Lean air fuel mixture

[rbradley]

Hi All
I have a question regarding a lean air fuel mixture at high speed (over 3000rpm) under load.
My newly installed air fuel gauge registers very lean at this speed. Otherwise the car runs
well with good driveability.
The set up is as follows
1.timing at 38 degrees
.linkage as per BBB
The gauge suggests rich at idle and balanced at light load.
I have tried adding a shim to the barometric compensator which gave me approximately three quarters of a turn. This has done little for the high speed mixture but did richer the idle.
The car runs well shoul I just ignore the gauge and monitor the plugs?
Suggestions would be appreciated.

[Tyler S]

What is your A/f reading?
And also where in the exhaust stream do you have the wideband sensor installed?
And most importantly, is your exhaust completely sealed upstream of the sensor?

[rbradley]

Hi Tyler
The sensor is located in the down pipe just below header. All gaskets fresh no leeks apparent.
All replaced after head overhaul. The gauge reading goes from 10 to 20 with 20 being the leanest.
Idle is between 10.5 and 12.5. High speed under load is 19.5.
I have no benchmark for this gauge but I know from the split linkage test that idle is a little rich.
The temp is normal and the engine sounds great and has good pick up now that I richend the mixture
using the BC.
Theplugs are a nice caramel colour but the mileage is very low..

[Tyler S]

19.5 under load is very lean. Im surprised it doesn't have hesitation issues. Good numbers should be around 12.5- 13 at idle. 14.5-16 at light cruise. And 10-12 under full load. Was the sensor/gauge calibrated? Depending on the type, there should be a calibration procedure.
If shimming the baro richens across all ranges and improves your under load readings, then you can go back and adjust the idle settings (knob at
back of IP) to bring idle specs back.- DO NOT adjust with engine running!!
If shimming only has an effect on idle and partial load but the full load doesn't improve, then you either have a fuel delivery (pressure or flow) issue or the injection pump's full load adjustment/setting has an issue.
Also be sure your linkage is set up correctly.

[Benz Dr.]

This system is supposed to run rich at idle, lean at mid range without load, and rich under full throttle or higher engine speeds. All of this, or at least most of it, is done right at the throttle valve with the way the linkage operates. The throttle valve opens faster during mid opening than it does coming off idle or approaching WOT. This quicker opening during mid range allows more air to enter the intake manifold thereby leaning out your mixture while cruising down the road a 60 MPH.

At WOT you need a rich mixture to protect your pistons from overheating and burning up. This system took into account every input consideration imaginable and responded with a very cutting edge answer in its day.

[rbradley]

Well, what a difference a day makes. I added a six thou shim to the BC and the numbers improved
immediately. After balancing the ip and air screw I have AF 12.5 at idle, 13.5-14 mid range and
10.5-12 at 65 mph cruise.
The gauge was out of the box so no calibration by me. I will leave it connected an monitor thing
including plugs. At least it is not cruising at max lean as before.
Thanks Tyler and Benz Dr.

[Tyler S]

You may want to put a few miles on it and then go back and lean it out slightly. 14.5-16 or so during light cruise is acceptable.
Pull your plugs again after 100 miles and give a look.