Fuel and Oil Specifications for two stroke paramotors
by Had Robinson.
Updated July 29, 2020 -- please see footnote 1 at the bottom of this page concerning Miniplane and its position on AVGAS.
This is the right page if you want facts and references rather than rumors and unsubstantiated assertions from blogs and misinformation published by some paramotor marketing departments. This UK web site has a good (but dated) discussion on all aspects of petroleum.
Table of Contents
- Premium ethanol-free automobile gasoline - Aviation Gasoline - Automobile gasoline with ethanol
- Poor quality fuel
- Vapor lock (winter vs. summer gasoline formulas)
- Fresh Fuel ONLY
- Sticking piston rings
- Increasing the oil mix ratio
- Approved brands of full synthetic oils
- Fuel oil ratio
- Mix the oil just before use
Fueling equipment You will need the right equipment to mix, store, and transfer your fuel. Please see the equipment page for information on what you need. Removing fuel from a paramotor fuel tank is easy if you have a good siphon. Metal containers are the best for storing gasoline because, unlike plastic, UV from sunlight cannot penetrate metal. UV and oxygen cause gasoline to deteriorate quickly. If you do use plastic jugs, especially clear ones, store them in a cool, dark place and never in direct sunlight. It is wise to put labels on fuel containers in order to prevent disasters!
A. Quick reference
Use full synthetic oils. In the U.S. the best oil to use and widely available is:
AMSOIL Dominator Synthetic 2-Stroke Racing Oil
Top 80 engine – 2% (50:1) full synthetic oil with premium ethanol-free MOGAS or AVGAS1
Polini Thor engines – 2% (50:1) full synthetic oil with premium ethanol-free MOGAS or AVGAS
Minari engines – 2.5% (40:1) full synthetic oil with premium ethanol-free MOGAS or AVGAS
Fresh Breeze Simonini 200cc – 2% (50:1) full synthetic oil with AVGAS (Fresh Breeze specifies that only AVGAS be used in their engines)
Simonini 200cc – The factory version of this engine has special fuel and oil requirements because of different tuning than the Fresh Breeze version.
Vittorazi Moster 185 – 2.5% (40:1) full synthetic oil with premium ethanol-free MOGAS or AVGAS
Honda has this quick reference on how to avoid fuel problems. They know a thing or two about small engines.
Chevron published this technical review on motor fuel which is helpful for those wishing to improve their knowledge of fuels rather than rely on hearsay and rumor of which there is way too much in the paramotoring community.
EXXON has these words on the lack of quality control in MOGAS (from their page on AVGAS):
Note that properties critical to aviation use (for example, vapor pressure and cleanliness) are not controlled to the same degree in automotive motor gasoline manufacture and handling. ExxonMobil aviation does not support or approve the use of Automotive Gasoline as aircraft fuel.
Best gasoline – Exxon Mobil has this primer on gasoline storage. Cheap or old gasoline can ruin engines, especially when run at or near sea level where engines run hotter. Make certain that the gasoline is fresh (less than 30 days old). Exxon Mobil, for example, notes that gasoline can be stored for up to (6) months but it is not easy to meet the storage requirements. Fuel preservative helps but it is still better to have everything fresh. Our workbench is full of burned out engines, many of which were ruined because of cheap/old fuel and water contamination. Water-saturated ethanol fuels will cause erratic engine performance because of different stoichiometric requirements. Paramotors are closest to chainsaws in design and carburetion. Modern automobile engines are fuel injected and the air, fuel, and exhaust streams are closely monitored so changes in the fuel type and/or the presence of water does not cause the problems which occur with carbureted engines i.e. paramotors.
- Unleaded premium gasoline ethanol-free
- Aviation gasoline (AVGAS) with the anti-fouling TCP or Decalin RunUp fuel additive. Decalin is substantially less expensive but is not FAA approved for use in certified aircraft engines. It must be used (according to the FAA) in experimental aircraft only (i.e. for ultralights).
- Unleaded premium gasoline with ethanol <10%. If you must use fuel with ethanol, follow the instructions below. Ignore anything published by Miniplane concerning >10% ethanol blends (here are the reasons why if you live in the U.S. or other areas in the Western Hemisphere).
Premixed racing fuel – Pilots who cannot find ethanol-free gasoline or AVGAS can purchase ready-to-use, pre-mixed fuel made by VP Racing Fuels. These fuels are 94 octane and use full synthetic oil. They have 50:1 and 40:1 mixes available. Most major distributors, including Wal-Mart and Home Depot, have these premixed fuels but they are not cheap. VP Racing fuels contain stabilizers so that the containers can be opened, exposed to the air yet, according to the manufacturer, the fuel will remain fresh for a year. Accordingly, additives are used to prevent the deterioration of the oil that has been mixed with the gasoline.
