How to Modify Jet Sizes in Small Engines
By Had Robinson
An annoying problem with small engine carburetors is adjusting the main
jet size for a particular altitude and temperature. Often enough, no
one knows where the jets can be obtained or who makes them? Calls to
the factory are useless. If you own a Walbro, you are pretty much
out of luck. Bing, thankfully, has easy access and sources for different
main jets and they can be precisely adjusted throughout their range in
ways unknown to Walbro.... Anyway, do you want to spend $50+ dollars on a collection of
jets you will not likely use, even if you could obtain them? With the
method below, you can safely modify a jet of any type of carburetor to see which size works the
best. Then, if the jets are available, you can buy what you need
and nothing more.
An alternative way to adjust Walbro type carburetors is to change the
pop-off pressure by changing the pop-off spring. This technique
works very well and is much preferred to snipping or stretching the
pop-off spring which is not recommended. If you want to change the
high speed fuel mixture this way, Mike Forbes has written an excellent
to" including a source for a pop-off spring kit.
My technique is an inexpensive, easy, and practical method to change
the jet size (a KISS technique) with reasonable precision that will not harm the existing
jet. It requires a fraction of the time and expense of modifying
the pop-off pressure. The modification can be reversed in less
than a minute with ordinary tools. The only drawback is that you cannot increase the jet size but
this is rarely a problem as main jets are calibrated for sea level.
#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. 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.
A jewelers loupe or magnifying glasses you must be able to carefully
see what you are doing.
For a complete list of the cross sectional area of various wire sizes,
Here are some coss sectional areas of some sample 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
Determine the jet size that you will
probably need. This can be done using the chart found
example, your existing main jet is a size #170 and you want to run
your small engine at 4,500 MSL 86ΊF. According to the table,
the conversion factor is .94. 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
CHT is the most reliable for figuring out what jet size to use but
you MUST be patient and NEVER do any radical changes to the jet size
and, additionally, only increase the load on the engine SLOWLY so
the CHT can "catch up". We sell a rugged and reliable CHT for $99
-- contact us if you wish to order one. I have personally used
Alex's technique and it works very well. You can also check the spark plug (it should have a
light brown color and not a white or grey color) but I would
strongly recommend that you install a CHT as it is the only reliable
and sure way to check the air/fuel ratio on your engine. Running an engine
too lean at full throttle can burn it up quickly!
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
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
2.27mm2 - 2.01mm2
= 0.26mm2. #30
wire has a CSA of 0.0511mm2.
= 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.
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
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 Walbros 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.
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.
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.
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.
Use a very fine pair of wire cutters to trim all but 1 or 2 coils
from the wire.
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
If you ever have to go back to the original jet size, its 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. The only way to be sure is to install a CHT on your
engine. We sell a rugged kit that will do the job -- contact
us for details.
If this has been helpful, let us know at: