74 914 Ljet injection AFR

engine specs:
2056
CR 9:1
Rabby 9550 cam
50 mm throttle body
Fuel pressure 32lbs. at idle

Currently I have the dynamic AFR set at 13.5 cruising at 3000rpm, but the idle AFR is 16.9! If I move the small arm 1.5 teeth and play with the idle set screw I can adjust the idle AFR to 13.9-14.2, but then that richens up the dynamic AFR is at 10.1! There is no way I can adjust the large wheel to get the dynamic AFR to move from 10.1 to 13.5. Any suggestions?

Try not to chase a specific idle AFR. Type IV's don't lend themselves to that very well.
Focus more on idle characteristics (speed, stability, smell, etc.).

My two cents.

i dunno

but fool pressure at idle ought to be at 28 lbs - vac operates the pressure regulator to drop it from the 35 lbs that is in giving it the gas otherwise.

the rest of it is so far off standard spec i cannot comment.

Problem could be injector size, are you using stock 1.8’s?

Yes, standard 1.8 injectors.

I think your injectors are crapping out at max delivery, look for a set of 912E injectors, you can bring the pressure back to normal.

Standard Motor Products email.Click to view attachment

As I have been reading up on these injectors for my own needs, I found that now "rebuilding" the injector is really simple.

There is an inlet screen, just like the screen in the fuel tank, inside each injector.
We can simply buy new screen now and remove/replace that screen.

Provided the injector still goes on and off properly, and does not leak, it seems that the screen replacement is what more modern folks are doing on cars much younger than our cars.


https://theinjectorshop.com

I am using new injectors, they only have 1200 miles on them.

i'd say those injectors are already at 912E level and more going on the quoted flow you received.

here is the numbers i have for the standard 1.8 injectors.
112 injectors on 1.8
Click to view attachment
other information i have from a chart on R Atwell's bus website quotes slightly lower flow rate of 185 cc/min slightly less than 190.2 and 18.1 lbs/hr.

standard injectors notably less than 19 lbs/hr. injector low rate shouldn't be your problem?

Atwell quotes 912E as 187cc/min

I'm personally not worried about a 16.9 idle AFR, though it may cause idle surging.

I'd be more worried about a lean AFR under load.

You've already exceeded the L-Jet's capability to accomodate with your mods; I'd say pick the lesser of two evils and live with it.

I have it set to 13.5 at 3000 rpm cruising and it dips down to 12.4 under load. Sealed the lid on the AFM. I live in a rural area of South Jersey so it's not like I am idling sitting in traffic. Thanks for everyone's input.

in a way you are no worse than a factory 1.8 L jet.
if its original as is mine.
the ignition timing is set up retarded at idle due to the dizzy.
so it runs hot due to that when its sitting in traffic.
i live in the inner city. my 914 hates hot days sitting at idle.
i avoid super hot summer day driving.
primitive emissions tech on these cars was not perfect.
so long as its good at cruise its going to be the best it was anyway, in a funny way.

@wonkipop shared this link a while back and I think it was a good reference for me whilst working through the tuning process.
http://www.itinerant-air-cooled.com/viewtopic.php?t=7761

That is the article I followed. But when you move the small arm to richen up the idle and adjust the AFR to 13.9-14.2, it makes the dynamic AFR overly rich, like 11.1-12.2! There was no way to adjust the dynamic AFR to get it to come back to 13.5. Like I said, if I lived in a high traffic area and spent time idling in traffic I would be more concerned about the idle AFR at 16.9. I feel better to have 13.5 at cruise and not worry about idling at a stop sign for 20-30 seconds

I think that you may want to consider that these initial old school EFI systems where a lot of compromise to ensure the car did not overheat due to lean conditions.

When I wanted to see exactly what I could get out of a 914 for MPG...there was certainly a lot of room to make the car run leaner.

I did find that even that had limits...but it could be done if you monitor the exhaust gases properly.

Rich

i think the way these early emissions systems were set up was a factor of the emissions laws and the available technology. particularly the L jets which come in just as the EPA and CARB were tightening and ratcheting up the standards.

from a historical perspective. and as a technical history.

very definitely with the early L jet 1.8s they were set up to at a minimum produce less NOX at standstill idle. this was a feature of all of them in 74. USA wide and california.
primarily that was done with the ignition timing retarded at idle. as a consequence you naturally got more unburned hydrocarbons in the exhaust along with CO. the EPA accepted that at first as NOX was the first primary focus to tackle photochemical smog.
but though they tolerated it as a consequence (until manufacturers could get CATs into production) they still set a standard to be met, which wasn't too lenient. i think the AFM units might be tuned a little to be a little leaner at idle as a result.

it wasn't ideal but they had to meet regulated standards.

it took until you got more powerful ECUs, closed loop systems and most importantly oxygen sensors (also developed and patented by bosch) that you could really properly control the mixture across the entire operational range of the engine.

these early systems in some ways are more like victorian clocks than computers.

and despite all the hoo hah about emissions laws and smogged cars in the early to mid 70s in the USA the EPA never did actually meet the goal set by the Nixon administration of a 90% reduction on 1970 vehicle emissions by 5 years (1975). not even close.
that took until the mid 1980s as it turned out.

i think you just have to live with what the AFM can do as its likely that those very early units which our cars have are designed with an inbuilt bias.