Types of gasoline
1. Premium ethanol-free automobile gasoline
It is the best because it burns with the least amount of deposits but only if it is fresh (<30 days old) or properly preserved and stored. It does not add lead to the atmosphere. Briefly, all automotive gasoline (MOGAS) will start to decay when it comes in contact with oxygen. Stored properly, MOGAS can last about (6) months. How old is the fuel at the pump? High volume stations will have fresher fuel. But, you will never really know. (Please see "Fresh fuel only" below for more information on this.) Modern automobiles will run on just about any concoction of unleaded gasoline and other chemicals, even fuel that is months old. Of course, the presence of ethanol makes things worse.
Unfortunately, ethanol-free fuel is getting harder to find.
ROTAX notes these important points about why pilots should use only premium fuel from a major (and busy) supplier and why a high octane value is needed:
- First, when fuel is premixed with 2-stroke oil, the octane rating is reduced by about 2 points. An 87 octane fuel would therefore become 85 octane.
- Second, fuel [except AVGAS] evaporates and loses its octane rating when it lays in your ultralight’s fuel tank or in a plastic jug. A “premium”, 91 octane fuel will see its octane rating reduced to unusable levels after as little as three weeks. Fuel with a lower octane rating would obviously have an even shorter usable life.
2. Aviation Gasoline
AVGAS may be used in paramotor engines and is preferred over fuels containing ethanol. All engine manufacturers (except Miniplane as of 2018 – see the 1st footnote below) note that AVGAS does not harm their engines, contrary to various rumors that continue to circulate on the blogs. In fact, some manufacturers including Fresh Breeze and Nirvana specify that ONLY aviation gasoline should be used in their engines or that it is the preferred fuel. Nirvana Systems of Czechoslovakia has this in one their engine manuals:
Solo and NIRVANA recommend using 100% synthetic Castrol TTS in correct oil/petrol mixture, which prevents combustion chamber and exhaust carbonization. Do not store mixed petrol longer than a few days, max 2 – 3 weeks. You can mix Castrol TTS with leaded or unleaded petrol. However leaded petrol is preferred. Always use quality petrol.
AVGAS is the best fuel if your ambient air temperature is often over 38C/100F, as it is where we fly. However, AVGAS is not easily available to paramotor pilots in Europe so they often must use the ethanol blends – and suffer the problems of ethanol. Europeans are often forced to purchase gasoline substitutes which are expensive but overcome the problems of ethanol in fuel.
- Has the highest octane of all gasoline. The higher the octane, the less damaging pre-ignition there will be in high compression ratio engines, like paramotors. Ordinary automobile fuel oxidizes quickly if not stored or preserved properly and, as a consequence, its octane value drops in just weeks.
- Contains the octane enhancing compound tetraethyl lead. When this compound is burned, it forms lead monoxide deposits which coat various parts of the combustion chamber, especially the spark plug. According to the multi-cylinder aviation engine manufacturer ROTAX, lead monoxide deposits can damage the crankshaft bearings due to the deposits flaking off from the combustion chamber and then being sucked back into the crankcase in their multi-piston two stroke engines. This is something that not occur in single cylinder paramotor engines. The use of the TCP or Decalin RunUp anti-fouling additive scavenges the tetraethyl lead in the fuel and coverts it to lead phosphate, a harmless substance that will not affect bearings or other parts of the engine.
Some claim that ring sticking is more common from the additional lead deposits in AVGAS than from the carbon deposits created by the use of any fuel. There is no proof of this. Ring sticking is caused by cheap oil or engine overheating, regardless of whether the fuel is leaded or unleaded. Suffice it to say, overheating is the most common problem in engines which are not properly maintained. Red Line, a manufacturer of 2 stroke synthetic oil lubricants, notes what happens in this excerpt from one of their technical documents: "The time indicated [in the graph] is the time required for the lubricant to decompose to a sticky mass capable of sticking a two-cycle piston ring."
- Is the most stable and pure of all gasoline. It can be stored for long periods (years).
- Will not be contaminated with water because of the strict standards for storage at airports. Whenever there is flooding from storms, there may be problems with water in the fuels sold at some service stations. Modern motor vehicles can handle a lot of water contamination but not paramotors. AVGAS, on the other hand, is carefully stored at airports so that water contamination is eliminated.
- Has a lower vapor pressure than ordinary gasoline and is much more resistant to vapor lock, a serious conditions that results in fuel starvation and engine overheating.
- Will not harm fuel system parts (despite the unsubstantiated claims by Miniplane) as does gasoline containing ethanol.
- Cost is about double that of premium unleaded, unfortunately.
- Mixes well with all types of (2) stroke oil.
- Causes increased fouling of spark plugs, earlier clogging of the decompression port (if there is one), and more deposits on top of the piston. The cylinder head, being much cooler, will have hardly any deposits. Lead fouling is greatly decreased by the use of aviation fuel additives. If AVGAS is used, even with the additives, the spark plug should be changed every (15-20) hours. Routine maintenance schedules must be done at shorter intervals than if unleaded fuel is used.
- Adds lead salts to the atmosphere. However, I hope it is obvious that paramotors using AVGAS add insignificant amounts of lead to the environment compared to the 1/2 million general aviation aircraft worldwide. The only reason that lead is allowed in aviation motor fuel worldwide is that general aviation aircraft still do not add significant amounts of lead to the atmosphere.
