How to build an air/fuel
guage for tuning carbs:
| Back in the days
when I was constantly converting dual Webers and Dellorto's for
use on Harleys, I bought a little gizmo from
K&N for about $350. It was a tiny plastic box with 10 little
lights (LEDs) and 3 wires, a positive(+), a negative(-) and a
wire that connected to a oxygen sensor (Lamda sonde) wich screwed
into a fitting in one of the exhaust pipes. The idea is that when
5 of the 10 lights are burning, the mixture is perfect*; less
then 5 means not enough fuel/ more than 5 means too much fuel.
*About 14 parts air to 1 part
|| Being the curious
guy that I am, I couldn't resist taking it apart to see what was
inside: not much! I am definitely not an electronics-expert but
I was able to build a working copy for about $20, and in the next
few weeks I will tell you how. As the pictures show I mounted
my homemade-version on the carb itself, so while riding I could
read my mixture at any rpm. Once the carb is jetted correctly,
you can take it off or leave it on to make people curious as to
what the hell it is;-)
| The most difficult
part to make was the print (03), all other parts can be easily
bought at Radioshack.
The guage you see in the photo's is the one I made myself, an
oxygen-sensor can be found at any scrapyard, most cars with fuelinjection
have one attached to the exhaust header(s). Use a small sordering-iron
to avoid damaging components. Part 02 & 06 are IC sockets, these
can be soldered onto the printboard,so that part 01 & 05 can be
clicked into place and remain replaceable without soldering.
02-20 Pin IC Socket|#:900-5752|$0.56
04-Capacitor 0.047 MF|#:900-7222|$0.49
05-Display Driver LM3914|#:900-6840|$2.85
06-18 Pin IC Socket|#:900-5751|$0.56
07-Resistor 2.2 K Ohm|#:900-0226|$0.07
08-Resistor 5600 K Ohm|#:900-0307|$0.07
(The part# are from Radioshack,
|| How to identify
red, red, red, gold
=2200 Ohm (5%)
1/4 Watt Carbon film
green, blue, green, gold
=5600.000 Ohm (5%)
1/4 Watt Carbon film
How does it work???
I am no electronics-expert myself, but I'll try to explain what this little
box does: actually it is nothing more than a volt meter, the more volts
coming from the Lambda-sonde the more lights go on.
0.45 volts is supposed
to be a perfect mixture.
The resistors determine the measuring range (how many volts trigger how
many lights) The capacitor reduces flickering of the lights, by "smoothing"
the input signal. The display driver does everything else.
As a matter of fact you could probably get pretty good results with just
the Lambda-sonde and an accurate voltmeter, but this would look less cool....
Where do I start???
1) Find a lambda-sonde at the junkyard (single wire type) and find a
nut it will screw into. The lambda-sonde in the picture is from a 1997
Suzuki Baleno 16v 1.3i, which was the smallest one I could find at my
local junkyard. Lamda-sonde's (or oxygen-sensors) can be found in the
exhaust pipes of most cars that have fuel-injection. A brand-new one
costs about $45 here in Holland. Drill a hole into one of your dragpipes
about 12 inches from the cylinderhead, and weld on the nut. Try it with
a rusty old pipe first to see if it works, before screwing up those
brand new pipes!
2) Make a printboard
looking carefully at the pictures on this page; this is the most difficult
part and I had a friend with electronics experience help me with this.
You buy a piece of print board with one side completely covered with
copper, mask off the places where you want to keep the copper and then
with acid dissolve the rest of the copper. Another possibility is using
predrilled printplate with a little copper "island" for each hole. I
will make a new print for the purpose of photographing it soon
3) Carefully, with the
smallest soldering-iron you can find solder on the components. Solder
on the IC-sockets without the bar graph and the display driver in them,
because the heat would damage the parts. Also solder on the 3 wires.
Plug in the display-driver and the bargraph carefully, look at the pics
because neither will work when plugged in upsidedown.
4) Take the bike for
a spin untill it is warmed up and ofcourse without using the choke test
your work by connecting it to the + and - of your battery and to the
5) If you doubt the results
compare the readout in a garage with the results from a professional
mixture-analyzer. Some may give 4 lights for a perfect mixture, some
6 because of slight variations in the accuracy of the resistors. Remember
it is better to have a mixture that is a litte bit to rich than one
that is to lean, so stay on the safe side with at least 5 lights burning!
6) Make a nice housing
to keep the electronics dry and vibration-free. I used a lump of solid
plexiglass and a vertical milling machine, but you can also buy a universal
housing at an electronics store.
7) Good luck!
||16.2 to 1
||15.7 to 1
||15.2 to 1
||14.7 to 1
||14.2 to 1
||13.7 to 1
||13.2 to 1
||12.7 to 1
||12.2 to 1
||11.7 to 1
| Red wire = +, Blue
wire = -, White wire goes to sensor
12 November 2000, some
- The Lambda-sonde starts
giving an accurate reading at 600 degrees Fahrenheit (=315 degrees
- A Lambda-sonde is also
sometimes called an Oxygen-sensor
- The Lambda-sonde works
best with unleaded fuel
- If you don't want to
build it yourself, car-versions are available starting at $30: