Midrange performance carburetor tuning for paramotors

by Had Robinson  UPDATED May 2019


Most paramotors run roughly in the midrange (4.5K-7.5K RPM) due to engine misfire from a rich fuel/air ratio.  It gets worse at higher altitudes.  At throttle settings near cruising speed, the engine may jump from misfiring to complete firing.  The result is a surge and then a fade requiring the pilot to move the throttle back and forth.  As the engine "4-cycles", the vibration can be annoying.  In addition, fuel economy is effected.  For example, a tuned Thor 130 will cruise (with a quality paraglider e.g. an Ozone Delta or Buzz and a lightweight pilot) straight and level at less than 0.5 gal/hr.  With an un-tuned engine, this value can double.  Thankfully, this problem does not harm the engine but it is a nuisance.  The only advantage is that the engine will be less like to overheat.

Why is this?

The paramotor manufacturers use stock Walbro carburetors which were designed for chain and concrete saws (e.g. Huskvarna 3120XP 3120K) and run rich in the midrange.  This is done on purpose (there is not the space to discuss this here).  Unfortunately, there is not some easy method to lean out the midrange, like with the Bing.  These carburetors were not designed for paramotors with fuel tanks way below the carburetor.

The WG-8 (and some other WG models like the WG-7,9,10) have a black lever on the throttle shaft which, over the years, has puzzled pilots.  It is part of the throttle system on the Husqvarna engines and is not used or needed on paramotors.  It can safely be removed/cut back.

The rich midrange issue is in the idle/low speed circuit and is caused by too much fuel and not enough air entering the engine during midrange operation.  The problem can be fixed either by increasing the amount of air entering the engine through a throttle plate (TP) modification or by decreasing the amount of fuel entering the engine by plugging one of the idle progression holes (IPH).  It is easier to plug an idle progression hole but they are not as accessible in the WG-8.  The TP modification is preferred because the fuel/air ratio can be varied whereas it is a set value for plugging an IPH.

Another problem is operating a paramotor in very cold air at low altitudes (sea level).  Instead of too much fuel entering the engine, there is too little due to the increased viscosity of the fuel/oil mixture.  In this case, it is often required that the main jet be increased in size.  However, it is a waste of time to do this unless it is certain that the entire fuel system is in perfect order and that a cylinder heat temperature (CHT) gauge is installed so the pilot can monitor the engine temperature during operation.  There is way too much guessing done by pilots in regards to engine troubleshooting and performance issues due to not having a CHT and a tachometer.

Also inherent in the design of the WG-8 and WB-37 is a problem with the fuel pumps.  These pumps were never designed to pull fuel from a tank far below the carburetor.  As a result, fuel starvation is a common and annoying problem under extreme hot or cold conditions.  We are working on an auxiliary fuel pump that will achieve adequate fuel pressure under all conditions.

To properly performance tune your engine, it will help if you can study the Zama technical guide (a Chinese knockoff of the Walbro) and know the internal parts.  It is much easier to understand and more relevant than the Walbro service manual (SM) which is not nearly as well illustrated or written.  Here are the parts  diagram of the WG-8 and the parts diagram of the WB-37.

If you do the modifications correctly, you will not harm your engine but greatly improve its operation.  Of course, we can do the mods for you, if you prefer.  Thanks are due to the kart racing guys (the world experts on small engine carburetion), Gerry Farell, and Scott Travers for their input.

A. WB-37 carburetor

Pilots should not attempt any modification of the carburetor (or use an HA jet) unless they have a cylinder temperature gauge (CHT) installed.  It is easy to burn up an engine.  Most of the engines that are sent to us for repairs have been overheated!

The easiest way to lean out the midrange with the WB-37 is by plugging the correct idle progression hole (red arrow in the 2nd photo below) with a piece of 0.5mm copper wire.  The photo below is of a WB-37 with the metering diaphragm cover (#5) and circuit plate (#48) removed with the piece of copper wire in the idle progression hole that is closest to the metering lever or center of the carburetor.

Modifying the throttle plate (TP), on the other hand, is more difficult to do but has the advantage that a pilot can more easily adjust the midrange mixture.  Increasing the size of the correct TP notch will increase the amount of air going into the engine at midrange.  The notch should be approximately the same as for the WG-8.  We can supply the blank throttle plates.  Contact us to order.  The technique for doing this type of mod is given in the WG-8 section below

Materials needed

A few inches of 0.5mm (22 gauge) copper wire or equivalent.

Plugging one of the idle progression holes

Note: If the correct length and size of copper wire used to plug the IPH is installed, in can later be removed without removing the Welch plug.  With needle-nosed pliers, carefully grab the small end of the wire protruding into the carburetor throat.  Pull gently and slowly and it will slip right out.

