Copyright WWW.DBBP.COM | updated 12-November-2000

How to build an air/fuel guage for tuning carbs:
Illustration by Duckman

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 fuel.

click to enlarge

click to enlarge 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. click to enlarge

01-Bar graph array|#:900-6147|$2.21
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, total:$6.81)

How to identify resistors:

Part 07:
red, red, red, gold
=2200 Ohm (5%)
1/4 Watt Carbon film

Part 08:
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???

Lambda-sonde 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 lambda-sonde

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!

of lights
  Small enough to fit anywhere
1 16.2 to 1 Lean
2 15.7 to 1  
3 15.2 to 1  
4 14.7 to 1 Perfect
5 14.2 to 1 Safe
6 13.7 to 1 Safe
7 13.2 to 1  
8 12.7 to 1  
9 12.2 to 1  
10 11.7 to 1 Rich

click to enlarge

Red wire = +, Blue wire = -, White wire goes to sensor

click to enlarge

12 November 2000, some extra notes:

  • The Lambda-sonde starts giving an accurate reading at 600 degrees Fahrenheit (=315 degrees C)
  • 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: