How-to: Tuning Carbs with an Oxygen Sensor and A/F Gauge

Routine maintenance for your XJ600's engine.
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This guide outlines the procedure for tuning carburetor pilot jets using an O2 sensor. This procedure requires that O2 sensor bungs be welded to the exhaust header to allow for the O2 sensor.

What You'll Need:
  • Four (4) weld-on O2 bungs with plugs
  • Small flat-blade screwdriver
  • For narrow-band:
    • Heated oxygen sensor (used is alright)
    • Autometer A/F gauge
    For wide-band:
    • AEM Wideband A/F gauge with sensor
Install the O2 bungs:
Before removing the exhaust, mark the locations of your O2 bungs. They should be in an area where they will not be easily damaged but where they are accessible. There should be sufficient clearance between them and the frame. There should be enough clearance to allow you to install & remove the O2 sensor and the O2 plugs. I located mine near the factory plugs. Mark their location with a Sharpie and remove the exhaust.

Using a 1" hole-saw type drill bit, drill a hole for each bung. A 1" hole is nearly as large as the header tubes so be very careful when drilling. Also, ensure your drill bit isn't long enough that it'll go through the other side of the tube. Once the hole is drilled, clamp each bung to the header and weld it in place. Your welds should be air-tight. If you do not have access to a welder, take your header and bungs to a custom exhaust shop and have them welded on.

Here are my O2 bungs welded in place on my black '92. You can seed the location and orientation I chose:

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Here are the O2 bungs welded in place on my stock red '92. I moved them more forward to allow better clearance with the frame:

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Choose an O2 Sensor/Gauge:

A note on Narrow Band vs Wide Band O2 Sensors
(Taken from Autometer's Website)
Narrow Band O2 Sensors began to appear on vehicles with the advent of fuel injection in the 1980’s. Their purpose was to monitor component degradation (i.e. fuel injectors, vacuum leaks) of vehicles as they accumulated miles. Their basic job was to let the computer know whether the vehicle was running at an air/fuel ratio of 14.7:1 under idle (ideal ratio for gasoline engines), moderate acceleration, and cruise conditions, and if it wasn’t, to “trim” the injector pulse-width to either slightly lean or richen the engine. When the computer is paying attention to the input from the O2 sensor, the engine is operating in a “closed-loop” capacity. Under heavier acceleration or wide-open throttle the computer ignores the O2 sensor because it requires an air/fuel ratio other than 14.7:1, which is outside the design parameters of the sensor. This is known as “open-loop” operation. The sensor lets the computer know if the engine is running above or below 14.7:1 by sending voltage to the computer in a range between 0 and 1 volt, usually sweeping between the two extremes of this scale. Auto Meter’s traditional narrow-band air/fuel ratio gauges are simply a voltmeter for this signal. This can be seen by the repeated sweeping back and forth of the gauge in most idle, light throttle, and cruise conditions. To summarize, a narrow band O2 sensor is only able to tell a computer (or gauge, for that matter) whether an engine is operating above or below a 14.7:1 air/fuel ratio.


Wide Band O2 Sensors where developed in the early ‘90s as vehicle manufacturers began looking to obtain air/fuel ratio information under all circumstances. This ranged from WOT to varying ratios, for example running air/fuel ratios leaner than 14.7:1 under cruise conditions. Volkswagen and Honda pioneered the development of the wide-band O2 sensors to provide accurate air-fuel ratios under these varying circumstances. They did this by broadening the voltage range in which feedback from the sensor was provided and making a linear scale that provided a fixed voltage that correlated to a specific air/fuel ratio.


High performance vehicle tuners discovered that wide-band O2 sensors are very helpful when accurate air/fuel ratio readings are required to maximize power, reliability, and mileage on modified vehicles. With these benefits in mind and to meet consumer demand, Auto Meter has created a wide-band air/fuel ratio gauge/sensor to augment our popular narrow band, gauge only offering. This unit uses the Bosch LSU 4.2 sensor that is known industry wide to be both accurate and durable, along with an advanced control unit that is encased entirely within the gauge head. Further, the gauge head utilizes both an LED readout, for accuracy, and a radial display for ease of reading. Combined with user definable stoichiometric ranges and voltage outputs, Auto Meter’s wide-band gauge creates a useful and greatly needed tool for the tuning market. As an added bonus, Auto Meter will be offering these gauge kits in our most popular gauge lines.


