Fuel system modification (FSM) for paramotors with diaphragm carburetors v2.1

by Had Robinson
updated January 6, 2022

Note: We are continuing field tests of the FSM by other pilots who live in various parts of the world.  It has to be adaptable to a large variety of engines and setups.  If you are interested in testing the FSM, please contact us.

Note: We are also testing a better method of reducing the compression when starting paramotors by adapting what Stihl and Husqvarna do with their big chainsaw and concrete saw engines -> a decompression relief valve (CRV).  It eliminates the need for the unreliable and delicate "flash starters".  It also greatly reduces the wear and tear on electric starter systems.  The steps are: a. set the CRV and prime the system  b. slowly pull the starter until the CRV puffs a little smoke  c. reset the CRV, if needed  d. pull the starter quickly.  With an electric start, steps "b" and "c" are eliminated.  Here is a demonstration of how it works.

1. Why the FSM?

The purpose

Transport 100% liquid fuel to the carburetor fuel inlet of the paramotor at a constant and very low positive pressure.

The problem – vapor lock

For many years I was plagued by quirky and unexplainable engine stalls while flying, especially when going to wide open throttle (WOT) at launch.  During some hot weather I launched and then, in 15 seconds, the engine quit and I had to land in a huge puddle, slid around on my feet, and fell flat on my face in the mud.  When I got up, the engine started right up again.  What is this?

I would rebuild the carburetor, replace ignition parts, check the reed valve, replace the spark plug, replace the fuel filter, and so on.  It would drive me crazy....  Anyone reading this knows what it is like.  As a result of this intense frustration, I got to work in our shop, invested in testing equipment, and purchased all sorts of random parts, most of which are now collecting dust on a rack.  But after about (3) years and all of the prototypes, I discovered something that works that I named the "FSM" short for "fuel system modification".  It has been tested it in all sorts of conditions, including sub-freezing and extreme heat.  All types and quality of gasoline have gone through the test engines successfully.  It is simple and reliable but it must be installed exactly per the instructions because the system has pressurized gasoline (<7 PSI/50 kPa) and is dependent on gravity (the FSM will not work if the paramotor is upside down).

Typical paramotors have the fuel tank far below the engine.  This creates negative pressure at the carburetor fuel inlet which can range from -0.3 PSI to -0.5 PSI (-1.8 to -3.3 kPa), depending on how are the fuel is below the carburetor.

Gasoline is *not* manufactured to remain in a liquid state at negative pressures.  Because of this normal chemistry of gasoline, we have serious problems with fuel starvation due to fuel vaporization.

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.  While this is at least a foot (30cm) greater than the distances between the carburetor and fuel tank in a typical paramotor, the problem of fuel vaporization is still present.  Gasoline in a gaseous state (bubbles) has trouble moving through pumps, screens, and passageways designed 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 along with the vaporization of more of the ingredients that make up the gasoline mix.  Winter mixes of MOGAS further complicate the problem due to the higher vapor pressure which causes the fuel to vaporize (form bubbles) more easily in the fuel lines and, in particular, in the carburetor's internal fuel pump where it will cause vapor lock.  Engines experiencing vapor lock will not achieve full power and even quit after running a few minutes, as this one did.  With hot or cold weather, for example, things get worse.

Diaphragm carburetors, like the Walbro and Tillotson, are engineered to have the fuel tank at the same level as the carburetor.  However, nearly all paramotors have the tank far below the engine, about 20" (50cm) or more below the carburetor fuel inlet.  Some paramotors have a distance of 29" (73cm) and more from the bottom of the tank to the fuel inlet.

In order for diaphragm carburetors to function as designed, the fuel pressure at the carburetor inlet should be close to zero.  This is because the internal fuel pump in the carburetor was designed to move fuel horizontally, not vertically.

At sea level, if the ambient temperature is moderate (21C/70F) and every part of the fuel and ignition systems is in good order, paramotors usually have enough fuel to operate correctly throughout the entire throttle range – but barely, as we all know.  However, an 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 often result in fuel starvation.  The carburetor is frequently named the culprit even though it is 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 WOT.  As the pump works harder, it can create vaporized fuel (bubbles) which paralyzes the fuel delivery system inside the carburetor.  Here is a video of a new paramotor experiencing WOT vapor lock.