1. Remove the air box.  Disconnect the throttle return spring, if there is one (usually not present).  Disconnect the throttle wire, the fuel line, and the fuel pump hose going to the crankcase.  Undo the nuts or bolts holding the carburetor to the engine.

2. Remove the carburetor and take it to a clean work area with plenty of lighting.  Remove the metering diaphragm cover (#5), the diaphragm, and the circuit plate (#48) which exposes the idle progression holes.  DO NOT USE ANY METAL TOOLS OF ANY KIND WHILE WORKING INSIDE THE CARBURETOR!  The soft metals used to manufacture carburetors are easily damaged, especially the openings into the throat which are of very precise shape and diameter.

3. Cut a piece of 0.5mm copper wire about 9mm long.

4. Make a loop with 5mm of wire at one end, as in the photo below.  The portion of the wire opposite the loop will be about 4mm long.  The loop in the end of the wire also aids in removing the wire with fine needle-nose pliers or tweezers if it is no longer needed.

WB-37 midrange modification

5. Put the wire through the idle progression hole closest to the center of the carburetor.  Orient the loop as shown here.  Your carburetor may have more holes than shown here.  Always use the hole closest to the center. 

WB-37 midrange modification

If the wire was cut the right length, it will stick into the throat of the carburetor about 1.5mm.  The view here is from the engine side of the carburetor. 

WB-37 midrange modification

6. Using a wooden object e.g. a toothpick, bend over the loop of the wire enough so that the wire cannot rotate.  This must be done to prevent the other end of the wire from contacting the throttle plate when it is opened. 

WB-37 midrange modification

7. Orient the carburetor so that airbox side is facing up. (Note: In the photo below, the engine side is facing up!)  Use a stick of wood or your finger to hold open the throttle.  Using the end of a toothbrush or a small wooden stick, push down firmly on the short piece of wire that is sticking into the throat of the carburetor.  The wire will bend sharply towards the engine end of the carburetor and keep it from coming out of the hole and then getting loose inside the idle circuit chamber.  A loose wire in there will not cause any harm but the midrange air/fuel mixture will be normal i.e. rich.  The bent over tip of the wire is barely visible in the photo below. 

WB-37 midrange modification

8. Ensure that the throttle plate does not contact the wire in the idle progression hole!

9. Reassemble everything, start the engine, and adjust the low speed system (go to this link for the WB-37).  You will probably also need to adjust the high speed system, as well.

10. Test fly the engine while carefully observing the CHT.  Most engines at sea level will be around 160-170ºC at full throttle and somewhat less when operating in the midrange.  At our altitude (4,500' MSL), the range is 140-160ºC.  Be alert to a quick increase of temperature or the engine having "hiccups".  These are signs that you are running too lean.  After the modification, the midrange operating temperature will increase.  During testing, if it ever starts climbing above 180ºC, STOP!  It is easy to overheat engines, even during midrange operation.  Different engines may react differently to this modification – and is another reason to be careful and patient when test flying the engine.  Remember that it is always better to be a bit rich in the midrange than too lean.

If the mixture is too lean i.e. the motor is running too hot, the wire can be removed and a razor blade (X-acto knife) used to scrape away 0.1mm or more from the side of the wire, as needed.  This will richen the midrange air/fuel mixture.

B. WG-8 carburetor

Pilots should not attempt any modification of the carburetor (or use an HA jet) unless they have a cylinder temperature gauge (CHT) installed.  It is easy to burn up an engine.  Most of the engines that are sent to us for repairs have been overheated!

The WG-8 has no easy way to plug one of the idle progression holes (IPH) like the WB-37.  Instead the throttle plate can be modified to lean out the midrange fuel mixture.  A downside of the modifying the TP is that the idle can become too high without a way of adjusting it.  However, plugging an IPH is easily reversible but you must have a supply of Welch plugs.  We stock the plugs.  Contact us to order.

Replacing the main jet in the WG-8 with a smaller one (or modifying it) should also be done if you are flying at high altitudes.  However, pilots do not (and should not) run at full throttle very often so tuning the midrange of the engine is more important.  Nonetheless, the main jet does affect the midrange as the throttle is opened further.

Plugging an IPH is NOT recommended on the Polini Thor 130 and 190 or other engines that are 130cc or larger..  These engines need a richer air-fuel mixture.

1. Plugging one of the idle progression holes

To plug one of the holes, the Welch plug must be removed.  (A replacement plug is included in most WG-8 repair kits.)  The photo below shows the idle progression circuit Welch plug (blue arrow).   The green arrow points to the brass cup plug that seals the hole to the nozzle jet assembly.  This plug must never be removed.  The OEM main jet (#116) has a piece of wire in it to effectively make the jet smaller for use at high altitudes.  This should be only a temporary fix until the correct jet size can be obtained.  It is so easy to overheat a 2 stroke engine and utmost care must always be observed.

