Fuel system modification (FSM) for paramotors with diaphragm carburetors

by Had Robinson
updated September 16, 2023

Note: Before making the FSM available to all, we are continuing field tests of the FSM by other pilots, including some who live outside the U.S.  It has to be adaptable to a great variety of climates, engines, and setups.  If you are interested in testing the FSM, please contact us.

Quick test to see if the FSM may solve your fuel starvation problem

If you have taken the time to go through the troubleshooting pages on our site and all looks good, you can do a quick test to see if your issue is vapor lock/fuel starvation and is something the FSM will fix.

Take a piece of 1/4" (6 mm) hose and connect it to the primer tube on your engine.  If you do not have a primer tube you will have to figure out some way of pressurizing the fuel tank while flying.  Make it long enough so that you can get to the end of it in order to pressurize the tank while flying.  If “priming” the system fixes the problem then you know that the FSM will fix your vapor lock/fuel starvation problem.  Make certain that you secure the end of the tube in a convenient spot where you can place it in your mouth AND also to be sure that it does not get in the propeller.

Using a primer bulb to "prime" the system will not work because it has the same problem as the internal fuel pump in the carburetor has in that it is sucking on the gasoline from a significant height.  Pressurizing the fuel tank is the only QUICK way to see if the FSM will work for you.

1. What is the FSM?

The FSM modifies the OEM fuel system on 2 stroke paramotors that have the fuel tank below the engine and have a diaphragm-type carburetor.  It corrects the problem of fuel starvation caused by vapor lock.  Paramotors with float-type carburetors already have an auxiliary fuel pump and the FSM would be redundant.

The purpose of the FSM is to transport 100% liquid fuel to the carburetor fuel inlet at a constant low pressure of about 0.2 PSI (1.4kPa). The major benefit of this is ending fuel starvation at high throttle openings and “4 cycling” in the midrange caused by overly rich main jet settings. s

It consists of:

  1. A leak-proof fitting installed at the base of the fuel tank

  2. A pulse-port driven auxiliary fuel pump installed externally at the base of the fuel tank

  3. Various valves and regulators that work together to maintain the correct fuel pressure at the carburetor inlet

  4. A simple manometer that is used to monitor the fuel system pressure

  5. Fuel recirculation through the entire system to help prevent fuel overheating and vaporization, even under positive pressure

The overall design of the FSM creates a slightly positive pressurized fuel system which helps prevent the gasoline from vaporizing.  Gasoline that has bubbles/fizz in it will not move through the carburetor and into the engine.  The most acute problem is the failure of the internal fuel pump inside the carburetor.  It will not pump fuel properly unless it is 100% liquid.  The greater the amount and size of the bubbles/fizz in the fuel, the greater the problem.

2. Why install the FSM?

The primary purpose of the FSM is to end fuel starvation.  Another purpose is to recirculate the fuel from the carburetor back to the tank.  This keeps the temperature of the fuel as low as possible, preventing vaporization due to fuel overheating before it enters the carburetor.  Lastly, the paramotor manufacturer's enriching the fuel/air mix often causes "4 cycling" or roughness in the midrange.  The FSM allows the main jet size to be reduced which eliminates "4 cycling" without causing overheating at or near wide open throttle (WOT).

Why do we have this problem?  Diaphragm carburetors are not engineered to suck fuel from a tank far below the carburetor inlet, as is the case with nearly all paramotors.  Furthermore, modern gasoline is not manufactured to remain liquid in the presence of a vacuum which it is in all but a small number of paramotors which have the fuel tank above the engine.  Hence, paramotors often have severe fuel supply problems if things are not perfect, especially if the carburetor pump diaphragm is old and stiff or there are any restrictions in the fuel line e.g., primer bulbs with spring loaded check valves.

The FSM fixes this fundamental problem by creating the conditions that the carburetor was engineered to operate in: the FSM moves 100% liquid fuel from the bottom of the fuel tank to the carburetor fuel inlet at a constant and very low positive pressure.  As a result, the engine runs more reliably, efficiently, and cooler, especially at WOT.  A bonus is that the engine also achieves greater output at full throttle with a reduced main jet size.

