Fuel system modification (FSM) for paramotors with diaphragm carburetors

by Had Robinson
updated February 26, 2021

Note: The FSM is still in the testing phase (February 2021).  This page is for information only.  Thank you.

Why the FSM?

Both the Walbro WG and WB-37 carburetors were designed for chain saws that have the fuel tank at the same level as the carburetor.  Nearly all paramotors have the tank far below the engine and the fuel level can be 18"/45cm or more below the fuel inlet to the carburetor.  This has caused fuel starvation problems in both hot and cold weather including vapor-lock in hot weather.  The carburetor fuel pump was designed to move fuel horizontally, not vertically.  When the engine is running there is a vacuum at the carburetor fuel inlet that ranges from -7.2 to -13 inAq (-1.8 to -3.3 kPa) depending on the fuel level height in the tank.  The vacuum increases if the fuel filters are clogged or there are other restrictions in the fuel supply system.  This causes fuel starvation and other problems when the engine is running under high load.

Miniplane IT currently offers a kit to replace the Walbro diaphragm-type carburetor with the Dell'Orto float-type carburetor.  It contains a pulse-port fuel pump which effectively removes some of the inherent fuel delivery problems of the Walbro diaphragm-type carburetors.  A downside of the kit is that tuning the Dell'Orto requires more skill.  In addition, a float-type carburetor must always be in the upright position or it will flood the engine.  The worst downside is that the conversion process is complex.  According to Miniplane, the response and fuel delivery are less than the WG-8.  In addition, a port must be installed in the crankcase in order to run the pump.  A float type carburetor must be specially mounted as it cannot endure the intense vibration of a paramotor.  In general, a float-type carburetor is not a good solution.

On the other hand, the Walbro is simple and easy to rebuild compared to float type carburetors.  Changing out at the high speed jet on a WG-8 takes minutes whereas changing the high speed and midrange settings on a float-type carburetor can easily take an hour.  All float-type carburetors must be well insulated from the engine with rubber boots including a special way to suspend the carburetor.  To sum up, the Miniplane kit is a "last resort" solution.

Here are benefits of the FSM:

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 through the entire throttle range to run the engine, but no more.  However, an increase/decrease of the temperature, addition of altitude, modest problems such as semi-clogged filters, aged carburetor parts, ignition problems, and out-of-spec carburetor adjustments will almost guarantee that fuel starvation will occur.

As the throttle is opened, fuel flow increases along with the volume of air.  The demand on the built-in 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.

The common fix (and also done by the paramotor manufacturers) is to install a larger fixed main jet (WG-8) or open the main jet further (WB-37).  The side effect is that the midrange is enriched.  This causes the engine to "4 cycle" i.e. misfire in the midrange.  One problem is traded for another.  With cold weather, things get worse.

In order for the WG and WB-37 carburetors to function as designed, the fuel pressure at the carburetor inlet should be slightly positive.

With the WG-8, a change in pressure of less than 1 kPa (0.15 PSI/4 inAq) affects the air/fuel ratio delivered to the engine and, as a result, engine temperature and performance.  The higher the pressure at the inlet valve, the more fuel moves through the valve when it opens.  This is why pressures inside the carburetor are critical.  During field tests, the inlet pressure was varied.  When the pressure was dropped slightly, engine temperature in a Polini Thor 130 increased 30C-40C very quickly, even while running in the midrange.

How can the problems be fixed?

An auxiliary fuel pump could be added to the existing system but its output pressure would be too high.  Most pumps have an outlet pressure of 7 PSI/48 kPa that is also too variable.  High pressure will force open the inlet needle valve, depending on the condition of the ML spring, and flood the engine.  A precision pressure regulator could be added to the system that will maintain a constant pressure regardless of other external factors.  However, commonly available regulators e.g. the Holley 12-804 do not have the correct range required and, even if they could be used, are expensive and heavy.

Hence, the need for the FSM – it eliminates the inherent problem with the fuel tank location and supplies fuel at a VERY LOW CONSTANT PRESSURE (VLCP) to the carburetor over the entire throttle range.  It maintains an average pressure of about 0.4 PSI/11 inAq (2.75 kPa) with the pressure control valve fully open and about 0.14 PSI/4 inAq (1 kPa) with the pressure control valve nearly closed.  These pressures vary little from idle to full throttle.

The greatest benefit of the FSM

Extensive testing has demonstrated that the FSM allows for a smaller main jet (fixed or adjusted).  Paramotor manufacturers have a larger main jet installed in the WG-8 or specify a richer main jet mixture on the WB-37 to compensate for the lower pump output at WOT in order to prevent overheating.  This, however, makes the midrange rich and causes "4 cycling" of the engine, especially with the WG-8.  Instead of some fraction of the fuel needed near or at full throttle, the FSM supplies 100% which eliminates the need for greater main jet sizing.  When I first starting studying these carburetors I wrongly assumed that the very rich midrange was deliberate for some reason.  When I installed the first FSM I realized the engineering brilliance of the Walbro diaphragm carburetor but only if the fuel tank is at the same level as the carburetor.

With the FSM, the common and annoying stall when quickly going to full throttle at launch becomes a thing of the past.

Larger engines have demonstrated a 10% increase in power at WOT. 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) and in cold weather.  Normal WOT for this engine at this altitude is about 8,400 RPM.  Engine RPM must be monitored after the FSM is installed so maximum specified RPM is not exceeded.  Throttle response is instant and engine running temperature is lower at WOT.

FSM increases power of a Polini Thor 130

The heart of the FSM is a Walbro pulse-powered fuel pump with the VLCP regulator assemblies.  An external pulse port is required to operate the pump.  However, some engines, including the Top 80, have an internal pulse port so an external port must be added.  It is a relatively simple task to drill and tap a hole in the crankcase near the reed valve assembly for a 1/8" barb x 1/4" NSF (6mm) barb fitting.  The right sized drill and tap are included in the FSM kit for engines without an external pulse port.

Why not use an electric pump?  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 might want to use an electric pump with the FSM.

Those who must use winter formula MOGAS will need to install a bulkhead fitting and pump at the base of the tank.  Otherwise, the suction needed to list 18" (45cm) of gasoline will cause it to vaporize and lock up the fuel system.  The FSM will be available in two different kits, one for AVGAS only and one for any type of gasoline.  The latter will cost more.

Below is a photo of an idling Polini Thor 130 with the FSM installed.  It pumps about a liter per minute at a 24" (60cm) of suction head, much more than would ever be required.  The pressure output is 7 PSI (48 kPa) which is why a VLCP system must be installed.

Other pulse-powered pumps cannot be compared to the Walbro.  Tests on clone pumps (Chinese knock-offs) had outputs <25% of the Walbro and cannot be used.  However, the Walbro pump is vented on the pulse port side and will collect oil (stopping the pump) unless the pump is mounted above the pulse port in the engine and the pulse port on the pump oriented in the "down" position.

Walbro fuel pump output

Drawbacks of the FSM

WARNING:  THE FSM CHANGES THE THROTTLE RESPONSE AND OUTPUT IN MOST ENGINES.  PILOTS MUST EXERCISE CARE IN GOING TO/NEAR FULL THROTTLE.  MONITOR ENGINE RPM SO AS NOT TO EXCEED THE MAXIMUM SPECIFIED BY THE MANUFACTURER.

Other solutions for the Walbro carburetor typically involve modifications of the carburetor, special metering lever springs, and over-sized jets.  But 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 specialized wiring.  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.

How the fuel pressure at the carburetor inlet is measured during field tests.

FSM test setup

The immediate availability of pressurized fuel right at the carburetor fuel inlet makes tuning more difficult because it must be precise.  In tests, a 1/16 turn on the idle adjustment screw was quickly noticeable.  Pilots will have to develop a new and somewhat different paradigm for priming their engines.  It is a small price to pay for a dramatic increase in performance and power.

Pilots flying at all altitudes and conditions will benefit from the FSM.  Tests at sea level demonstrate that throttle response and engine power are the most dramatic improvements.  Nonetheless, high altitude and/or cold temperatures will still require cold weather operation adjustments.

Because of decreased main jet settings, midrange performance modifications are not required.  This is one of the most important benefits of the FSM.

If ethanol fuels are used, pilots must allow for more frequent maintenance of the system.  Ethanol, a powerful solvent, tears up fuel system parts, especially the VLCP regulators.