How are you reading the AFR and how strong is the idle when its running 16+?

Curious as I have seen a few of the 914 exhaust systems out there will cause falsely lean idle readings due to the lack of exhaust pressure.

13.5-12.4 between cruse and WOT is perfect so unless it is stalling out at idle dont even worry about what the O2 is reading.

With the Rabby 9550 cam the idle is set at 1100 rpm, The exhaust is SS heat exchangers with a Triad West muffler.

My approach would be to set dynamic fueling as close your target as practical via cog and lever arm offset. Then use the AFM screw to adjust idle fueling. The TPS commands the ECU to provide full throttling enrichment.

@r_towle

How do you replace the screen? Does it pull out of the inlet? The other problem with used injectors is t bgg at they wear and it affects the flow rate. I try to send 8 injectore for cleaning and testing and match the clocest 4.

There are some videos out there on removing screens.
Insert proper size screw
Yank on screw

Really looks simple, yet it has been kept a costly secret for years.

Go to the link I posted, they have a YouTube channel

No one has mentioned this yet so I thought I might...

The early TPS units were basically just two microswitches that are configured so one set of contacts are closed at idle and the other set goes closed at WOT.

The idle contact richened the mixture at idle and the second richened at WOT.

If the ECU isn't seeing the idle contacts closed it would do what the OP is seeing.


My question is "have you verified your TPS is showing continuity on two of the pins at idle?"


Early Z stuff was adjustable. 914 is not but it would be easy to slot the mounting holes to allow some adjustability.

You can take the cover off of the TPS to bend/clean these contacts if needed.

Super fine nail file works great to clean the contacts, steal one from your wife!

I love the fact that smart people have stepped up to make some parts.

the TPS replacement circuit boards are available, I just recently saw them on PMBperformance website. I recall when someone figured out how to make new ones years ago.

Rich

If you are getting your O2 reading from a sensor in the bung in the riser between the crossover and the mufflers on that Triad that would explain the lean readings at idle. It isn't actually lean, just a false reading from the sensor as there just isnt enough pressure in the mufflers at idle to keep fresh air from reverting back to the sensor. I encountered this same thing tuning a Megasquirt car with this exhaust.

A lot of 914 mufflers have this issue just by nature of their design. A lot of the single can type mufflers will see the same as well due to there not being a good location for the O2 sensor. The Bursch with an O2 sensor right after the collector is probably your best chance of getting clean O2 reading at idle, but its not important enough at this point to swap your exhaust. Just know that your idle O2 reading is wrong so dont worry what the gauge says if your idle is good.

I've had an exhaust that did the same thing to me. Read crazy lean on fhe O2 but you could smell it was still way rich.

BTW, when I had a D-Jet w/9950 and 123 distributor that's about as low as I could get the idle (1100 RPM).

Click to view attachment

Several years ago, I received this info from a member. The car really performed well too

"The 1975 914 uses the L Jetronic system. It is the best of all the injection systems used in the 70’s and 80’s. You will find the L system on Opals, Range Rovers, Porsche 911’s, BMW and others. You may also use parts off these cars if the pins are the same for the airbox. You can also use the throttle body off these cars if the pins are the same.

In the articles from opeltuners, you will notice that a small change in throttle body and airbox means a large change in available horsepower. The throttle body that is on the 1.8 is good for 160 hp. Just clean up the rough edges inside the throttle and airbox with a grinder and you will be set. Next step is to increase the fuel flow to the engine. You can do this by larger fuel rails, and lowering the full on timing of the injectors to 3000 RPM. Down from the normal 3500 – 3800 stock setting.

I replaced the injectors with stock Bosch; increased the fuel pressure to 43 #; lowered the full on timing of the injectors to 3000RPM. Then I reduced the spring tension in the airbox by a -4clicks and increased the injectors on-time in the lower RPM by one contact slot on the inside dial of the airbox ( inside the tension spring adjuster ) This inside adjuster set the idle to 1000RPM, but made the idle rich so I leaned the low RPM using the bleed screw on the top of the air box. This lets the air by pass the airbox and go directly into the intake after the throttle body, leaning the mixture at low RPM.

So, I have increased the airflow through the engine by exhaust and small mods to the sir box; increased the fuel flow at low and high RPM; and leaned the mixture at idle.

The car seems to get with the program. Raced a 914/6 in Costa Mesa and went through 2nd and 3rd walking away from it”

Adapted a Vanagon 50mm throttle body. Modified the original 1.8 throttle shaft to accept the larger 50mm throttle plate and had to drill a hole to mount the TPS. Used a tail pipe expander on the intake manifold to open it up to 50mm.