WG-8 carburetor metering diaphragm side

Begin by following steps #1-#3 given in the TP Modification steps below (removing and preparing the carburetor).  Plugging an idle progression hole on the WG-8 is similar to how it is done on the WB-37.  Once the carburetor is removed/away from the engine, go to the Welch plug page and follow the steps for removing the Welch plug.

There are (4) idle progression holes (IPH) and a small passageway (just to the right of IPH #4) that goes to the chamber underneath the main jet and is the source of fuel for the idle circuit.  The IPH which should be plugged is the hole closest to the center of the carburetor (#4 in the photo below).

idlle progression well on the WG-8 carburetor

Hole #4 is 0.68mm in diameter.  A piece of 0.5mm (22 gauge) magnet wire works.  However, a single strand from a length of stranded #10 THHN electrical wire will completely block it.  A (1) foot piece of #10 can be easily purchased at Lowe's or any hardware store for less than $1.

Follow the steps for plugging an idle progression hole in the WB-37 above.  If things get too lean, it is always possible to use a smaller sized wire in the hole, such as the 0.5mm magnet wire or an even smaller wire.

The wire (black arrow) in its correct position in hole #4.  Changing the main jet to the HA size of 112 should also be done if flying at high altitude.

WG-8  modification of an idle progression hole

2. Throttle plate modification

The throttle plate modification will work on both the WB-37 and the WG-8 carburetors even though the throttle plates are different.  However, it is better to plug an IPH.  Only modify the throttle plate if you cannot obtain a new Welch plug to replace the one that must be removed.  It is a good idea to have some spare throttle plates on hand if you are going to implement this modification.

By modifying the throttle plate (TP), the air/fuel mixture can be leaned out in the midrange.  However, pilots should also lean out the main jet at the same time because the transition from midrange to full throttle is gradual per how the jetting works.

The photo below shows a WG-8 carburetor from the engine side.  The black arrow points to the notch in the TP that controls the amount of air going past the idle progression holes in the carburetor throat.  As the TP opens, the main jet begins to take over but fuel is also coming into the engine via the idle circuit.  The green arrow shows the brass ball used to plug one of the bore holes of the idle/low speed circuit.  The notch (red arrow) on the TP that is modified is the one opposite the plugged bore hole.  This notch does NOT have the number "305" between it and the center hole of the TP.

Walbro WG-8 from the engine side showing the throttle plate

This fix decreases the amount of air going directly over the idle progression holes and increases the air going into the engine as the TP opens.  Increasing the size of the notch will allow more air through the carburetor while lessening the venturi effect on the idle progression holes.  The net result is that less fuel and more air enters the engine at the midrange throttle setting, thus leaning out the fuel/air mixture.  Thanks are due to the kart racing guys for this way to modify a stock diaphragm carburetor.

Materials needed

For those who do not want to modify the throttle plate themselves, we can supply a modified throttle plate for some Walbro carburetors.  The cost is $25 with free shipping within the U.S.  Contact us to order.  Please tell us the carburetor model, and at what altitude you will be using your engine.  We can also supply unmodified throttle plates.

I installed the 112 jet, already had the throttle plate modified and took the first flight. It ran so much smoother at half throttle. Really big difference. CHT only got to 150 at full throttle. – K.R. Utah, Top 80 owner, high altitude

Note: There is no guarantee that modified throttle plates supplied by us will perfectly fix your midrange problem.  Much depends on the type of gasoline and oil used, what your exact cruising speed is, your weight, and the altitude where you fly.  However, there will be a great improvement, in any case.  It is always better to modify your own TP in an incremental fashion but it takes a significant amount of time to do it.

Modification steps

1. Remove the air box, if there is one.  Disconnect the throttle return spring from the throttle lever that is on top of the carburetor.  Disconnect the throttle wire and fuel line.  Undo the two carburetor screws or nuts.

2. If there is a choke, disconnect it from the carburetor and get it out of the way.

3. Slide the carburetor off.  You may be able to access the TP without having to disconnect the fuel line or the throttle cable.  Use a felt tipped pen (Sharpie) to mark the correct notch (the notch with the red arrow in the photo above).

4. REMOVE the idle speed adjustment screw (the screw with the cone) It must be completely removed in order to reseat the plate later.

5. Remove the screw that holds the TP to the throttle shaft and remove the TP.

6. Place the TP in a vise.  Sandwich the plate between two pieces of wood.  The TP can be on the side or the top, the latter makes it a bit easier to symmetrically enlarge the notch.  Do NOT clamp directly on the TP with anything but soft wood or you may bend the plate and ruin it.  It is a good idea to put some masking tape on the TP to further protect it from being bent when it is put in the vise.  Only clamp the TP just enough to secure it.  TAKE EXTREME CARE NOT TO BEND THE THROTTLE PLATE!

