Hello fellow Datsun owners.

I am trying to help my father get his 1986 720, Z24 standard cab 2 wheel drive, Calif p/u to pass Calif. smog inspection.
He has owned this truck for 12+years and has not had trouble passing smog until 2 years ago. At that time when it failed (High CO I think) he was told that the truck was running rich and he needed a new catalytic converter. He replaced the Cat and the truck passed. He had the Cat outfitted with flanges so that it could be removed easily and replaced with a straight pipe. I know, I know...not the right thing to do...but he did it anyways. So the truck was driven in town only for the next 2 years until it was time for smog testing. The Cat went back on and the truck fails. The carbon monoxide (CO) measures 2.96% at idle and 4.00% at speed. The smog shop said he need a new carburetor and quoted him $700.00 bucks plus installation. So my drops the truck off with me (300 miles north of where he lives) and he takes my 2001 Altima for transportation.
So here is what I have done so far:
Replaced plugs (fuel fouled) wires, cap, rotor, air cleaner, thermostate (180 degree F) oil and filter, and all small vacuum lines, verifying their routing with the under hood sticker and Factory Service Manual (FSM).
I then took it to a local shop for a pre-test and it failed again. High CO again.
So I then started testing the various sensors and switches connected to the ECU. My results were:
Neutral safety switch – Fail – Nissan dealership says this is only used for the “starting circuit”. “If the truck starts then don’t worry about it”…I have my doubts. My concern here is that the switch is functioning completely opposite of how the factory service manual states it should work. Essentially, when the trans is in gear, the switch should be open and it tests closed. When it is in neutral, it should test closed and it tests open. So what I am not totally sure about is when the truck is sitting at idle for any period of time, I think air/fuel is controlled “open loop”. When the RPM increases, it changes to a closed loop system. If it is running “at speed in neutral” and the neutral safety switch is backwards, does it think it is in gear and therefore dumping more fuel???And if so, why am I unable to see any changes to the dwell meter??? Totally frustrating. I have looked for a replacement switch with no luck
Clutch disengagement switch – Pass
Temp Sensor – Pass (20C measures 2.56Kohm, 40C meas 1.25Kohm, 50C meas 0.93Kohm, 60C meas 0.66Kohm & 80C meas 0.33Kohm)
O2 Sensor – Pass (measuered twice and still bought a new NTK out of frustration)
Throttle position switch – Pass
Engine speed (negative side of the coil) – Pass
Full throttle switch – Pass
I have rebuilt the carburetor and tested the Mixture Control Solenoid (MCS) for continuity (40 ohms through the solenoid). I also applied 12 volts to the solenoid when I had it outside of the carb and it functions (moves in and out). I sprayed carb cleaner and compressed air through both the fuel side jet and the air bleed jet. I also confirmed it had the correct part number stamped on it GV113-20. The float level was a bit high when I rebuilt the carb but it is spot on now. I have sprayed around intake and carb base for vacuum leaks and found none. I also put a vacuum gauge on the truck and it has 20.25 - 20.75 inHg with the needle sweeping slowly in that range.

There is mention or a “High Altitude Switch” in the FSM which this truck was not, and is not equipped with.
No other inputs to the computer.

I also have performed several tests on the MCS using procedures I found on line. The premise is that by connecting your dwell meter to the input side of your MCS, a reading of 30 degrees dwell angle on the 6 cylinder scale would equate to a 50% duty cycle pulse from your ECU to the MCS. A lower number means the ECU is sending a "rich" signal and a higher number means a "lean" signal is being sent.

When I use the dwell meter on the input side of the MCS, it reads 20˚ steady. If I rev the engine to about 3200 RPM and hold, the dwell meter needle will hold steady for a second or two, then peg over to full lean for maybe a second, the engine will stumble, and the needle swings back to 20˚ quickly.
What I think I gain from this is that the computer does seem to be sending a signal to the MCS and it tries to make some sort of a correction for that short period of time. Thus leads me back to is the Neutral Safety Switch playing a roll???

I performed a compression test, cold, and all 4 holes read 175psi. So low compression is not a culprit…I had several people ask.

I took the truck back and had another pretest done with no significant change in CO. The spark plugs are still being fuel fouled and black soot comes out of the tail pipe

I spoke with a carburetor rebuilder in Martinez CA. and he told me that from my testing that he did not think the carburetor was the issue. He mentioned a similar problem with a Mazda that he rebuilt the carb on, ended up refunding the customers money because it would not pass smog, and later found out that it was the high altitude switch. Which is why I double checked that this vehicle does not have one those.

Well I hope that is enough data for the experts on this site to help troubleshoot this issue. I am a bit long winded...just like my Pop.
I guess my questions are:
Could the Neutral Safety Switch be effecting the tail pipe emissions?
Why would my dwell meter read that the ECU is trying to send a "rich" pulse to the MCS?

Any help would be greatly appreciated.

Well I was able to get this 720 to pass California emissions test. We put a different carburetor on it from a donor truck from Pick-n-Pull. I performed the same dwell meter test as noted above. With the new carburetor, the needle sits at 20˚ at idle. When I start to raise the RPM to say about 1200, the needle swings to full lean and stays there. I am not sure how to interpret this test but it is definitely different than the test results from the original carburetor. My only conclusion is that the MCS in the original is not functioning correctly. I am planning on sourcing a new MCS and installing it into the original carburetor. Once this is done, I will install the original carburetor and have another tail pipe test performed. I will post the results from this activity.
Based on all of this work, my conclusion is if your vehicle is failing emmisions, at speed, for high HC and high CO, I would suspect the MCS as the problem.