An example is our test engine – a Polini Thor 130.  The stock jet is a #130.  With the FSM installed and tuned, we were able to decrease the jet size to a #125.  We did go down to a #116 but, at sea level, it was too easy to overheat the engine.  (Our reward was a piston with a hole melted in it.)  The #125, in this case, is a reduction in jet size sufficient to eliminate midrange roughness (4 cycling) while not causing overheating at WOT.  On engines with the WB-37 (or its clones), the adjustable main jet can be leaned out in the same manner.  Our installation instructions explain how to balance leaning out the main jet while not overheating the engine.

Why not have a slightly pressurized fuel tank?  Read this page for the reasons why this may not be a good idea....

3. What is the problem?

For many years, yours truly was plagued by quirky and unexplainable engine stalls while flying, especially when going to WOT at launch.  Once during hot weather (over 100º F/38º C), yours truly launched and, in less than 15 seconds, the engine quit.  Has anyone had something like this happen?  After a few minutes, the engine would restart and another attempt was made to launch but the engine quit soon after becoming airborne.  Time to go home....

What was going on?

I would rebuild the carburetor, replace ignition parts, check the reed valve, replace the spark plug, replace the fuel filter, and so on.  It was crazy.  Nothing helped.  Anyone reading this knows what it is like.  The frustration forced me to take the time to think it all out and apply my training as an organic chemist.  I studied published material on the manufacture and composition of the various kinds of gasoline.  It was an eye opener (but should not have been).  After a couple of years of study and experimenting, things started to get clearer.

Typical paramotors have the fuel tank far below the engine.  The fuel from the tank is sucked up to the carburetor by a small fuel pump that is inside the carburetor.  This negative pressure at the carburetor fuel inlet can range from -0.3 PSI to -0.5 PSI (-1.8 kPa to -3.3 kPa), depending on how low the fuel level is in the tank.

How easily the fuel turns to fizz in the fuel lines and inside the carburetor depends on many things including the ambient temperature, altitude, and the formula of the gasoline manufactured in your state or country.

Typically, the negative pressure at the carburetor inlet will increase if everything in the fuel system is NOT perfect, especially at or near WOT.  Fuel filters (the tank clunk, the fuel inline filter, and the fuel inlet screen in the carburetor) can become restricted in various ways that are more noticeable at or near WOT.  Some manufacturers make things worse by putting a primer bulb in the system that is inline with the movement of fuel to the carburetor.  Another issue is the presence of a minimum sized pulse port which causes a problem at WOT, in cold conditions, or when the air temperature is high.  To reduce restrictions in the fuel system, some manufacturers will eliminate the inline fuel filter (!) or modify the stock carburetor.  Then there is cheaply manufactured gasoline which is notorious for vaporizing at the least negative pressure.

Sometimes, a pilot might switch to aviation gasoline (the finest gasoline in the world) but it, too, can cause problems in some of the larger paramotors, specifically fouling of the piston lands with lead deposits.  Like MOGAS (motor vehicle gasoline), AVGAS is not manufactured to remain liquid at negative pressures.  At the least, we know that AVGAS has a strict formulation for safety reasons and its formula does not change worldwide.  As noted further on in this essay, the last resort for the OEM setup is to purchase premixed "racing fuel."  It is called this because it is for off-road use and not subject to the pollution rules for motor vehicles.

Another fuel issue can be a tiny air leak where the fuel line joins the fitting that comes out of the fuel tank.  This will introduce air into the fuel system which can haphazardly paralyze the fuel system.  It is may only be noticeable when at or near WOT.  I had to put a video camera near the carburetor on one engine in order to discover this problem.

These problems in the inherent design of nearly all paramotors work together to create fuel starvation and endless headaches.

Here is a video of what happens when gasoline is subjected to negative pressure while being siphoned from a large container into a paramotor fuel tank.  The siphon head is about 3' (90cm) above the receiving fuel container.  Gasoline in a gaseous state (bubbles/fizz) has serious trouble moving through the pump valves, filter screens, and passageways designed exclusively for a liquid.