Here is another example of vapor lock occurring in a Top 80 (courtesy of a helpful PPG pilot).  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.

The only solution to this problem is to supply fuel to the carburetor at zero or a slightly positive pressure so the fuel always remains a liquid.  This is exactly what the FSM does.

Why haven't the paramotor manufacturers implemented a system like the FSM?  It is pretty simple:  It is called LIABILITY, both for them and their dealers, especially those in the U.S. where the Courts urge people to sue for any reason.

For those who want to understand the science of gasoline and vapor lock, in particular, may study this technical review paper from Chevron Oil.

The benefits of the FSM

More details on the benefits:

In order to prevent overheating due to lower fuel pump output at WOT, paramotor manufacturers install a larger main jet in the Walbro WG carburetor or specify a richer main jet mixture on the WB and other adjustable carburetors.  This, however, makes the midrange rich and causes "4 cycling" of the engine, especially with the WG.  Instead of some fraction of the fuel needed near or at full throttle, the FSM supplies 100% which eliminates the need for increasing 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 first FSM.

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 much less from fuel starvation.  However, when temperatures approach freezing and below, the FSM should also be accompanied with a preheat system.  Just as in any piston driven aircraft, carburetor icing occurs, among other things, and a preheat system is necessary.

Below is a tachometer reading of 9,080 RPM at WOT 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 WOT for this engine at this altitude 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 WOT.

FSM increases power of a Polini Thor 130

Below is the CHT on the same engine but with a main jet (8) sizes less (!) than stock.  The cylinder head temperature after a WOT 1,000' climb-out was not significantly higher, just 12C above the usual running temperature.

FSM CHT reading

Same engine as above and at WOT.  It has the FSM v2 operating near sea level.  The FSM rams fuel into the carburetor so it can get into the engine.  The stock engine at this altitude and WOT goes to about 8,800 without the FSM. 

FSM Thor 130 data

2. How can the problems 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 would force open the inlet needle valve in the carburetor and flood the engine.  There are electrical auxiliary pumps ($80+) that have the right pressure output but they will require a battery, on-off switch, and wiring.

Nonetheless, even a quality auxiliary fuel pump can fail to achieve sufficient output at all throttle settings because of vapor lock.  In field tests with everything perfect, engine fade at WOT from fuel vaporization was still experienced under certain conditions.  Even modern automobiles will experience vapor lock in the fuel system if conditions are right (high altitude, high temperature along with cheaply manufactured gasoline.)

Hence, the need for the FSM – it 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 or its location.  It maintains an average pressure of about 0.28 psi/2.0 kPa.  The fuel inlet pressure to the carburetor is adjustable with the FSM, as needed.

The FSM can be safely installed on all paramotors.  However, it should not be installed by pilots who have little mechanical abilities e.g. have never taken a small engine apart, rebuilt a carburetor, or used a torque wrench.  The only option is for them to find a competent mechanic (usually at chainsaw or go-kart shops) and have them install it.  Southwest Airsports also offers a complete installation and testing of the system.

The heart of the FSM is a pulse-powered fuel pump and the fuel inlet located at the base of the fuel tank.  This eliminates the problem of sucking gasoline 50cm/24" or more from below the pump.  An external pulse port is required to operate the 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.  Note: the FSM will not work properly with a crankcase pulse port smaller than 3mm.

Why have the paramotor manufacturers not created something like the FSM?  Firstly, the bulkhead fitting must *not* leak.  It took some time to find one that would do the job for an HDPE tank.  The additional cost of the FSM may also be a cause.  Thirdly, it adds complexity to the fuel system – but this is inevitable if we want a permanent and bulletproof solution to the tank location.  Fresh Breeze put the fuel tank above the engine and added a float-type carburetor ($$$).  These features not only added complexity but greatly increased the cost of their paramotors.  To their credit, fuel vaporization and vapor lock do not exist on a Fresh Breeze.

Why not use an electric 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, I do not recommend this option unless it is absolutely necessary because of the additional complexity it adds.  Get a local machine shop mechanic to drill and tap the pulse port hole in the crankcase if you are not able to do it.  We can do for you for just the cost of shipping, both ways (a free service to all purchasers of the FSM).

Other pulse-powered pumps cannot be compared to the Walbro.  Tests on clone pumps (Chinese knock-offs) had outputs <25% of the Walbro.  The Mikuni pumps, though well engineered and made, had outputs that were 50% less.