While the FSM greatly improves the overall fuel supply system, engine performance can still suffer due to the same reasons as with the stock 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 fuel supply system and the inherent weakness of the Walbro carburetor fuel pump used in typical paramotors.

Parts list – external pulse port

Parts list – no external pulse port

Installation and maintenance

The photos of FSM installation are for illustrative purposes only, from early test setups, and are *not* a guide for installation.  Current setups of test engines are different.

Large engine FSM (>130cc)

Important parts identification (the Polini Thor 130 shown here)

  1. VLCP regulator #1 (vented to the atmosphere)
  2. fuel overflow line going to the main fuel tank
  3. VLCP regulator #2
  4. fuel line to carburetor
  5. fine pressure adjustment valve
  6. inline OEM fuel filter between fuel tank and fuel pump
  7. Walbro fuel pump (This photo shows the pump mounted below the engine pulse port.  To stop it from becoming filled with oil, it was moved to a location above the engine pulse port.)
  8. pipe from engine pulse port to fuel pump port

The VLCP regulators are only available from Southwest Airsports.  They have a limited life, the price for being lightweight and inexpensive.

FSM install on a Polini Thor 130

Unless the Walbro pump is higher than the engine pulse port, it will fill up with oil from the crankcase after about (30) minutes and the engine will quit.

oil clogged FSM fuel pump

Small engine FSM (<100cc)

The red arrows point to external pulse ports installed in the crankcase of a Top 80.  The upper port is for a 1/8" barb X 1/4" NSF fitting.  In the photo below this port is plugged.  The bottom 6mm barb port requires less work to install.  I chose to use the 6mm barb.  Installation of the port does not require extensive skills other than being able to use an electric drill.  I would practice on a piece of hardwood or, better, a 1/4" piece of aluminum if I had never drilled and tapped a hole before.  Instructions on how to use a tap in aluminum, a soft metal that easily galls, will be included in the kit.

FSM installation Top 80 paramotor

Installation of the important parts (a Top 80 shown here)

  1. fuel overflow line going to the main fuel tank
  2. VLCP regulator #1 (vented to the atmosphere)
  3. VLCP regulator #2
  4. fuel line to carburetor
  5. fine pressure adjustment valve
  6. fuel line from tank
  7. Mikuni or Walbro fuel pump – the Mikuni does not have as high an output as the Walbro but is sufficient for small engines.  It will not collect oil like the Walbro and can be mounted below the engine pulse port.  The Walbro MUST be mounted above the engine pulse port in all applications.
  8. inline OEM fuel filter
  9. pipe from engine pulse port to fuel pump port

This photo has the Walbro fuel pump mounted below the engine pulse port.  It filled with oil after about 30 minutes of flying.  It was later moved above the engine pulse port.

FSM installation Top 80 paramotor

This engine has the Mikuni fuel pump which is mounted below the engine pulse port.  The lower the pump is, the less chance there is of causing fuel vaporization, a particular problem when winter formulas of MOGAS are used.  Note the fuel return line on the top right side of the fuel tank which has clamp around the line.  It is critical that all fittings past the fine pressure adjustment valve be secured with clamps if there is any chance the fuel line might become detached.  These lines have pressurized gasoline in them!

FSM install in a Top 80

Top 80 with the FSM installed

NOTICE:  ALL MODIFICATIONS TO THE FUEL SYSTEM ARE DONE AT YOUR OWN RISK!  ANY FUEL SYSTEM CAN LEAK FUEL WHICH CAN BE A FIRE HAZARD!  YOU MUST BE ABSOLUTELY CERTAIN THAT ALL HOSES AND FITTINGS HAVE BEEN PROPERLY INSTALLED SO THAT NO HOSE CAN SLIP OFF AND LEAK FUEL.  ANY ENGINE MODIFICATION MAY VOID THE MANUFACTURER'S WARRANTY.  YOU MUST HAVE THE SPECIAL TOOLS LISTED ON THE TUNEUP PAGE AND KNOW HOW TO USE THEM.  IF YOU HAVE ANY DOUBTS OF YOUR MECHANICAL ABILITY HAVE AN EXPERIENCED 2 STROKE ENGINE MECHANIC DO THIS MODIFICATION.  IMPROPER INSTALLATION OR MODIFICATION OF FUEL SYSTEM COMPONENTS COULD CREATE A HAZARD.

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