Carb Tuning the ALMOST Scientific Way - Oxygen Sensor!

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radare
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A few years ago, I built an A/f gauge for my '78 Yamaha XS400 to tune it's horrible Mikuni carburetors. I've used this setup on and off on the 400 with pretty good results. Last night, I decided it was time to adapt it to the XJ.

For starters, I welded an O2 bung onto each one of the exhaust pipes; this was done by cutting a hole and welding an adapter on. The silver you see is anti-seize grease that's spooged out.

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These bungs let me screw an O2 sensor into the pipes and measure the air/fuel ratio for each cylinder.

Once the bungs are in place, I used my A/F gauge to set the idle screws. I made the 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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To tune with it, I installed the O2 sensor in the #1 pipe and connected the gauge. I started the bike and let it warm up to operating temperature. I then turned the pilot screws until I was on the line between the rich-most stoic and the rich LED on the gauge. I then proceeded with #2, #3 and #4 in like fashion. I found that if the screws were set to the mid-stoic setting, the bike would become too lean just off idle as the airflow through the carbs changed; going more toward the rich side of stoic remedied this problem entirely.

After settling on the best settings, the #1 and #2 screws were set about the same, 1 3/4 turns out from closed. The #3 screw was tuned leaner, say 1 turn out and the #4 screw was tuned much more rich, nearly 2 1/2 turns out. 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.

Tomorrow, I'll pull the tank and VERY precisely sync the carbs to see if the results can be even better. All-in-all, however, this is a much recommended way to tune your carbs if you have the ability to weld bungs to your exhaust header.

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