'77 280z running rich at idle

[Mitchel0407]

Hello everyone, maybe somebody can help me. My 1977 280z is running rich at idle and the idle mixture on the AFM doesn't seem to help.

 

I spend the last few days cleaning the electrical connections and timing the ignition again as it was out of spec. The car runs way better but the AFR gauge still shows 11,5 at idle. I'd like to bring that down (or up depending how you look at it) to 14,7 as the car stinks of gas. The previous owner mentioned that that was the reason it didn't pass inspection (auction company didn't mention that). I tried messing with the idle air mixture screw but that didn't help. When turning clockwise a bunch the car did start to run richer but when turning anti clockwise it didn't get past 11,5 AFR.

 

 

Does anybody have any tips or things I should look at? The AFR does seem to be working alright as far as I can tell.

 

I did notice the ignition coil is a 1,5 ohm unit with a total resistance of 2 ohm with the external resistor. I believe it needs to be closer to 1 ohm total so I'll probably replace the coil.

 

[Mitchel0407]

I also posted about this on the Classic Zcar forum but forgot to update this one. Here is a link to that post: 

https://www.classiczcars.com/forums/topic/69530-77-280z-running-rich-at-idle-afm-screw-doesnt-work/#comments

In short, I tracked the problem to a faulty AFM (Air Flow Meter). The carbon track inside the AFM looks fine but when hooking it up to a battery and measuring with a multi meter there was a break in the track. Unfortunately it’s not repairable and I couldn’t find a Datsun AFM in Europe or somebody that could repair or recreate the circuit board (the carbon resistive track part that is).

This has lead me into a pretty undocumented part of the Datsun / Bosch EFI system that is the AFM. It’s pretty much just a voltage divider (brings down the battery voltage) and a potentiometer. However nobody makes a potentiometer I could jerry rig inside of the AFM to recreate the original signal.

However in my goal to make the car run (not rich) again I found a possible solution. Why not just use a different AFM? I mean it’s basically just a generic Bosch / Denso AFM module (5 pin) with fuel cut off (which makes it a 7 pin). The only problem is that the Datsun’s use the earlier > 5V AFM otherwise u could just grab one from a BMW or Opel. I did find an Australian forum post that mentioned a Toyota AFM that was plug and play with some spring tension adjustment with a part number that (over here) came back to one from a 90’s Ford Probe / Mazda 626. The part number was  F201 13 210 (197100-3420) for the record.

 

As this AFM was only €25 at the local wrecker I got one as well. I will be testing this AFM over the weekend to see if it actually works in the Datsun and also works well. I did notice the Mazda AFM is much bigger then the stock one. Maybe that second hand 60mm throttle body I saw online might be an good upgrade as well if this AFM works. 
 

[Mitchel0407]

So little update and probably the end of this thread. Although feel free to revive it if somebody has any questions and I’ll try to help out.

 

First off, good news!! The Ford/Mazda/Toyota and so on AFM actually works and giving of the correct signal! I my opinion this is a huge win for everybody that wants to keep these cars alive. My intake both and the rubber booth between the AFM and TB where both flexible enough to stretch over the larger AFM unit without trouble. The new AFM unit unfortunately doesn’t have bolt holes in the bottom like the Datsun unit but mounting can be fixed by a custom 3D printed or metal bracket. I just left mine loos as I just needed to test it for now and drive to the inspection.

 

 

Opening the new AFM was a bit more difficult as they used a ton of silicon sealant and plugged the idle air screw from factory. The sealant can be carefully removed with a knife and the plug can be drilled out. The new AFM was also way easier to dial in as you can rotate the spring gear with a flathead screwdriver and a bit of finagling. In my case I had to tighten the spring tension a lot but I do have more problems concerning the car running rich. Even though I just cleaned and rebuild them I think my injectors are bad.

 

 

I had the car inspected yesterday and the CO gas values where down to 2,4% (4,5% allowed) and all other values where good so I’m happy. Although the car ended up failing due to rusted thru frame rails and a crooked rear axle probably caused by a twisted chassis. I started a legal dispute with the auction company and previous owner as the both sold the car as “without rust and free from structural damage” but that is something for another thread.