Wide band O2 sensors are here and we no longer have to depend on seat of the pants or plug readings to obtain the correct air fuel ratio. Lets assume those reading this are aware of how to tune thier carbs idle, part throttle and power circuits, as it varys by carb.

Let's discuss what numbers we should be tuning to achieve. Most are thinking, duh, 12.5 power circuit, 14.7 cruise (stoichiometric). Well, it aint that easy. Those air fuel ratios were at one time suggested for performance cars operating on gasoline. The 10-15% Ethonol they add to the fuel changes that, and depending on whose chart you believe,the target is now about 14.08 to 1 stoichiometric at cruise. So we think, tune to 14.08. But wait, the current A/F wide band meters really read lambda and interpret this into a A/F ratio for the fuel selected in the gauge. My Innovate MTX-OL guage can be set for gasoline, alchohol, LP, ect, but not specifaccly for 90% gasoline, 10% ethonol. So we know these guages determine Lambda, regardless of fuel type. So with a guage set for gasoline, and you run 90/10 pump fuel, the guage will still show 14.7 relating to Lamdba of 1.

SO know we know the ratios have changed with pump gas, and the guages arent perfect. But nobody is offering much to what to tune to. Seems like the old timers dont like the guages as they "know" what the engine needs by watching it run. And that may be, but for the rest of us this tool should help us get close to the right A/F ratio without years of testing. I was surprised to see my 1968 Q-jet putting down 15.5 to 16.5 to 1 A/F ratios at cruise with stock rods/Jets. However, since the fuel has less BTU's in it now with the 10-15 Ethonol, thus it gets less gasoline, it makes sense with the same jetting it would be lean.

What are you setting up your carbs to do at part throttle cruise A/F ratio wise with pump gas? How lean is too lean at cruise?

F/A ratios will be different for different builds, a 10.5-11:1 compression ratio will need a richer setting at power than an 8:1 ratio car. Mine pings at 12.5 power pulls so I've got it at 11.9 - 12.1.

Some of the Q-jets had APT (at part throttle) adjustments in the front of the base plates that were sealed OFF at the factory once set per application.

Cliff sells an adjustment kit for them and detailed instructions in his book on how to adjust the APT.

With today's gasoline, the old way of looking at plugs to determine rich/lean running is not accurate.