If the fuel filters are clogged or the bottom of the fuel tank is significantly lower than 20" (50cm), the negative pressure on the fuel system increases and so does the possibility of vaporization of the gasoline.  Winter mixes of MOGAS further complicate the problem due to the higher vapor pressure of the mix which causes the fuel to vaporize more easily in the fuel lines and, in particular, in the carburetor's internal fuel pump.  Engines experiencing vapor lock cannot achieve full power and even quit after running a few minutes, as this one did.  With hot or cold weather, things get worse.  Remember that fuel vaporization is progressive as the vacuum and temperature increases.  That is, the engine may still run with some fizzing of the fuel.

All diaphragm carburetors are engineered to have the fuel tank at the same level as the carburetor.  Note: Chinese carburetor clones suffer even greater problems from vaporized fuel.  NEVER USE THESE CLONES ON A PARAMOTOR!

In order for diaphragm carburetors to function as designed, the fuel pressure at the carburetor inlet should be close to zero or slightly positive.

At sea level, if the ambient temperature is moderate (21C/70F) and every part of the fuel and ignition systems is in perfect order, paramotors usually have enough fuel to operate correctly throughout the entire throttle range – but barely.  Any increase/decrease of the temperature, addition of altitude, winter-blend gasoline, modest problems such as semi-clogged filters, low fuel levels, aged carburetor parts, ignition problems, and out-of-spec carburetor adjustments result in fuel starvation.  The carburetor is usually named the culprit even though it is probably functioning correctly.

As the throttle is opened, fuel flow increases along with the volume of air.  The demand on the internal carburetor pump also increases.  Rather than just ceasing to pump fuel, it pumps proportionately less fuel the further the throttle is opened.  That is, if fuel demand doubles, fuel supplied may be only 175% instead of 200%.  If fuel demand triples, fuel supplied may be 250% rather than 300%.  These figures vary but this explains the common problem of engines leaning out as the throttle is opened and sometimes stalling or fading at wide open throttle.  As the pump works harder, it can create vaporized fuel which paralyzes the fuel delivery system inside the carburetor.  Here is a video of a new paramotor experiencing vapor lock from wide open throttle.

Here is another example of vapor lock occurring in a Top 80.  This is the classic case of what typically happens when our engines quit or fade.  It has nothing to do with some defect or problem with the carburetor but is due to the chemistry of gasoline and the location of the fuel tank.  The bubbles of vaporized fuel choked the movement of fuel through the carburetor and prevented the engine from reaching full power.

After a few years and a bunch of prototypes, I discovered something that works which I have named the "FSM" which is short for "fuel system modification".  It supplies fuel to the carburetor at a slightly positive pressure so that the fuel always remains a liquid.  It has been tested it in all sorts of conditions, including sub-freezing, extreme heat, and high altitude.  All types and quality of gasoline have gone through the test engines.  The FSM is simple and reliable.  Nonetheless, it must be installed exactly per the instructions because the fuel system now has pressurized gasoline which can create a hazard.

Why haven't the paramotor manufacturers implemented a system like the FSM?  It is because of LIABILITY, especially in the U.S.  A leak in any fuel system, especially in aviation, is always possible.  No fuel system is 100% safe from leaking.  Only the German Fresh Breeze paramotors have an emergency method of disconnecting the engine and fuel system from the harness in case of a fire.  As of yet, there has never been an inflight fire with a paramotor and we want to keep it that way.  The FSM must be installed ONLY by pilots that have modest mechanical skills e.g., have done routine maintenance on small gasoline powered engines, such as changing the spark plug or replacing the carburetor.  If you have never used a torque wrench or a spark plug feeler gauge, the FSM is not something you should probably install.

For those who want to better understand the science of gasoline and vapor lock, they may study this technical review written by Chevron Oil.