When the pump is mounted at the base of the fuel tank, additional lines and an oil scavenger system are required in order to prevent the pulse chamber of the pump from filling up with oil and stopping its operation.

Below is an idling Polini Thor 130 with the FSM installed.  It pumps about a liter per minute, far more than would ever be required, even of the largest paramotor.

Walbro fuel pump output

Other solutions for the problem of locating the fuel tank so far below the carburetor inlet typically involve internal carburetor modifications, special metering lever springs, and over-sized jets.  However, 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 would be well over $250, add significant weight, and would require a battery and wiring.  I would prefer an in-tank pump because there would not be any fittings installed on the tank that are below the fuel level.  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.

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.

FSM test setup

The gauge manometer in the figure above was used to monitor fuel inlet pressure of the VLCP system.  In later tests, a vertical tube manometer was used and is now a part of the kit.

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) will still require cold weather modifications.

Because of decreased main jet settings, midrange performance modifications are not required, another important benefit of the FSM.

The figure below illustrates how we measured some of these pressures inside the carburetor.  It was fascinating watching how the WG carburetor works.  It is a brilliant design.  As the throttle was opened (no FSM), the fuel pressure at the metering lever inlet valve dropped.  The pump could not keep up.

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!

While the FSM greatly improves the fuel supply system, 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 negative pressure in the fuel supply system that if found most paramotors.

3. Testing and analysis of the FSM

Chris Jokinen (Darwin, Australia) has been testing the FSM on an Air Conception Hybrid 130cc.  Here is his report.  His engine had a Chinese clone of the Tillotson.  He recently obtained a WB-37 and will test it with the FSM.  Instead of an open oil drain for the fuel pump, he mounted a catch bottle which is much handier and there is less mess.  We added this improvement to the FSM kit.  "Thank you very much for helping improve the FSM!"

Version 1.0 of the FSM did not have a fuel inlet for the pump at the base of the fuel tank.  The fuel pump was located close to the crankcase pulse port fitting far above the bottom of the tank.  This sometimes resulted in vapor-lock and fuel starvation, even with AVGAS.  So, we had to re-locate the pump at the lowest level that might be experienced (near the bottom of the tank) and place a bulkhead fitting adjacent to it to ensure that all types of gasoline would *never* be exposed to negative pressures even in the hottest weather.

One pilot who tested the FSM for us asked why not just have the fuel pump at the base of the tank and loop a line down from the existing tank outlet to the pump?  He suggested that fuel, like water, would siphon down to the pump and so on.  Doing this would eliminate the step of cutting a hole in the tank.

The answer is that if gasoline had the same vapor pressure as water, we could do it.  When the fuel tank is full, this might work but even then, there will be a negative pressure on the gasoline.  This negative pressure on a cocktail of petroleum distillates (i.e. gasoline) can cause it to form small bubbles which block the flow of the liquid fuel through a pump (the pump cannot pump a gas, only a liquid).  Much depends on the gasoline formula from the refinery, the ambient temperature, and atmospheric pressure.  Here is a video of treated and stored ethanol-free gasoline being siphoned out of a just unsealed 55 gallon tank.  Being stored under its own vapor pressure and then having the pressure released caused certain volatile compounds to become gaseous, as the video demonstrates.  While the changes in pressure on the gasoline was extreme (probably about 8 psi), the principle stands.  The common paramotor tank is about 13" (33cm) high.  When the fuel is near the bottom of the tank, there will be a negative pressure at the top of the siphon which can be a enough to cause the formation of bubbles.  Those bubbles will not be instantly reabsorbed as they travel down the other side of the siphon to the fuel pump (the video demonstrates this).  When the bubbles enter the valves and diaphragm inside the pump, they paralyze its operation.  The degree of failure is dependent on the aforementioned, especially temperature.  A greater issue is: gasoline is *not* manufactured to remain in liquid state at negative pressures in a closed system.  Just the same, having the pump source fuel through a siphon would improve the problem, but only some.  That is why the FSM is designed to move fuel at positive pressures throughout the entire fuel system.

4. Requirements for installing the FSM

5. FSM v2.1 Schematic

FSM schematic

figure 1 schematic by Chris Jokinen  Note: basic schematic does not include the manometer tube nor other options.