Other AVGAS problems? There are some competent mechanics who flat-out state that AVGAS will ruin paramotor engines. Yours truly wishes he could know the circumstances under which they believe engines were harmed (other than the need for increased maintenance). We have used AVGAS for over many years in our group here in the southwest U.S. because we have no access to ethanol-free gasoline (due to serious air pollution from Mexico). If paramotor manufacturers Fresh Breeze and Nirvana prefer that AVGAS be used in their 2 stroke paramotors, what can the issue be?
Wiki has a long article on aviation gasoline for those who desire to increase their knowledge of engine fuels. The dirt-bike guys have commented on some of the questionable assertions made about AVGAS that circulate on blogs. I have repeatedly requested more information, photographs, and/or the affected parts that have been supposedly ruined by AVGAS, but have yet to receive anything. I will even pay shipping. I could be missing something, so please help out here if you have some special situation I am unaware of. If AVGAS is the best for four stroke aviation engines, what could it do to two stroke engines and why? (Note: we already know that it should not be used in multi-cylinder two stroke engines because of the possibility of lead salt deposits getting in the main bearings.) General Aviation News has this page on MOGAS myths.
Where to purchase AVGAS? Almost all airports sell AVGAS. Please review this article on airport access for ultralights on how to purchase AVGAS.
Fuel treatment for AVGAS is important. If treated with the additive TCP or Decalin RunUp, the problem of lead oxide fouling is greatly decreased. A quart of TCP (cost <$40 US) will treat 320 gallons. A quart of Decalin (not "certified" for aviation use) costs $35 and will treat up to 640 gallons, depending on the engine. For TCP, this comes out to about $ 0.13/gal and $0.05 for Decalin) and is well worth it considering that less engine maintenance will be needed. For info on how TCP works, read this pamphlet from Alcor. The active chemical in these additives is known as a lead scavenger. During combustion, the tetraethyl lead is converted into lead phosphate instead of into lead monoxide. Lead phosphate is a non-conductive compound that does not foul spark plugs. Rather than build up on the piston and spark plug, lead phosphate is carried harmlessly out in the exhaust stream.
Below is a Thor 130 engine with approximately 130 hours of run time. AVGAS was used exclusively in this engine without any anti-fouling additives. The cylinder head is nearly 100% free of any lead deposits. Lead deposits do not tend to form on the cooler internal parts of the combustion chamber, like the cylinder head here. The red arrows point to small flakes of lead monoxide that were probably lodged on the edges of the spark plug and broke off when the plug was removed. They are soft and harmless in single cylinder 2 stroke engines. Note that the spark plug is covered with lead monoxide deposits (light yellow in color). Large 4 stroke aircraft engines have expensive spark plugs that get just as fouled as the plug in this photo. On the other hand, our plugs cost a few dollars rather than 10X that and are easy to replace.
Here is the same engine with a view of the top of the piston. It runs much hotter than the cylinder head and has more lead deposits. 20 minutes with a razor blade took care of the deposits in this engine.
This is another photo of the same engine as above. Typical in paramotor engines run with untreated AVGAS, the spark plug has the most lead fouling. This plug has about 50% more life left but should be replaced because it is badly fouled. As in general aviation engines, lead fouling can short out a spark plug. It is a rare event, thankfully.
Below is a spark plug from an engine that used AVGAS treated with the TCP additive. The plug has about 25 hours on it and is at the end of its life. It has a trace of lead monoxide deposits which is normal.
This spark plug is from an AVGAS burning engine that did not have an anti-lead fouling additive mixed in. Note that lead monoxide is present close to the grounded electrode and absent from the cooler parts of the plug. The cylinder head had no lead deposits and the piston had minimal deposits. The only symptom the engine had was hard-starting because the plug is worn out. The decompression port was also clogged. Engines that use ethanol blends will not last as long and why any other kind of fuel is preferable. Regardless of what kind of fuel is used, the spark plug will wear out or foul. It is so inexpensive to change out the spark plug and improve starting and high end performance and is why it should always be done.
Here is a another spark plug from a Polini Thor used for tandem flying (it ran at full throttle most of the time). AVGAS treated with the TCP additive was used in this engine. From tests, engines run under heavy load will have less lead monoxide fouling, like this one. It has about 30 hours on it and is at the end of its life. Even so, the plug has a light fouling of (white) lead phosphate, the byproduct of the chemical action of the TCP additive on tetraethyl lead. Compare the plug below with the plug above. Both engines had about the same number of hours on their respective plugs. It shows what a difference the TCP fuel additive makes.
3. Automobile gasoline with ethanol
All (2) stroke engine manufacturers continue to issue warnings about the problems with fuels containing ethanol. Ethanol, in a word, rots fuel system parts from the inside out, especially metal and most plastics. It also attracts moisture and leans out the air/fuel mixture. As the marine and kart racing guys note, ethanol-free gasoline should be made available for all off-road users. Ethanol also lowers the vapor pressure of gasoline and increases the problem of vapor lock in hot weather. (I have personally experienced engine failure during launch from vapor lock.)
