Cold weather operations
by Had Robinson
Cold weather operations require changes to the fuel-oil used and modifications to the carburetor in order to ensure that the correct fuel-air ratio is maintained and that enough fuel is
delivered to the engine.
Before modifying anything in the fuel air delivery systems or checking the many different items below, perform a thorough fuel system test
on the engine. You will be wasting your time if you do not do this test.
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 or tube more slowly as the temperature
decreases and more rapidly as the temperature increases. We are not so worried about restrictions in air flow through the engine. Fuel delivery is, however, becomes a severe problem as
the temperature drops. This is mostly due to the inherent problems with the fuel pump design in the WG-8 and WB-37 carburetors. (We are currently working on an auxiliary pump that will
be effective, small, and trouble-free.) Cold air has a higher density and, accordingly, requires more fuel to burn to prevent the mixture from being too lean and overheating the engine.
In all 2 stroke engines, the unburned fuel helps to cool things and is necessary to prevent damage.
For hot operating conditions, the problems are different. The most common of which is "fizzing" of the fuel which prevents it from moving through the fuel system, especially the fuel pump
of a paramotor. This presents a lean condition but it is usually so drastic that overheating of the engine is not an issue. Instead, the engine just quits at WOT.
Here are things to check when operating when it is cold (<15C or 60F). The colder it is, the more pronounced the problems are and the more likely a lean condition will damage the engine.
- Fuel pump (vacuum side of the pump) – 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 if
filled to the top.
- Diaphragms – Stiff/old/perforated fuel pump diaphragms cannot pump the correct amount of fuel. 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.
- 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 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. 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 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.
It is true that this filter can 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 all kinds of other problems. You must test all gasoline that contains ethanol 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 it for the presence of water
- Metering lever height – if it is excessive it will cause a lean fuel/air condition when at or approaching 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, step #19. For some reason,
Walbro does not include this spring in their rebuild kits. It can be ordered from Miniplane-USA.
- Main jet – 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. 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-8. Contact us for more information and to order.
- Ignition system – Maximum engine output in marginable conditions is greatly 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 greatly 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.
In sum, everything changes as air density increases due to cold conditions. Note that the optimal propeller pitch in warm ambient conditions (22C or 72F) is different than at freezing
temperatures (less pitch is required).
As pilots of simple aircraft like a powered paraglider, we cannot expect the same performance in all ambient conditions as a $100,000 USD general aviation aircraft. It is the price we pay
for greater simplicity, attractive cost, light weight, and less maintenance. Think of it as tent camping versus a travel trailer.