4. Benefits of the FSM

5. Negatives

6. Why haven't the engine manufacturers use something similar?

It is likely due to liability.  Below is a photo of what happens after a few years to the "100% made in Italy" fuel line that is stock on most paramotors.  This line was never exposed to ethanol fuels but still split at the fitting.  Why don't they use premium fuel line?  A failure of the fuel line in a stock engine will only cause it to shutdown so a significant amount of money can be saved by using cheap fuel lines but with the FSM.

Italian fuel line

Such a failure to fuel lines in a pressurized system, on the other hand, could be hazardous.  Thankfully, liquid gasoline being ignited after coming in contact with the hottest parts of the engine is still unlikely.

In light of this, the FSM is a "do it at your own risk" product.  We sell the parts, you do the installation.  Suffice it to say, the fuel line used in the FSM kit is premium quality for the above reasons.

7. More details on the FSM

ENGINES THAT CAN BENEFIT FROM THE FSM   These would include Top 80, Minari, Vittorazi, Polini, Air Conception, Simonini, Cors-Air or any other 2 stroke paramotor that has a diaphragm carburetor and the fuel tank below the engine.  Some engines have an internal pulse port, some have an external port.  If internal, an external port must be installed, a not particularly difficult task for those who, for example, have gapped and installed spark plugs and know how to use a torque wrench.  Some paramotors have fuel tanks that have limited access to mount the auxiliary fuel pump and might also have limited locations where an external tank fitting can be safely installed.  Pilots may contact Southwest Airsports via email and photos for suggestions on what to do and whether the FSM can be installed on their engine.

An example of a 2 stroke paramotor that would not benefit from the FSM would be the FreshBreeze which not only has a fuel tank above the engine but also has a float-type carburetor.

OVERHEATING  In order to prevent overheating due to lower fuel pump output at wide open throttle, paramotor manufacturers install a larger main jet in the Walbro WG carburetor or specify a richer main jet mixture in the WB and other adjustable carburetors.  Minari actually modifies the metering lever spring in the Walbro WB-37.  These mods, however, make the midrange rich and cause 4 cycling of the engine, especially with the WG and its clones.  These are temporary and inadequate solutions.

Instead of some low percentage of the fuel needed near or at full throttle, the FSM supplies 100%.  This eliminates the need for increasing the main jet size.  When I first starting studying these carburetors I wrongly assumed that the rich midrange was a deliberate design for some reason.  That changed when I installed the FSM.

COLD WEATHER  Do you fly in cold weather?  As temperatures drop into the 40'sF/4-9C, engine stall, hiccupping, and power loss become a problem due to the increase in oil viscosity in the gasoline as well as an increasing amount of un-vaporized fuel which enters the combustion chamber.  With a pressurized fuel delivery system, those flying in cold weather suffer less from fuel starvation.  However, when temperatures approach freezing and below, the FSM (including all stock paramotors) should also have a preheat system.  Just as in any piston driven aircraft, carburetor icing occurs, among other things, and a preheat system may be necessary for best engine performance.  In general, air-cooled engines run better when HOT.

BETTER FUEL ECONOMY  Our test engine with a reflex glider averages about 2.4 liters/hour of fuel consumption.  This is almost 25% better than without the FSM.

MORE POWER  Below is a tachometer reading of 9,080 RPM at wide open throttle with the FSM installed on a Polini Thor 130, stock carbon fiber prop, at high altitude (4,500' MSL) in cold weather (45F/7C).  Normal wide open throttle for this engine at this altitude (5,000' MSL) is about 8,400 RPM.  Also notice the relatively cool operating temperature of the engine.  There is plenty of fuel available when running at or near wide open throttle.

FSM increases power of a Polini Thor 130

Below is the same engine running in summer conditions with a #122 main jet (eight sizes less than stock).  The cylinder head temperature was not significantly higher, just 12C above the usual running temperature, after a wide open throttle 1,000' climb-out.

FSM CHT reading

Same engine as above but at wide open throttle.  The FSM rams fuel into the carburetor so it can get into the engine.  The stock engine, on the other hand, at wide open throttle goes to about 8,800 without the FSM.

