Printable Version of Topic

Posted by: malcolm2 Sep 30 2026, 04:04 PM

Hey hey... I need some suggestions. I went for a nice drive last nite. I have a AFR gauge that has not been working. But the PLX Devices system has a blue tooth transmitter so i can review the live data on my phone.

I was getting 10.5 A/F reading. It did jump to 12.5 as i went to neutral and coasted to a stop.

To start with, I was thinking i would pull the sensor and take a look, maybe clean it up and re-drive.

Any ideas about the 1st steps? The car seemed to run fine.

I will need to look them up here, but years ago I had a reply from, I believe, Mark Henry that gave me the following:

13.5-12.9 while accelerating of climbing
12-8-12.5 at WOT
16-17 going downhill or decelerating.

1911 cc L-Jet. I did tweek the AFM back in the day. 62,000 miles ago.

http://www.914world.com/bbs2/index.php?act=ST&f=2&t=304320

Posted by: JamesM Oct 1 2026, 01:19 PM

10.5 is way rich

Probably not impacting to much other than gas mileage, but I wouldnt want it there long term, thats rich enough to potentially be getting black smoke out the tail pipe on acceleration enrichment.


Ideally
~13.5 at idle
~12.8 full load

No reason to ever see anything richer than 12s. May be able to go leaner at idle if you have an ECU with the adjustment knob but 13.5 is usually around the strongest there.

light load/cruise is a lot less particular, not sure you are going to see anywhere near Mark Henrys targets with d-jet, light load/cruise usually winds up mid-high 13s. while leaning out this area is totally acceptable it can require additional ignition advance to avoid missfires which most of the stock setups dont give you. 14.7 is the HOTEST place you can run and you generally want to be either richer or leaner than that. Best for emissions, and totally fine on water cooled cars, but then that is why we dont have air cooled cars anymore.



EDIT: I missed that you are L-jet, you may be able to setup leaner cruise numbers.

Posted by: emerygt350 Oct 2 2026, 06:11 AM

Not sure what brand of gauge you have but my auto meter is super picky about rich mixes and just sits there and wags at me when I am running too rich or if it doesn't like the voltage it's getting. It also does that when my cheap O2 sensors get tired.

I shoot for the numbers JamesM posted. I am getting pretty good at Djet, but no clue on ljet. 13.5 at idle really does seem like the sweet spot for a strong reliable idle. At cruise I use advance (and now I have a bluetooth 123 so I am having fun with pushing the numbers around), and like it to sit around 16.

Posted by: malcolm2 Oct 2 2026, 06:24 AM

Yes, the reading is low. Question is how can i change them, short of the delicate AFM spring adjustment? Yes on the fuel mileage too.

1st i wanted to go over the sensor, wiring, and gauge to see if it was working. Maybe clean up the sensor or buy a new one from PLX Devices.

So looking for pointers on the AFR system and making FI adjustments.

Posted by: JamesM Oct 2 2026, 08:24 AM

On the AFM itself you can adjust the spring preload of the air meeting flap.

There is also a static position adjustment as well as the idle mixture screw.



Not sure what you can do as far as wiring other than testing the continuity and resistance readings of the sensors at the ECU connector.



The AFM adjustment is not a touchy as you might think, especially with a 50+ year old tired spring. You can mark the starting position and return it there if needed.

There is however the possibility that it was previously adjusted to get around a wear spot in the track, in which case you just need a new AFM.

Posted by: Literati914 Oct 2 2026, 09:33 AM

On my last build I had similar issues - turned out I needed to add a fused (small glass tube, 250Ma I think) wire to the heater input of my Lamda processor (brand 14point7 btw), that was not included with my kit for some reason. I added it, and it immediately corrected.

Posted by: malcolm2 Oct 4 2026, 10:56 AM

https://www.itinerant-air-cooled.com/viewtopic.php?f=50&t=7761

I used that link some 10 or 15 years ago. Guess I will read thru it again. That spring could POP at any second during my operation.... IIRC I had to adjust, then adjust once more. So i did it twice. It was very scary.

So that being said, this car has run with that spring setting for more than 10 years.

What would all of a sudden make it so rich?

After i check the AFR set-up, I might go to the 5th injector. Maybe it is stuck.

Any other thoughts, let me know. I have a few days of home time before the kids fall break.

Clark
615-504-1086

Posted by: emerygt350 Oct 4 2026, 12:55 PM

I am not an ljet guy but there must be a temp sensor or two in the system, not just the flapper. If one of those goes, or the ground/wiring is funky, it might send it rich. That is the first place I look on fuel injection cars.

Posted by: JamesM Oct 4 2026, 01:53 PM

The the "joy" of analog injection systems, especially 50 year old ones, is that anything that impacts the resistance of several key circuits impacts your mixture, and you may not even be looking at a single source.

So:
Oxidized connectors
loose connectors
broken wiring
corroded wiring
shorted wiring
bad ground paths
worn AFM traces
Weakening AFM flap springs
Partial failure of anything in the ECU itself
EMI from bad ignition components

Because you have no centralized visibility of the system the only way to diagnose is to individually test every component and circuit through the harness itself.

It is not a small task.

I used to be a purist when it came to retaining stock injection systems however as someone that owns 914s specifically to drive them the cost(in dollars, time, and reliability) vs benefit has shifted to favoring replacing with modern electronics.