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Midrange performance problems in paramotors

by Had Robinson


Most paramotors run roughly in the midrange (4.5K-7.5K RPM) due to engine misfire from an incorrect 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 greatly effected.  For example, a tuned Thor 130 will cruise (with a quality paraglider) straight and level at less than 0.5 gal/hr.  Untuned, this value can double.

Thankfully, this problem does not harm the engine.  The paramotor manufacturers use stock Walbro carburetors that have been tuned in the midrange for chainsaws not paramotors.  There is not some 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 at midrange operation.  The problem is fixed either by increasing the amount of air entering the engine through the midrange (throttle plate modification) or by decreasing the amount of fuel entering the engine through the idle progression holes in the carburetor throat.

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 because it is much easier to understand than the Walbro SM.  The Zama is a Chinese knockoff of the Walbro.  Here are the parts  diagram of the WG-8 and the parts diagram of the WB-37.

A. WB-37 carburetor

The WB-37 can be modified in two ways.  The first way is to enlarge the notch on the throttle place but this is a more technical modification that cannot  be undone without replacing parts.  However, this way may lean things out over a broader range.  To do it this way, see "WG-8 carburetor" below.  This way works by increasing the amount of air going past the idle progression holes in the carburetor which, in turn, leans out the midrange.  Note: The throttle plates are different in the WG-8 and the WB-37.

The second way to lean out the midrange is to plug one of the idle progression holes (red arrow in photo below) with a piece of 0.5mm copper wire.  This is a quick and easy way to lean things out in the midrange.  However, you will not be able to adjust midrange air/fuel mixture as precisely as with the first way.  The photo below is of a WB-37 with the circuit plate removed with the piece of copper wire in the idle progression hole that is closest to the metering lever.  Make sure you use this hole.  Photos courtesy of Fresh Breeze.

Walbro WB37 carburetor idle progression holes

In this photo, the small piece of 0.5mm copper wire is visible that has plugged the idle progression hole closest to the center of the carburetor.  The wire must be long enough (5mm-8mm) so that it does not fall out of the hole when the engine is running.  The small coil in the end of the wire gives a way to remove the wire if it is no longer needed.  Copper magnet wire or a single strand from some electric wire that has the correct diameter works well.

Walbro WB37 carburetor idle progression holes

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.

For those who do not want to modify the throttle plate, 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.  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 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

Screw in the idle speed adjustment screw five (5) complete turns.  This is an approximate setting.

10.  Reassemble everything, start the engine, and adjust the low speed system of WG-8 or the WB-37.  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.

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 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.