Walbro WG8 throttle plate

7.  The existing notch (marked with the pen) is cut at a slight angle and is not perpendicular to the plane of the plate.  The depth of the cut is the same distance from the perimeter of the plate.  For best results, modify the notch at the same angle.

8.  Using a round needle file (3.25mm), enlarge the notch.  Go SLOW and compare the values below with what you are doing.  Use a digital caliper to take the measurements.  Use a safety razor blade over the top of the cut and measure from the blade to the bottom.  The measurement is not from the tangent of the cut but it is good enough and provides a consistent reference point.

Because there are so many variables, use the starting cut value below and then test the engine.  It is a bit of trouble but you will be able to lean out the midrange mixture to exactly the right amount by this method.  If you make the cut too deep and wide the engine will have a runaway idle.  If this happens, carefully measure and record the cut depth and width.  Use another TP and decrease the cut dimensions by at least 10%.

Maintain the same cut angle (15º) as the original.  Smooth any rough edges on the notch with a flat needle file or a wire brush on a Dremel-type tool.  Use a scribe to note that the plate has been modified.  It is important to measure the plate carefully and then note the dimensions somewhere.  Every engine is a little different.  For different carburetors, a good approximation would be to start with 50% increase in the stock notch dimensions.

Here are some starting values for engines that use the WG-8.  At this time, we do not have sufficient data for the WB-37 but adding 20% to the values here might be a place to start.  Start with the lesser value and see what the slowest idle speed will be.  If you overdo things, you will have to start over with a new throttle plate.  As I have observed, doubling the dimensions results in a runaway idle speed (clutch engagement).

All altitude measurements are MSL.

Stock value of the notch (#305) opposite the idle progression holes/bore hole:  1.85mm wide x 0.75mm

Starting cut value: 2.75mm x 1.13mm

Polini Thor 100, 130
     Sea level:       best value not available yet
     3.5K' - 5.0K    2.5mm x 1.0mm

Top 80
     Sea level:      3.5mm x 1.5mm    Max is starting value +15% = 4.0mm x 1.8mm.  A greater value than this gives a runaway idle.
     3.5K' - 5.0K':  3.8mm x 1.5mm
     5.5K' - 8.0K':  4.0mm x 1.5mm

Pilots can be a great help to our flying community by sending us the values that worked with your engine.  Be sure to note the altitude at which you fly.

9. Replace the plate on the throttle shaft.  The numbers on the plate must face out and the "305" must be adjacent to the plugged bore hole (green arrow in the photo above) and NOT the open hole on the carburetor (the hole opposite the plugged bore hole).  Do not install the TP screw yet.  The plate will ONLY center fully in carburetor throat if the idle speed adjustment screw is removed.  In addition to the advice from Gerry Farell below, snap the throttle shut a number of times from just barely open to be certain that the throttle plate is centered in the throat of the carburetor BEFORE tightening the screw.  The TP can be rotated very slightly with a toothpick so that the notch is centered over the 1st idle progression hole.

"When you place the throttle flap back, ensure it closes completely before tightening it to its shaft, and don't push too hard while tightening it or you will bend the delicate shaft.  Add one small drop of [blue] threadlock to its central screw. Mount the carb on the engine and make sure the throttle cable permits the complete throttle assembly to open and return 'all the way' against the stop!" – Gerry Farell

10. Screw in the idle speed adjustment screw until the TP just starts to open.  This is an approximate setting.

11. Reassemble everything, start the engine, and adjust the low speed system (go to this link for instructions WG-8).  The more the midrange performance improves, the more chance that you will not be able to adjust the idle low enough to keep the propeller from spinning in a clutched engine.  If you cannot adjust the idle properly (too fast with the cone screw not contacting the throttle lever), get another throttle plate and start over with the notch dimensions 10% smaller.

12. Test fly the engine while carefully observing the CHT.  Most engines at sea level will be around 160-170ºC at full throttle and somewhat less when operating in the midrange.  At our altitude (4,500' MSL), the range is 140-160ºC.  Be alert to a quick increase of temperature or the engine having "hiccups".  These are signs that you are running too lean.After the modification, the midrange operating temperature will increase.  During testing, if it ever starts climbing above 180ºC, STOP!  Some engines, like the Minari, can tolerate much higher operating temperatures.

It is easy to overheat engines, even during midrange operation.

Different engines may react differently to these modifications – and it another reason to be careful and have extra throttle plates on hand in case you made a mistake.  Better to be a bit rich in the midrange than too lean.

Turkey Vulture