Air Fuel Mixture adjustment Using LM-2 O2 sensor, MPS Adjustment Options

[Steve_R]

So now that I have about 1,000 miles on my car it’s time to adjust the air fuel ratio (mixture).
It generally runs good. However, it is a little rough before the engine is fully warmed up. Also, when I took it in for the initial wheel alignment the mechanic told me he thought it was running a little lean.

My plan is to use a wideband oxygen sensor to measure the AFR (air/fuel ratio) and adjust the MPS (manifold pressure sensor) to achieve an AFR of 12.5 at full load or WOT (Wide Open Throttle).

I purchased a new Innovate Motorsports LM-2 Air/Fuel meter on ebay for $300.00. I originally wanted to log the results on the LM-2 SD card , however I could not get the LM-2 to log data on the supplied SD memory card. After a few calls to their tech support I was able to connect a laptop to the LM-2 via the supplied USB cable. The laptop then displayed the data (AFR) and record it to a file. I also wanted to record RPM but I could not get it to read an accurate value. The LM-2 would be a great unit if it only worked. Also, the customer support and manual are not good. In the end, I was able to get it to log the AFR which was what I really needed.

I drilled a hole in the exhaust muffler collector and welded in the O2 sensor fitting. It was cheaper to weld in the fitting (since it came with the kit) than buy the exhaust clamp for 80 bucks. Since this is not a permanent install I just ran the wires into the engine compartment and thru the engine lid, thru the passenger window to the passenger seat. The oxygen sensor is connected to the LM-2 and the LM-2 is connected to the laptop via a USB cable.
The laptop is running the supplied Logworks software and can display (and record) the AFR in real time on a graph as well as a digital gauge.

Once I got the Logworks software to log the AFR results real time, I made some initial baseline runs up a long hill to get a steady AFR plot at full throttle or WOT.
The baseline WOT AFR was about 13.5 which means it was running lean.

Next, I removed the MPS from the car and removed the epoxy plug covering the Full Load Stop screw. There are two adjustment screws, one inside of the other. I needed to adjust the inner screw without moving the outer screw. Luckily, I was able to turn the inner screw without turning the outer screw. If the outer screw turns there is a tool available from Tangerine Racing that allows you to turn either screw. I initially loosened the inner screw CCW 90 degrees and made another run. It helped but the AFR was still high. I then turned it another 90 degrees tor a total of 180 degrees. The WOT AFR was right on 12.5 and the car ran so much better. It was a lot smoother and the roughness when the engine was cold was gone.