Fuel system modification (FSM) for paramotors with diaphragm carburetors

by Had Robinson
updated July 1, 2022

Note1: 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.

Note2: The supply chain for critical parts e.g., valves and pumps, is getting tighter and more disorganized which requires evaluations of substitutes.  The hardest part is finding ethanol resistant items.  Substitutions will not be as good in this way = more frequent change outs if pilots only have access to gasoline containing ethanol.  The special fuel tank valve we use which has proven 100% leak-proof is almost of out-of-stock at this time.  We cannot ship out the FSM without it.  We planned to open up sales of the FSM this summer but, so far, it may not happen.  The FSM is engineered to have the absolute minimum amount of pressurized gasoline in the system and any changes MUST be thoroughly tested here first.  Simplicity is golden and we want to keep it that way.

1. Why the FSM?

The purpose

No diaphragm carburetor is engineered to suck fuel from a tank far below the carburetor inlet, as is the case with nearly all paramotors.  Nor is gasoline manufactured to remain liquid in the presence of a vacuum, however slight.  Hence, paramotors often have severe fuel supply problems if things are not perfect, especially the pump diaphragm material inside the carburetor.  The FSM fixes this fundamental problem by creating the particular environment that the carburetor was engineered to operate in.  That is, 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.  A bonus is that the engine also achieves greater output at full throttle.

The problem –> vapor lock

For many years, yours truly was plagued by quirky and unexplainable engine stalls while flying, especially when going to wide open throttle at launch.  Once during hot weather I launched and, in less than 15 seconds, the engine quit and I had to land.  Anyone had something like this happen?  After a few minutes, I was able to restart the engine and attempted another launch which was a sputter experience.  I gave up and went 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 terrific frustration forced me to take the time to think it all out.  Once-upon-a-time, I knew some organic chemistry so I began to review a bunch of it.  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, 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 is at the carburetor fuel inlet can range from -0.3 PSI to -0.5 PSI (-1.8 to -3.3 kPa), depending on how far the fuel level in the tank.

The key to the problem is that gasoline is *not* manufactured to remain in a liquid state at a negative pressure.  How easily the fuel turns to fizz in the fuel lines and inside the carburetor depends on the ambient temperature, altitude, and the formula of the gasoline manufactured in your state or country.

Typically, the negative pressure at the carburetor inlet increases if everything in the OEM fuel system is not perfect, especially at or near wide open throttle (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 using a minimum sized pulse port which, again, is only noticeable at WOT, in cold conditions or when the air temperature very hot.  Then there is cheaply manufactured gasoline which is notorious for vaporizing at lesser negative pressures.

As a result, the pilot might have to switch to aviation gasoline (the finest gasoline in the world) but it, too, can cause problems in some of the larger paramotors (fouling of the piston lands in 2 stroke engines).  Like MOGAS (motor vehicle gasoline), AVGAS is not manufactured to remain liquid at negative pressures.  At the least, we know what we are getting with AVGAS because it has a strict formulation and standards for delivery for safety reasons.  As noted further on in this essay, the last resort is to purchase premixed VP Racing fuel but it is expensive!

Another fuel issue, for example, can be a tiny 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 Go Pro camera near the carburetor to discover this problem).  But who wants to look at a 200cc Minari running at full throttle on the ground to see if there are bubbles in the fuel system?

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

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 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 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 run with some small fizzing of the fuel.

All diaphragm carburetors, including the Walbro and Tillotson, are engineered to have the fuel tank at the same level as the carburetor.  Chinese carburetor clones suffer even greater problems from vaporized fuel.

In order for diaphragm carburetors to function as designed, the fuel pressure at the carburetor inlet should be close to zero or little positive.  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.  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 usually 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 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 (3) years and a bunch of prototypes, I discovered something that works that I have named the "FSM" 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.  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 with modest mechanical skills.

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

Some benefits of the FSM

More details on the FSM

OVERHEATING  In order to prevent overheating due to lower fuel pump output at wide open throttle, paramotor manufacturers often 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, makes the midrange rich and cause "4 cycling" of the engine, especially with the WG.  These are all Band-Aid type solutions.

Instead of some fraction of the fuel needed near or at full throttle, the FSM supplies 100% which 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.

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 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 main jet (8) 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.  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, on the other hand, at wide open throttle 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 (>10 PSI) will force open the inlet needle valve in the carburetor and flood the engine at idle and in the low mid range.  There are electrical auxiliary pumps ($80+) that have the right pressure output but they will require a battery, on-off switch, and wiring.

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 or its location.  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 but only if the pilot has intermediate or better 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, find a competent mechanic (usually at a chainsaw or go-kart shop) and have him install it.  Southwest Airsports also offers a complete installation and testing of the system but the complete paramotor must be sent to us.

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.  This arrangement eliminates the problem of sucking gasoline 50cm/24" or more from below the pump.  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.

Other than the liability issues, why have the paramotor manufacturers not created something like the FSM?  Firstly, the tank fitting must *not* leak.  It took some time to find one that would do the job for an HDPE tank.  Secondly, there is the additional cost of the FSM.  Thirdly, the FSM adds complexity to the fuel system – but this is inevitable if we want a permanent and bulletproof solution to the tank location.  Lastly, the FSM must be installed and maintained by a competent mechanic.  Sadly, most paramotor dealers do not have the skills to properly install and maintain the FSM.

Why not use an 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 to the system.  Get a local machine shop 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).

The FSM auxiliary pump must be 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.  Note the low level of fuel in the tank.

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.  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.  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.

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.

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!  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 greatly increase the routine maintenance of the FSM.

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.

3. Testing and analysis of the FSM

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

Version 1 of the FSM did not have the fuel inlet for the pump at the base of the fuel tank.  Version 2 did not include the manometer tube.

4. Requirements for installing the FSM


5. FSM v3.0 Schematic

FSM schematic by Chris Jokinen, Darwin AustraliaNote: this schematic does not include all available 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.

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 identical to quality made items but are not.  The worst abuse is carburetors.  These fakes will 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

G. Full installation and flight testing by Southwest Airsports: $475

7. 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.




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

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.