precise tolerances.
- Ignition system – Maximum engine output in marginable conditions is affected if the ignition system is not
in perfect working order.
- Type of 2 stroke oil used – 100% synthetic oil retains its viscosity better through temperature changes than other types of oil.
- Gasoline – AVGAS or ethanol-free premium unleaded gasoline should be used. Ethanol decreases the viscosity of the fuel but increases the amount of fuel needed to prevent the engine from running in a lean condition. If you use gasoline with ethanol, it must be tested for the presence of water. See our fuel-oil specifications page on how to do this.
- Pump diaphragm type – Bill Stoll did some tests at and below freezing temperatures and discovered that the Teflon fuel pump diaphragm (the whitish-clear one) solved the fuel starvation issues he and others were experiencing. Apparently, this type of diaphragm pumps the fuel more efficiently in cold ambient temperatures. Some of the karting blogs also note this. In any case, the carburetor must always be carefully adjusted and rebuilt if there are any questions.
- Water in fuel – It will give a high pop-off pressures and, if it is below freezing, may cause the engine to stop running. Remove the fuel pump diaphragm cover and diaphragm on the carburetor and you will immediately know if there is water in your fuel system.
Note that the optimal propeller pitch in warm ambient conditions (22C or 72F) is different than at freezing temperatures (less pitch is required).
I am continually testing paramotors in all sorts of conditions and will post new information on these pages.
- We cannot expect the same performance in all ambient conditions as a $100,000 general aviation aircraft. It is the price we pay for simplicity, attractive cost, light weight, and far less maintenance. Think of it as tent camping versus a travel trailer.