Almost all paramotors use various models of the Walbro diaphragm carburetor. Walbro continues to warn about the potential damage from ethanol: THE ENGINE KILLER. On another page on their site they have this warning.
The simple fact is, ethanol in gasoline can damage Your fuel system. Gasoline which is organic will begin to decay due to high amounts of oxygen in fuel containing ethanol. Ethanol containing fuel will also absorb moisture from the air because ethanol is hygroscopic and will attract moisture. This added moisture can lead to phase separation. With phase separation comes corrosion to critical engine and fuel system components. The fuel system can also be impacted by contaminants carried in the gasoline itself, as well as gasoline containing ethanol. When used in mowers and other power equipment, ethanol can lead to higher operating temperatures, erratic running, and engine-part failure.
Mercury Marine has this warning about the use of ethanol fuels in their 2 stroke outboard engines. If you must use ethanol fuels, be sure to add the correct additives to help lessen that damage (see below).
Water in the fuel system clogged the carburetor fuel inlet screen and made it impossible to start or run the engine. After the saturation point with water is reached, additional water remains separated from the fuel. When it reaches the inlet filter screen in the carburetor, it takes 30-60kPa (5-10 PSI) to move it through the screen. That is, the water acts as a dam to fuel in the system.
Ethanol fuels should be avoided, if at all possible.
Automotive gasoline that contains ethanol should only be purchased from a major company, like EXXON, ESSO, BP, Chevron or Shell, because their fuels contain important additives, especially detergents, that lessen the buildup of carbon inside the cylinder and gunk in the fuel system caused by the presence of ethanol. Lesser grades of fuel than premium can be used but Miniplane notes that you may have higher running temperatures i.e. you may overheat your engine at higher loads and experience damage.
WARNING: DO NOT USE GASOLINE WITH MORE THAN 10% ETHANOL.
Miniplane claims that higher concentrations will not damage their engines but what about the Walbro carburetors used on all of their motors? If E85 works, then they should include an E85 fuel kit with their engines. Walbro notes on their webpage that gasoline containing more than 10% ethanol should not be used in their carburetors. Various U.S. oil makers (e.g. Bel-Ray) warn users not to mix their oils with ethanol blends greater than 10%. As every RC, go-kart, and dirt-bike group including paramotor manufacturers note, the Walbro carburetors that most engines use are NOT designed for fuels other than pure gasoline. Ethanol, for example, must have a lower fuel/air ratio because per volume it contains less energy than gasoline so ethanol configured carburetors must have larger main jetting so as not to run lean.
Here is a fuel filter that started to rust because of water in the fuel.
Anyway, who wants to find out what happens when a 15% ethanol blend is used in a paramotor? Furthermore, the 2 stroke oils available in the U.S. and non-EU countries are different than those in the EU. Has Miniplane tested the U.S. oils? Do the engineers there understand English well enough to heed the warnings from Walbro? Do they even look at Walbro's site? Do they know any organic chemistry? It is a great puzzle....
Here is a water (and debris) clogged inlet filter screen of a carburetor and fuel system that had become saturated with water. Fuel was unable to pass through the filter preventing the engine from running.
Observe these precautions when using ethanol fuels:
- Fuel stabilizer Add Sta-Bil, SeaFoam, or an equivalent
fuel treatment to keep the fuel fresher. Unless Chevron premium gasoline is used, it is helpful to add their Techron additive (or an equivalent) periodically. It does an amazing job
cleaning the fuel system of gunk and other residue.
- Fresh fuel If you are using premium (as you should), did you pump a gallon into your vehicle first? If you do not do this, the fuel in your portable tank
could be 50%
regular grade fuel. Do NOT allow gasoline with ethanol to sit for more than a month in
the fuel system as it can damage the metal and non-metal parts of the engine. If fuel stabilizer is
used, blends can be stored in ordinary gasoline containers for many months IF NOT
MIXED WITH OIL. If pilots store
motors for more than a few
weeks, they should purge the fuel lines and carburetor. Ethanol is a powerful solvent that causes non-metal fuel system parts (tubing, gaskets,
diaphragms, filter material) to deteriorate. It can cause some makes of fuel line to swell enough to allow the pickup tube filter in the fuel tank to fall off. Ethanol is hygroscopic and
also conducts electricity. This causes electrolysis of the metal parts in the engine and is why those using two stroke engines in a marine environment have endless problems. The
metering lever spring inside the carburetor can be affected from corrosion in the presence of ethanol blended fuels (which is why it should be replaced every time the carburetor is rebuilt).
Remember that fresh fuel (< 30 days old) is always better than old fuel, even when stabilizer is used.
- Carburetor maintenance Regularly test the
pressure of your carburetor because of corrosion and weakening of the
pop-off spring caused by electrolysis and/or the presence of water that is mixed with the fuel. Testing the pop-off and, more importantly, the rest pressure is the only way to tell if the spring or ML valve seat is bad. Miniplane-USA stocks the
springs. Low pop-off pressure dramatically affects
the high-end output of an engine due to a richening of the fuel mixture. High pop-off pressure is usually caused by water in the fuel. Rebuild the carburetor often.