Narrow-band O2 Sensor:

If you are interested in tuning your Seca II's pilot jets, at idle, with the throttle closed, a narrow-band O2 sensor will get you close and will save you the expense of a wideband O2 sensor. Note: When using a narrow-band sensor, a 4-wire sensor should be used so that you get the built-in heater.

Build the harness for the A/F gauge and O2 sensor:

The O2 harness consists of the A/F gauge, wiring and an O2 sensor. When choosing an O2 sensor, look for a 4-wire (heated) sensor. If you get one from a local pull-it yard, be sure to snip off and take the O2 connector as well.

Wire the A/F gauge according to the following diagram. The two white wires that run out of the O2 sensor are for the heater. Connect one white wire to 12V + and connect the other to ground. The grey wire coming out of the O2 sensor is ground and the black wire is signal:

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I made my narrowband gauge using an Autometer A/F gauge from craigslist, a heated O2 sensor from a late model Ford Focus ($4 from the local pullnsave) and a pair of eyelets to connect it to the battery. If you mount your O2 bungs closer to the cylinder head, you may get away with a non-heated sensor.

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Wideband O2 Sensor

If you wish to monitor the A/F ratio across the throttle band, if you wish to datalog your A/F data or if you simply want a better, easier to use and more accurate option than the narrowband O2 sensor, a wideband sensor is the way to go.

AEM makes a good sensor that comes already assembled with harness and include a new wideband O2 sensor. I found mine on EBay for around $150 USD.

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Tuning the Carburetors:

I have used both a narrow-band and a wide-band O2 A/F gauge to set the idle screws. I prefer the wideband sensor to the narrowband sensor and recommend it if you can afford the expense. The following procedure shows the process with a wide-band sensor. From experience, I have noted that the bike likes to be slightly rich at idle. When tuning, an A/F ratio of 13.8 to 14.7 is ideal at idle.

To tune with O2 sensor, Install the O2 sensor in the #1 header and connect the gauge to the battery. This will power the gauge and will also power the sensors heater. Start the bike and let it for 10 minutes to warm up; if it will idle and accept throttle without grief, its warmed up.

Note the reading on the A/F gauge. If it is reading a number lower than 14.7 (rich), you will need to screw the pilot jet for that cylinder's carburetor inward to lean it up. If it is reading a number higher than 14.7 (lean), you will need to screw the pilot screw for that cylinder's carburetor outward to richen it up.

Here is a short video showing the wideband gauge reading the A/F ratio of the #1 cylinder of my stock 1992 Seca II at idle. This is with the stock pilot-screw setting. The initial reading was so lean that it was right at the edge of the sensor's range, about 17.8 to 18. Also note the sound of the bike's idle:

[youtube]2DG4h-NcIM8[/youtube]

To remedy this, I backed the pilot screw out slowly while monitoring the gauge. My target A/F ratio is in the high 13's to low 14's. Here's the same #1 cylinder after adjustment. Note that the bike's idle already sounds better:

[youtube]nEtRv2RdOVs[/youtube]


Once you have cylinder #1 adjusted, proceed with #2, #3 and #4 in like fashion.

Here are my #2, #3 and #4 cylinders before and after tuning:

#2 Cylinder: Initial reading slightly lean.
Before:
[youtube]vgFuoa02tNI[/youtube]

After:
[youtube]dv1oithf94w[/youtube]

#3 Cylinder: Initial reading frighteningly lean
Before:
[youtube]DxG0TmIw8XI[/youtube]

After:
[youtube]2BNQCWVhWIA[/youtube]


#4 Cylinder: Initial reading slightly lean
Before:
[youtube]A5GKGq4iYCg[/youtube]

After:
[youtube]2K647oshVcE[/youtube]

After tuning, the bike's idle is now at the best it's been since I've owned it. It's quite smooth and steady with no more jumping around. Here's a short walk-around vide post tuning. Note the consistency of the idle.

[youtube]VpT0YeQScoY[/youtube]

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