6. Parts

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

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

International shipping costs:  If the total shipping weight is less than 450 grams/1 lb., the FSM kit can be sent 1st Class Parcel rate and is about 1/3 the cost of the international parcel rate for many countries.  If international customers can obtain Tygon® LP1100 fuel line (or equivalent) in their countries, it will ensure that the kit can be sent the 1st Class Parcel rate.  We will discount the FSM cost accordingly via a partial refund.


A. FSM kit $172 (not yet available for purchase on our shop page)

Note: A failure of the FSM auxiliary pump will not ordinarily result in an engine failure. The fuel system will return to its pre-FSM state but with some reduction in power, depending on the state of the internal fuel pump in the carburetor.

B. External pulse port kit option $14.50 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 $167

E. Tank quick-disconnect option $22

F. Options for the FSM kit

G. Full installation and flight testing by Southwest Airsports: $449

7. Installation, troubleshooting, and maintenance

A. FSM parts identification

  1. VLCP Green regulator
  2. Fuel overflow line to the main fuel tank (identified as #11 in figure 3)
  3. VLCP Red regulator
  4. Manometer tube for measuring the pressure of the system
  5. Coarse pressure adjustment valve (marked WHITE in kit)
  6. Fuel line from pump
  7. Fuel pump port on the carburetor (Just below is the tee that goes to the pulse port in the crankcase and to the external fuel pump port #14.)
  8. Pulse port line from the engine crankcase port to fuel pump port
  9. Fuel line to carburetor

B. Warranty note  Because we have kooks and the incompetent running the U.S. government at this time, wholesale supply chains for various FSM parts have been severely disrupted and we have had to substitute some items.  We regret any inconvenience this may cause.  For the time being, all parts for the FSM kit are warranted against defects/failure for (1) year.

The figures below show the general locations of the FSM parts on early test engines.  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 which is critical for the optimal function of the FSM.

Some of the fuel lines used in the test engines below are *not* the premium grade lines included in FSM kits.  Only premium fuel line should be used with the FSM for safety reasons.


figure 2

  1. Capped-off original fuel supply fitting going to the tank clunk (may not be present on some engine configurations).  The FSM kit includes this silicon fuel line cap.
  2. Fuel overflow line going to fuel tank (identified as #2 in figure 2)


figure 3

  1. Pump output fuel line going to coarse pressure adjustment valve (#5)
  2. Pulse line from engine pulse port
  3. Walbro fuel pump
  4. Oil drain valve for the fuel pump (modified tee marked w a white dot)
  5. Tank bulkhead fitting with an in-tank filter clunk attached
  6. Oil drain line to bottom of frame.
  7. Premium inline fuel filter (10 micron)
  8. Tank fuel line quick connect/disconnect (optional, not shown).  This fitting is installed in the fuel line between #16 and #18 in the figure below.  Note: without this fitting, fuel tank removal/installation is not as easy.

Fuel System Modification (FSM)

figure 4

  1. 30ml fuel pump oil drain catch bottle – Chris Jokinen suggested the catch bottle which improved an important part of the FSM.  Thank you!
  2. CHT and Tachometer – (not shown).  Pilots must have these gauges in order to tune and monitor the FSM.

oil pump drain catch bottle

figure 5

FSM installation detail

figure 6

Figure 6 shows the upper sections of the FSM.  VLCP GREEN (#1) must have the upper connector terminated with a 4" (10cm) or longer piece of fuel line vented to the atmosphere.  Recommended: an inexpensive piece of rubber windshield wiper tubing can be used from this VLCP to the bottom of the paramotor frame.  This will ensure that any gasoline leaking past this VLCP will not get near the engine and create a hazard.  The vertical piece of yellow 3/16" (4mm) fuel line on this particular engine is used to accurately measure fuel pressure at the carburetor inlet in the same manner as a water column is used to measure gaseous pressure but here it is a "gasoline column".

oil clogged FSM fuel pump

figure  7

Pulse operated pumps must have an oil scavenging system that prevents oil from filling up the pump when it is located below the engine pulse port.  If this is not done, the pulse chamber of the pump will quickly fill up with oil and fail, as happened to the pump above.

C. 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 when custom aspects of the installation must be addressed

3. Operating manual – how to operate the FSM

4. Troubleshooting manual – not every possible paramotor configuration can be anticipated.  This manual will help debug problems.