- Water contamination Check that the gasoline does not
contain either water (mixed in with
the ethanol) or more
than 10% ethanol. Wiki has a good
article on the stability of gasoline, especially ethanol mixes, and what happens as these mixes age.
- Percent ethanol Test the ethanol percentage in the fuel.
For about $10, pilots can buy an
percent tester. If 10% ethanol fuel contains water the fuel
tester will indicate an ethanol content <10%. This is another reason
why pilots who live in humid parts of the world should not use ethanol fuels
in their engines unless they are certain that it is not contaminated with
water. For example, if you test fuel that is supposed to contain
ethanol and the tester shows it to be ethanol free, the fuel has been fully
saturated with water. Such fuel can ruin a 2 stroke engine.
- Air-fuel mixture Be sure that the engine is not overheating or running too rich. The Walbro Service manual notes that ethanol blends will cause carburetors designed for pure gasoline (like the WG8 and WB37) to run leaner and cause the engines to run hotter. This is another reason for pilots to install a cylinder head temperature gauge. Ethanol fuels require less oxygen than the same quantity of pure gasoline. This means that ethanol fuels may require different carburetor jetting i.e. a larger main jet size. This aspect of ethanol fuels is largely ignored.
Miscellaneous notes on ethanol blends from manufacturers:
ROTAX warns that their engines will experience damage with ethanol fuel blends greater than 5%. This warning from ROTAX should alert paramotor pilots of the problems caused by ethanol. All U.S. blends have 10% or more ethanol. Your region may have more or less ethanol. Be certain to find out what your local blend ratio is. (Because ROTAX engines are 2 stroke multi-cylinder they should not be run with AVGAS.)
Recreational Power Engineering, the U.S. distributor of the 2 stroke Hirth engines, has these recommendations on fuel and oil for the Hirth engine. They warn against using ethanol fuels and synthetic oils in humid environments. They approve the use of AVGAS if anti-lead fouling additives are used, like TCP or Decalin RunUp.
Marine engines and small ultralight engines have much in common. MarineMechanic.com has this informative article on coping with the poor quality gasoline we have to use these days. They emphasize the importance of additives and making sure the engine is tuned up correctly and advice on how to reduce carbon buildup inside the engine. For paramotors this often translates into having "stuck" piston rings which quickly ruin the cylinder.
Moeller Marine Products, manufacturers of fuel system components, has this warning,
Ethanol is a common additive to blend the fuels with, and while ethanol may not pose the environmental hazards of other gas additives, the chemical properties of ethanol have the potential to cause severe damage to engine systems.
Walbro, the manufacturer of the carburetors used on Miniplane engines, warns users in this video about using ethanol blended fuels and how to prevent damage to the carburetor and fuel system.
Here are some photos of a carburetor that stopped working due to water and gunk from gasoline with ethanol. In the left photo, water and gunk covers parts of the fuel pump diaphragm. The pump could not do its job. In the bottom photo, the fuel inlet screen is clogged. This carburetor could not deliver fuel. The pilot was lucky that he did not burn up his engine.
I discussed ethanol fuels with Bel-Ray, a manufacturer of 2-stroke motorcycle racing lubricants. They noted that their oils mix correctly with ethanol fuels but warn users that the fuel must not have ethanol >10%. FRAM, a manufacturer of filters, warns users not to use ethanol fuels that have more than 10% ethanol because it will damage their filters. Poulan, a chain saw manufacturer, warns of the serious problems caused by blends with ethanol >10% or which have been contaminated with water. Seastar Solutions, a major manufacturer of marine fuel system parts, warns owners to replace their fuel filters every 50 hours or once a year because of the effects of ethanol in gasoline. They also warn owners that ethanol causes deterioration of fuel system lines, making them hard and brittle from the inside out.
Bottom line: Use gasoline without ethanol if you can.
The piston pictured below seized as a result of a lack of lubrication. The owner of this Top 80 stated that he added oil to the gasoline in his motor. The likely explanation is that the ethanol fuel had been contaminated with water, causing the lubricating oil to separate out. Chris Barker of Royal Purple synthetic oils confirmed in a telephone call to me that water contaminated ethanol fuels will do this.
To find sources of ethanol free fuel, go to this site http://pure-gas.org. If you find ethanol free fuel at a service station, check it before filling up as it may be mislabeled. Unfortunately, ethanol free fuels are becoming more difficult to find.
Use of poor quality fuel
Cheap fuel has all sorts of problems, including widely varying quality, lack of cleanliness, low vapor pressure, and the presence of contaminants, especially water. ONLY USE GASOLINE FROM MAJOR REFINING COMPANIES IN YOUR PARAMOTOR.
If you must use low octane/poor quality fuel, increase the thickness of the washer under the spark plug which lowers the compression ratio of the engine and also lowers the running temperature. Increasing the thickness of the washer can be quickly and easily done by adding a second or third washer from a used spark plug to the existing one. If you get too much spacing, you will not harm anything except lower the engine output. Alternately, the special aluminum spark plug spacer washers (2mm) are available from Miniplane-USA. Poor quality fuels will also vaporize (form bubbles) in the fuel system more readily which will lean out the air/fuel mixture.
Vapor lock or winter vs. summer gasoline formulas
It is important for pilots to note that automotive service station gasoline (MOGAS) is formulated differently for the cold and hot seasons of the year. We have learned – the hard way – that gasoline refined during the winter months has problems with vapor lock when used in hot weather. That is, the fuel can turn to "fizz" (bubbles) inside the fuel system and cause vapor lock. This will starve the engine of fuel and can cause lean-out under high loads which can damage the engine. Therefore, it is a good idea not to use fuel refined in winter for use in the summer. It does not matter whether fuel preservative is used or not.
For those who want more of the science of the problems with vapor lock, Chevron Oil has this helpful article. It is obvious to see why the hot area around the fuel lines and carburetor can cause vapor lock in a paramotor, especially in hot climates.
This video of fuel vapor bubbles forming in gasoline when a vacuum head of about 3' (1m) demonstrates the inherent problem in gasoline subject to negative atmospheric pressure i.e. being sucked rather than pumped out of a fuel tank. (This is why small aircraft have ONLY gravity fed fuel systems.) This can be done first hand by using a jiggle-siphon to move fuel from an elevated tank to another tank 3' below. As the fuel is rapidly moving through the siphon tube, jam the jiggle valve against the bottom of the upper tank and you will see a sudden appearance of bubbles in the siphon tube. This is caused by a negative pressure in the tube which causes the fuel to vaporize and form bubbles. These bubbles in the fuel system will stop the movement of fuel through a carburetor.
Aviation gasoline (AVGAS), on the other hand, has less problems with vapor lock because of its high quality and that its formulation does not change from summer to winter. Note that most paramotors use a vacuum to pump fuel from the tank to the carburetor, unlike other aircraft. This creates a low pressure on the fuel and it is easier for the fuel to reach its vapor pressure – and form bubbles which block the flow of fuel.
High flow rates in the fuel system discourages vapor lock and is why small engines like the Top 80 experience this problem more often than the bigger engines.
Fresh Fuel ONLY
Honda Motor Co. has these words about the importance of fresh fuel,
Fuel deterioration and oxidation can occur in as little as 30 days and may cause damage to the carburetor and/or fuel system. ...Deteriorated gasoline will cause hard starting, and it leaves gum deposits that clog the fuel system. If the gasoline in your engine deteriorates during storage you may need to have the carburetor and other fuel system components serviced and replaced. ...You can extend fuel storage life by adding a gasoline stabilizer that is formulated for that purpose, or you can avoid fuel deterioration problems by draining the fuel tank and carburetor. – from the 2011 Owner's Manual for the GX120, GX160, and GX200 engines
Unless AVGAS is used, be certain that fuel is fresh (less than 2 weeks old is best). Those who live in humid climates must exercise much greater care when using fuels mixed with ethanol because of the hazard of water contamination. Typically, water contamination occurs when humid air enters a fuel storage tank through the main vent. As the barometric pressure lowers (e.g. from a storm) water in the air will condense on the wall of the tank and wind up at the bottom – and eventually into your engine. If fuel stabilizer is used, ordinary gasoline can be stored for a year or so (but I would never store it this long).
Here is what MOGAS looks like after it is stored for a few months in an ordinary gas can. The gas can was less than 1/2 full i.e. there was plenty of air (oxygen) available to oxidize the fuel. Fuel stabilizer was not used. Fresh gasoline is as clear as water.
All service station gasoline (other than AVGAS) deteriorates quickly and loses some of its octane value after just a few weeks because of oxidation that starts immediately after the tank trunk leaves the refinery (not so much) and is delivered to the storage tanks at the gas station. All fuel storage tanks are vented to the atmosphere which is the source of oxygen which begins the oxidation process when it comes in contact with the stored gasoline. If you store gasoline, add a fuel stabilizer like Sta-Bil which can keep it mostly fresh for a year in sealed containers. There is also a product from Sta-Bil that is specifically for fuels used in marine (humid) environments called Marine Formula Sta-Bil. If I lived in a particularly hot and humid climate, I would add this special version of Sta-Bil to my fuel if AVGAS or ethanol-free fuels are not available.
Sticking piston rings
Running any engine too hot (lean conditions) will cause the lubricating oil to burn at the lands, including the sides of the cylinder and piston, due to piston overheating, regardless of what kind of fuel is used. This creates the black gummy deposits which can bind the ring. Red Line (a two stroke oil manufacturer) notes this in one of their technical documents.
This piston came from a fuel starved engine which overheated. Even though unleaded fuel was used, the piston was hot enough to burn the lubricating oil that is on the surface of the piston. The piston ring was stuck in the lands due to the burnt oil. The pilot flew this engine at the beach. The long vertical gouge was probably caused by a sand grain getting sucked into the engine and then caught between the cylinder and the piston. If the pilot had used 100% synthetic oil there would not have been as much damage.
Cheap fuel/oil, old fuel, and/or ethanol fuels contaminated by water will leave damaging deposits that can find their way into the piston ring and cause it to stick. Another problem with the cheap oils is that they tend to rapidly clog the decompression port, if present, on 2 stroke engines. Lead deposits from AVGAS are not black and gummy but light yellow (lead monoxide) or white (lead phosphate), if TCP or Decalin RunUp is used.
C. 2 STROKE OILS
Increasing the oil mix ratio Increasing the amount of oil in the gasoline above the manufacturer's recommended value does not prolong the life of the engine. Increasing the oil mix increases the viscosity of the mix and, as a result, leans out the fuel mixture which can cause overheating. Oil also burns hotter than gasoline. Outboard motor manufacturers (e.g. Evinrude, Johnson) recommend increasing the oil mix in new engines. Why not in paramotors? This is because an outboard is water cooled and is virtually impossible to overheat. This is unlike an air cooled engine which are used in most paramotors.
Type of oil – I no longer recommend using semi-synthetic oils in paramotors. Use a 100% full synthetic oil, such as Amsoil's Dominator, which is more resistant to the effects of heat. Overheated oil cannot lubricate! The piston below (inside view) shows what happens to cheap oil when things get hot. The black gunk will also get in the piston lands and cause the rings to stick – and ruin the cylinder wall.
When the cylinder wall temperature exceeds 180ºC, the lubricating properties of ordinary oil begin to deteriorate. It is one of the most important reasons to use 100% synthetic oils.
100% synthetic 2 stroke oil: Amsoil DOMINATOR 2-Stroke Racing Oil; Bel-ray Si-7 Synthetic 2-Stroke Engine Oil; Fuchs Silkolene Comp 2 Plus Pro full synthetic; Motul 710 2T; Motorex Cross Power 2-T; Red Line 2 Stroke Racing Oil; Shell Advance Ultra 2; Valvoline 2-Stroke Racing Oil. Many of these oils are hard to find in the U.S. because we have virtually no 2 stroke scooters in use compared to most other countries outside the western and northern hemisphere.
Motul oils are available from BikeBandit. Amsoil is sold in most motorcycle shops. You have a $6,000 paramotor and you want to save a few bucks on oil? Does this make sense?
1. Miniplane Thor and Top 80 engines – Full synthetic 2-cycle oil at 2% mix (50:1 ratio). Miniplane has reported that using full synthetic oils at the 2% mix ratio may cause engine overheating under certain conditions i.e. running at full throttle for extended periods. Therefore, it is a good idea to keep an eye on your CHT while running the engine.
Higher mix ratio – Miniplane allows a 1.5% mix (67:1) for the Top 80 and Thor models if full synthetic oil only is used. I do not recommend it.
In any case, the brands of oil Miniplane recommends for a 1.5% mix are not available in the U.S. Why use the lesser oil mix? Engines using the 1.5% mix will run cooler and more efficiently because of the lower viscosity of the fuel mix. In addition, the lower ratio has less negative impact on the octane value of the fuel. That is, the addition of oil to the fuel mix lowers the octane value of the fuel. Lowering the octane of the fuel can cause overheating and pre-ignition. I have observed that the 1.5% mix results in a smoother running engine and a small improvement in power output. The downside is that long term use of the higher mix ratio appears to cause premature engine wear2.
If you do use the 1.5% ratio, do not use gasoline containing ethanol. You must use either AVGAS or premium ethanol-free unleaded.
2. Minari – Full synthetic oil at 2.5% (40:1)
3. Fresh Breeze Simonini Mini 2 Plus – Full synthetic oil at 2% (50:1) AVGAS only. Fresh Breeze recommends that pilots use Castrol TTS oil. However, Castrol TTS oil is not recommended by some engine manufacturers because this 100% synthetic oil does not stay mixed in unleaded gasoline containing ethanol. Our own tests with the oil demonstrate that it mixes well with AVGAS. Our own experience with this oil is that it can be hard to find at times in the U.S., to say nothing of the controversy surrounding it.
4. Simonini Mini 2 Plus – The factory version of this engine has special fuel and oil requirements because of different tuning than the Fresh Breeze version.
5. Vittorazi Moster 185 – 2.5% (40:1) full synthetic oil with premium ethanol-free MOGAS or AVGAS
Approved brands of full synthetic oils
Amsoil Dominator Synthetic 2-Stroke Racing Oil (Racing oils are not to be used on public highways because they don't burn 100% so pollute more but they are the best oils.)
Bel-ray Si-7 Synthetic 2-Stroke Engine Oil
Fuchs Silkolene Comp 2 Plus Pro full synthetic
Motul 710 2T
Motorex Cross Power 2-T
Red Line 2 Stroke Racing Oil
Shell Advance Ultra 2
Fuel oil ratio
A good mixing bottle is from Shoreline Marine and is sold by Sports Academy and others. It is simple, well marked, and can be sealed to keep dirt out.
Some mix their fuel at a lesser ratio than the recommended 50:1 for the Top 80, e.g. 40:1, thinking that it helps lubricate engine parts better or is necessary when breaking in a new Top 80. Both Miniplane and Polini warn against this practice as it can damage an engine due to the increased fuel mixture viscosity which can lean out the fuel/air mixture and overheat the engine. Always be sure to follow the recommendations of the engine manufacturer. Pilots have to resist rumors and hearsay and stick to the engine manuals and information from well-researched sources, like this web site. Note that outboard motors are water cooled and will not overheat!
Here is a discussion of what can happen when lower fuel oil ratios are used in engines where this is not recommended. It will lean out the fuel burn and can foul the combustion chamber, including the piston ring, regardless of the type of gasoline used.
Minari has this note in their user manual, "WARNING: Please note that an excessive amount of oil does not stretch the life of the engine. a wrong quantity of oil leads to seizure." That is, too much oil in the gasoline can cause overheating in some engines.
Polini has this note in their user manual for the Thor engines, "...mixes that contain too much oil do not extend the engine’s life."
Everything depends on how the engine is engineered and the jetting used in the carburetor. Do not experiment with the fuel/oil ratio but do exactly what the manufacturer specifies. See this page on labeling fuel containers properly which can help prevent a disaster!
Mix the oil just before use
Per Miniplane, other paramotor manufacturers' recommendations, and the experience of many pilots, it is always best to mix small quantities of oil and fuel just before use and not store the mix more than (2) weeks no matter what kind of gasoline is used. The lubricating properties of the oil are affected by long term contact with any kind of gasoline, with fuel additives, and especially with ethanol blends. I mix fuel and oil (2) gallons at a time. It is a nuisance but it means my fuel mix is fresher.
Always fly safely and courteously.
1As of August 30, 2019 there has been no response to my questions (4th paragraph below) from Miniplane.
In July 2018 Miniplane released this cryptic, unsubstantiated bulletin (next paragraph below) concerning the use of AVGAS in the Top 80. Apparently, whoever made most of this up or confused the warnings about ethanol from Walbro and oil manufacturers with AVGAS. Who knows?
"...although in our user manual we have never
recommended the use of AVGAS, many customers have decided to use it following, according to current consuetude [sic], information randomly collected from the web. This type of fuel has never been
tested by us and we can not guarantee that it can not cause problems, not least that of a poor compatibility with the oils that can be easily purchased in the motorbike, boat, etc. market. In any
case, as explained on many occasions, the use of AVGAS is not necessary for the operation of the engine, at the moment we are not able to confirm, even if we suspect it, that it is even harmful.
One reason why we are now certain that we can advise against its use is that we have received a warning from one of our suppliers, he confirms that AVGAS gasoline causes problems to some
components of the petrol circuit and consequently to the engine. No problem for lead-free commercial gasoline, or with alcohol up to 20%, in this case, however, we advise you to check the
compatibility of the oil."
"Poor compatibility with the oils"? What's the chemistry here or where is the reference? "Easily purchased [2 stroke] oils [for a paramotor]"? The Miniplane people have obviously never been to the U.S. which has next to zero 2 stroke motorbikes on the roads. Which components in the fuel system? What did it do? What parts of a Walbro carburetor would AVGAS damage that MOGAS would not? Why doesn't Walbro note this in their service bulletins if AVGAS will harm their carburetors (which they do not)? In over twelve years of AVGAS use, our group has not experienced any damage to the fuel systems or engines whatsoever other than the usual done by all gasoline i.e. aging of fuel lines and diaphragms. What is the mystery component in AVGAS that does this? AVGAS is 100% petroleum but not most MOGAS which contains the powerful solvent/oxidizer ethanol. 2 stroke oils may have incompatibility with ethanol but not with any 100% petroleum product like AVGAS. Also, our MOGAS is not the same as that in Europe. Has Miniplane tested U.S. oil brands and fuels? Not likely. What part does the supplier make? It is possible that Miniplane has a new part in the fuel system that may be affected by the mystery additives in AVGAS but which? The formula for AVGAS is easily available and is the same worldwide because mistakes in its making could jeopardize aviation safety. In sum, what Miniplane has written above is vague, without reference to published works on gasoline, and unhelpful. How does anyone check "the compatibility of the oil"? Does Miniplane have a laboratory with chemists? If so, where is the evidence? AVGAS stores forever (unmixed) and has the highest standards of manufacture of any fuel. For this reason (and that it is ethanol free) we use it in our very hot environment, despite its high cost and some additional maintenance. (We have no source of ethanol-free gasoline, unfortunately.) Why hasn't Miniplane tested AVGAS, a simple and easy thing to do rather than indulge in hearsay? Our group has thoroughly tested it. As a degreed organic chemist, yours truly was surprised by the bulletin. I have contacted Miniplane and am trying to find out what they are talking about and will let everyone know if I hear anything but it is doubtful THAT will ever happen.
2CAUTION! I no longer recommend using a 1.5% mix even if the engine manufacturer allows it. It appears that the higher mix ratio may increased wear of the upper connecting rod bearing and wrist pin in a test engine that had been run exclusively at the 1.5% mix for over 300 hours. This part of the engine has the poorest lubrication which may be why the increased wear with the higher fuel/oil mix ratio. However, this is conjecture as I have not done any sort of extensive testing on multiple engines.