FSM Thor 130 data

8. How can the problem of fuel starvation be fixed?

An inexpensive auxiliary fuel pump could be added to the existing system but its output pressure would be too high and too variable.  Excessive fuel pressure (>10 PSI) will force open the inlet needle valve in the carburetor and flood the engine at idle and in the low mid range.  The high pressure can also damage the fuel pump diaphragm and valves.  There are electrical auxiliary pumps ($80+) that have the right pressure output but they will require a battery, on-off switch, and wiring.  Electrical systems in paramotors increase the risk of an accidental fire from a short which is why we do not recommend anything electrical on paramotor engines.

The FSM eliminates the inherent problem with the low fuel tank location and supplies fuel at a very low constant pressure (VLCP) to the carburetor over the entire throttle range, regardless of fuel level in the tank and its location below the carburetor.  It can be adjusted to maintain an average pressure of about 0.28 psi/2.0 kPa.

The FSM can be safely installed on all paramotors if the pilot has intermediate mechanical skills e.g., has taken a small engine apart, rebuilt a carburetor, correctly used a torque wrench, installed gaskets, etc.  If this does not describe your skill level, do not install the FSM.

The heart of the FSM is composed of: 1.) a pulse-powered fuel pump located at the base of the fuel tank and 2.) A system of regulators to keep the fuel pressure just slightly positive.

An external pulse port is required to operate the auxiliary pump.  Some engines, including the Top 80, have an internal pulse port so an external port must be added.  It is a relatively simple task for amateur mechanics to drill and tap a hole in the crankcase near the reed valve assembly for a 6mm x M6 brass barb fitting.  The fitting, the right sized drill, and tap are included in the FSM kit for engines without an external pulse port.  The pressures in the crankcase are not more than a few PSI, thankfully.

Other than the liability issues, why have the paramotor manufacturers not created something like the FSM?

Why not use an external electric pump instead of a pulse-powered pump?  As noted above, it is an option but it would require a battery, wiring, and add extra weight to the paramotor.  Those pilots flying engines without an external pulse port and who have little or no mechanical skills to drill and tap a hole in the crankcase can choose the electric pump option for the FSM.  However, it is not a recommended option unless it is absolutely necessary because of the additional complexity it adds.

The FSM auxiliary pump must be mounted at the base of the fuel tank.  Additional lines and an oil scavenging system are required in order to prevent the pulse chamber of the pump from filling up with oil, stopping its operation.

Below is an idling engine with the FSM installed.  It pumps about a liter per minute, far more than would ever be required, even of the largest paramotor.  Note the low level of fuel in the tank.  The average FSM system consumes 4-5 liters/hour, depending on the glider and the weight of the pilot.

Walbro fuel pump output

Other solutions for the fuel tank location problem typically involve internal carburetor modifications, special metering lever springs, and over-sized jets.

None of these fix the root problem.

An electric in-tank pump (as is used in snowmobiles and jet-skis) could solve the problem but it would still require the VLCP regulator system.  The cost of a premium pump would be well over $250, add significant weight, and would require a battery and wiring.  But such a configuration is neither practical nor cost effective for the small volume production of paramotors. Having electrical wiring and terminals around gasoline also adds risk.  Note: this is an INTERNAL pump, not an EXTERNAL pump.  The latter is an option for those who lack the skills to install a pulse-port in the crankcase.

Band-Aid type fixes may slightly help but they only work well under some conditions (cold/hot/high altitude) but not another.  With these partial/temporary solutions carburetor tuning becomes difficult (or impossible) and unpredictable.  It also makes changing an OEM carburetor for a replacement more expensive when an ordinary stock carburetor will work just fine.

The only company in the world that has solved the problem of fuel starvation that is addressed by the FSM is Fresh Breeze of Germany.  They did this by moving the gas tank above the engine.  In case of a fuel system leak, they have an ingenious system which allows the pilot to jettison the entire paramotor from the harness by pulling a couple of pins.  My opinion is that this is overkill for something that has, of yet, never happened.  I think it would be sufficient to jettison only the fuel tank in an emergency.  Hopefully, the jettisoned tank would not land on top of someone's house, in a crowded parking lot, or in a stadium full of people.  Thankfully, we do not usually fly over these things, in accordance with the regulations.

