Modifying the fixed high speed jet in a 2 stroke paramotor

By Had Robinson
updated September 28, 2020

An annoying problem with small engine carburetors is adjusting the size of the fixed main jet for a particular altitude, temperature, humidity, and/or fuel and oil type.  With the method below, you can safely modify a jet of any type of carburetor to see which size works the best.  The downside is that it is difficult to know what the final and functional size of the jet is.  This is because the wire ends affect the flow of fuel through the jet but how much?

It is better to have a non-modified main jet for this reason.  Just unscrew the original main jet and replace it with the exact size you need.  We can supply any jet size required (they are custom machined in our shop).
An alternative way to adjust the high range is to change the pop-off pressure.  Changing the pop-off pressure by changing out the spring is difficult because the range of springs available is limited and accurately measuring and evaluating the force of the spring is not practical.  Adding shims under the spring is more predictable but the entire range of the fuel/air ratio is changed.  That is, the midrange may run too lean if the high range is leaned out and vice versa.  For these reasons, I do not recommend changing out or modifying the OEM metering lever spring or adding shims. 
This technique is an inexpensive but a less precise method to change the jet size will not harm the existing jet.  The modification can be reversed quickly with ordinary tools.  The only drawback is that you cannot increase the jet size, only decrease it.  But this is rarely a problem as main jets are calibrated for sea level and when we fly at higher altitudes the jet size needs to be decreased.  Operating in colder air requires a larger jet because of an increase in fuel viscosity.

Supplies needed

DO NOT ATTEMPT CHANGES TO THE MAIN JET UNLESS THERE IS A CYLINDER TEMPERATURE GAUGE INSTALLED ON THE ENGINE.  It is easy to burn up engines.  My workbench is littered with them.  This method is NOT precise and requires more vigilance per decreasing the jet size too far (and overheating the engine).

#30 or #27 AWG magnet wire insulated or non-insulated (the paint insulation will eventually be dissolved by the fuel running through it).  You can always use what you have on hand but you must have some means of measuring its diameter e.g. a digital caliper.  Any copper wire will do as long as you know its size and it is small enough.  Electronics supply stores generally have magnet wire in stock.  Ordinary lamp cord often has strands that are very close to the right size i.e. #27.
A jewelers loupe or magnifying glasses – you must be able to carefully see what you are doing.

Here is complete list of the cross sectional area of various wire sizes. Here are some samples of the cross sectional area of magnet wire in mm2:

#30 – 0.0509 mm2
#27 – 0.102 mm2 – This is the ideal size as its insertion in the jet will reduce the jet size by about 5 with the larger jet sizes (>100).
#22 – 0.326 mm2

1. Determine the jet size that you will probably need

This can be done using this chart.  For example, your existing main jet is a size #170 and you want to run your small engine at 4,500’ MSL 86ºF.  According to the table, the conversion factor is 0.94.  0.94 x 170 = 159.8    The nearest whole jet size would be a #160.  This is the approximate jet size you would need.

It is important that you carefully check whether your engine is running too lean after any modification of the jet size.  Use a CHT (cylinder head temperature) or an EGT (exhaust gas temperature) gauge.  The EGT is the best to use because of its quick response which will help prevent the pilot from leaning down the engine too far and burning it up.  However, it is difficult to install.  The CHT, on the other hand, is easy to install but it has a slow response and, accordingly, must be used carefully.  Be patient and NEVER do any radical changes to the jet size.  Only increase the load on the engine SLOWLY so the CHT can catch up.  The TTO Temperature Digital Gauge is an inexpensive CHT that will do the job.  It is available from Miniplane USA.  Running an engine too lean at full throttle can burn it up quickly.

2. Determine how many strands of wire are needed to decrease the jet size

We need to know the cross sectional area (CSA) of the original and the proposed jet sizes as this is what determines how much fuel flows through the jet.  (Refer to this this chart for the CSA of standard jets.)  When we know the difference between the two sizes, we can reduce the jet size by inserting strands of fine wire in the jet.  In the example above, we want to reduce the jet size from a #170 (2.27mm2) to #160 (2.01mm2).  2.27mm2 - 2.01mm2 = 0.26mm2.  #30 wire has a CSA of 0.0511mm2.  0.26mm2/0.051mm2 = 5 strands of #30 wire would be needed in the throat of the jet.

The addition of multiple strands of wire and the method to keep it in place will decrease the effective CSA of the original jet a bit more – about 15%. So, in the above example, try 4 strands of the #30 wire and see how the engine runs.

Generally, a single strand of #30 magnet wire will reduce the jet size by about 2 and #27 will reduce it by 5.

3. Add the wire to the jet

Carefully remove the main jet from the carburetor using a premium screwdriver that has a perfectly flat blade or use one that has small horizontal ridges up and down the tip, like Sears Craftsman screwdrivers.  Do NOT use the poor quality screwdrivers sold at auto supply stores.  Make sure the blade completely fills the slot of the jet (that it is exactly the width and length of the slot).  This way you will not strip the slot when you remove the jet.  Press the blade firmly into the slot and just turn it a little.  It should break free easily.  If it does not, apply a little WD40 or equivalent.  If you have a micro-torch, you can quickly heat the carburetor body for a few seconds.

Note which side of the jet housing has the most space.  Usually, the jet screws into a hollow space that has less clearance than the top of the jet housing.  However, with the Walbro, the diaphragm is right above the jet so the greater clearance is below the jet in the housing.

The arrow in the photo below points to the main jet in a Walbro. (Note that some Walbro's have an adjustable main jet, unlike here.) If you look carefully you can see the loop of magnet wire coming out of the top of the jet.

How to Modify Jet Sizes in Small Engines

Below is the main jet from the Walbro above with 2 strands of magnet wire.  The loop is just visible in the upper left and is folded down next to the jet.  If it had been only a single strand that was needed, there would be coils at both ends.  The coils (about 1 mm in diameter) are visible to the lower right.  They should be centered over the hole in the jet.

How to Modify Jet Sizes in Small Engines

Cut about 1½” of the wire needed so you have slack to work with.  If it is more than one strand, cut enough of the wire so that it will make loops that are about 1½” long as in the photo below. This modification required 2 strands of wire.  The loop end, in this case, will be placed at the particular end of the jet that has minimum space.

How to Modify Jet Sizes in Small Engines

If it is only a single strand, you must first make a coil at one end similar to the photo below by using a small drill bit (1/32” or smaller) or a toothpick.  If 2 wires are needed, you won't have to do this as the loop will keep the wire from slipping out at that end.  Now, feed the wire through the jet.  Take the wire that is now coming out and wrap it around the drill bit a few times.  You can work the wire so that the coils are tight and close to the jet opening.  Remove the bit.  In the photo below, there would ordinarily be a jet between the coils and the loop but it is not shown here for clarity.

How to Modify Jet Sizes in Small Engines

Use a very fine pair of wire cutters to trim all but 1 or 2 coils from the wire.

You’re done

CAREFULLY re-install the jet back into the carburetor.  The blade tip of the screwdriver can be manipulated around the magnet wire with care so that the blade does not cut the wire.  If necessary, use a slightly smaller screwdriver.  Make sure that the wire does not get caught in the threads while inserting the jet into the carburetor housing.
If you ever have to go back to the original jet size, it’s easy to fix.  If you make a mistake, just start over.
Test run the engine to be sure that you have not leaned out the mixture too far.  Be sure not exceed the CHT of the engine.