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Midrange performance tuning for paramotors

by Had Robinson

Introduction

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 like the Ozone) 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 in any way.  The paramotor manufacturers use stock Walbro carburetors that have been tuned in the midrange for chainsaws not paramotors.  There is not some easy method to adjust the midrange on these carburetors, unlike some other brands, like the Bing.  Most paramotors use either the Walbro WB-37 or WG-8 carburetor.

The issue is in the idle/low speed circuit of the Walbro carburetors 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 (throttle plate modification in the WG-8) or by decreasing the amount of fuel entering the engine by plugging one of the idle progression holes (the WB-37).  It is easier to plug an idle progression hole but these are only accessible in the WB-37 so this cannot be done in the WG-8.

Replacing or modifying the main jet with a smaller one should also be done if you are flying at high altitudes.

Study the Walbro service manual (SM).  (Note: it is a very large file and should be downloaded first and then read.)  Pilots might want to also study the Zama technical guide (a Chinese knockoff of the Walbro) because it is much easier to understand than the Walbro SM.  Here are the parts  diagram of the WG-8 and the parts diagram of the WB-37.

DO NOT MAKE ANY CHANGES TO THE CARBURETOR UNLESS YOU HAVE INSTALLED A CHT AND LEARN HOW TO READ IT.  IT IS EASY TO BURN UP THE ENGINE.  BE EXTREMELY CAREFUL.  NOTE: MODIFICATIONS TO THE CARBURETOR WILL VOID ANY WARRANTY BY THE ENGINE MANUFACTURER AND THEIR U.S. DISTRIBUTORS.  USE ONLY PREMIUM 100% SYNTHETIC OIL AND ETHANOL-FREE GASOLINE/AVGAS.

A. WB-37 carburetor

The WB-37 midrange can be leaned out by plugging the correct idle progression hole (red arrow in 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.

The throttle plate can also have a round notch made that will increase the amount of air going into the engine at midrange.  The notch must be perfectly aligned with the progression holes.  However, at this time I do not have any data on the exact shape and size of the notch.  The notch would be approximately the same as for the WG-8.  If anyone wants to try it, we can supply the blank throttle plates.  Contact us to order

Materials needed

A few inches of 0.5mm copper wire.  We can supply 0.5mm wire and other sizes, if needed.  Contact us to order.

Installation

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. 

WB-37 midrange modification

If the wire was cut the right length, it will stick into the throat of the carburetor about 1.5mm. 

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

8. Orient the carburetor so that airbox end is facing up. (Note: in the photo below, the engine end 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

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

10. Reassemble everything, start the engine, and adjust the low speed system (go to this link for the WB-37).

11. 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.  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.  We can supply a 0.25mm wire, if needed.

B. WG-8 carburetor

The WG-8 has no easy way to plug one of the idle progression holes so the throttle plate must be modified to lean out the midrange fuel mixture.

This will work on both the WB-37 and the WG-8 carburetors even though the throttle plates are different.  It is a good idea to have some spare throttle plates on hand if you are going to implement this modification.  Pilots should not attempt this modification (or use an HA jet) unless they have a cylinder temperature gauge (CHT) installed on the engine.  It is easy to burn up your engine.  Most of the engines that are sent to us for repairs have been overheated!

By modifying the throttle plate (TP) at the right location, the air/fuel mixture can be leaned out in the midrange.  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 on the throttle plate must be adjacent to this bore hole and NOT to the open hole on the opposite side of the carburetor.

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 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.

Materials needed

(1) WG-8 throttle plate

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 for most Walbro carburetors.

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. I'm really grateful for all your assistance. -KR 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.

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.

Installation

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

3. Slide the carburetor off.  You usually will be able to access the TP without having to disconnect the fuel line or the throttle cable.

4. Unscrew the idle speed adjustment (the screw with the cone) six (6) complete turns.

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.  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.

Walbro WG8 throttle plate

7.  There are two notches on the TP.  The notch that is just to the lower right of the TP number (e.g. 305) is the one that will be modified.  The existing notch is cut at a slight angle and is not perpendicular to the plane of the plate.  The cut is more shallow on the side opposite the number.  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.  Because there are so many variables, use the starting value below and then test the engine.  It is a lot 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, you will not be able to adjust the idle speed.  If this happens, carefully measure the cut size, use another TP, and decrease the cut by 10%.  You must monitor your engine temperature at all times.

The depth should be measured from the shallower side of the notch.  Maintain the same cut angle (15º) as the original notch.  Smooth any rough edges on the notch with a flat needle file.  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 notch.

Here are some 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 good start.

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 modified notch must be adjacent to the plugged bore hole(green arrow in the photo above) and NOT the open hole on the carburetor.  Do not install the TP screw yet.  Make certain the idle speed adjustment screw was backed out (6) turns. The plate will ONLY center fully in carburetor throat if the idle speed adjustment screw does not touch the throttle lever!  In addition to the advice from Gerry Farell below, snap the throttle shut a number of times from 1/4 open to be certain that the throttle plate is centered in the throat of the carburetor BEFORE tightening the screw.

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 five (5) complete turns.  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.

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.  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 throttle plate 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.

Thanks are due to the go-kart guys, Gerry Farell, and Scott Travers for their help with this issue.

vulture