Pilots flying at all altitudes and in all conditions will benefit from the FSM.  Nonetheless, high altitude operation will still require adjustments of the carburetor jetting.  Cold temperature operation (near or below freezing) require cold weather modifications i.e., increasing the high speed jet size and/or installing a preheat system.

Because of decreased main jet settings, midrange performance modifications are unnecessary, the most important benefit of the FSM.

The figure below illustrates how we measured some of these pressures inside the carburetor.  It was fascinating discovering how the Walbro carburetors work and their brilliant design.  As the throttle was opened (no FSM), I could watch the fuel pressure at the metering lever inlet valve drop.  The pump could not keep up.  I am unsure why the Italians have not documented this problem....

Walbro WG-8 test carburetor

Note: If ethanol fuels are used, pilots must allow for more frequent maintenance of the FSM system.  Ethanol, a powerful solvent, tears up all flexible fuel system parts including the VLCP regulators and fuel lines.  If at all possible, *avoid* ethanol fuels!  Gasoline in some parts of the world also has corrosive additives to prevent mildew and other weird problems often found in the tropics.  These additives increase the routine maintenance of the FSM.

While the FSM fixes the root fuel supply problem, engine performance can still suffer due to the same reasons as with the stock fuel supply system such as:

The FSM is not a cure-all for a poorly maintained or incorrectly adjusted engine.  It is a permanent fix of the negative pressure in the fuel supply system.

9. Testing and analysis of the FSM

Aside from in-house testing of the FSM, Chris Jokinen (Darwin, Australia) has been testing it on an Air Conception Hybrid 130cc.  He flies in grueling conditions for a paramotor – very hot ambient temperatures and high humidity using gasoline formulated for the tropics.  Here is his report.  Instead of an open oil drain for the auxiliary fuel pump oil scavenging system, he mounted a catch bottle which creates less mess.  We added this improvement to the FSM kit.  Thank you, Chris, for helping improve the FSM!

10. Requirements for installing the FSM


11. FSM basic schematic

FSM schematic - by Chris JokinenNote: this schematic does not include some minor parts of the FSM.


12. FSM parts

The parts for the FSM are constantly being tested.  Replacement parts including pump rebuild kits are available from Southwest Airsports.  The VLCP's should be replaced each time the carburetor is rebuilt as they both share the same types of materials that eventually harden and decay in the presence of gasoline.

The FSM and the kits listed below include FREE shipping to continental U.S. addresses ONLY.

WARNING: the Chinese are the worldwide masters-of-cheap.  Pilots must be sure of their sources.  We continually discover their new imitations of pumps, fuel line, and carburetors which look like quality made items but are not.  The worst Chinese products are the carburetors, filters, and fuel valves.  These fakes may not work properly in the FSM.

A. FSM kit $190 (not yet available for purchase on our shop page)

B. External pulse port kit option $16.50 with free shipping to U.S. addresses

C. Fixed carburetor jets for the Walbro WG-8 $12 ea. with free shipping to U.S. addresses

D. Electric fuel pump option $187

E. Tank quick-disconnect option $25

F. Options for the FSM kit

There is no charge for option 1, 2, or 3.  Pilots MUST choose at least one of these (3) options.

13. Installation, troubleshooting, and maintenance

The figures below show the general locations of the FSM parts.  Current locations vary as we improve the functionality and reliability of the FSM.  Installation instructions sent out with the FSM have the latest and best locations of parts.

Fuel tanks can have many variations.  Some tanks must have the tank fitting installed on the bottom.  Other tanks may require another vent/drain fitting at the top of the tank.  We can supply additional leak-proof fittings, as needed.




Technical manuals included with the FSM kit

The manuals below are emailed to pilots at the time of FSM purchase and when updated.

1. Installation manual – detailed information on the FSM installation procedure

2. Operating manual – first time use and setup of the FSM

3. Operating manual – how to operate the FSM after initial setup

4. Troubleshooting manual