Kuroki924

Hello fellow Porsche owners and all, I hope you're having a great day. So I'm having an issue with the fuel/air mixture tuning of my '82 924 N/A, mind that it's an early model with CIS. The car is behaving like the fuel/air mixture adjustment goes out of tune after a couple of weeks, making the car begin to run rich, to the point where it will make little pops out of the exhaust, at an idle. It had always done it upon deceleration which I thought was to be expected with the cat being cut off and all (Previous owner's move). Anyways, my dad and I have tuned the system (The 3mm Allen key on the back of the carb and the flathead screw on the front of the throttle body) three times now to get the desired rpms at idle (About 950 +- 50 rpms according to the Hayne's repair manual) whilst not running too rich and having adequate throttle response, and every time it goes out of tune after only a couple weeks. This time however, I think (or at least hope) the detuning was instigated by the fact that I think we may have flooded the carb. We had the battery on, tinkering with the headlights, then when I tried to start it, she gave out a loud pop and that was it. I tried again, she started, but only ran for a second or so before dying. Tried it again, same thing, and if I tried to give it a bit of gas to keep her awake, it would fight me until eventually dying again. I left her to sit overnight, and the next day she fired right up, but was idling pretty hard, almost as if it had a big-cammed V8 in it. It smoothed out a little bit after a few minutes of running, but never quite calmed down to what it was before all of this happened. Still runs okay, but due to whatever reason it's running rich, is a little louder than usual and as mentioned before, pops repeatedly at an idle. Any ideas?

V2Rocket

If your car actually has a carb that might be problem #1
CIS has a big plate on the end of a lever that "weighs" the incoming air which then determines the correct amount of fuel to let through to the cylinders.

I'd check fuel pump, fuel filter, fuel pressure.

Kuroki924

Thanks for the reply. And it does have a carb, so what would be my first option in terms of what to look at? Could it just need cleaning, or could there be a larger issue behind it? And the fuel filter is good, can't have any more than a hundred miles on it. Pumps work and haven't had an issue with pressure yet, so I should be okay there

V2Rocket

Could you please put a picture here?
CIS cars don't have carbs, unless someone converted your car after the fact.

Kuroki924

My mistake, the 3mm Allen key adjustment is actually just behind the fuel distributor, bit of a user error there. But here are some pictures, one with a view of the bay, one zoomed in on the flat head adjustment screw on the throttle body, and one of the previously mentioned Allen key adjustment behind the fuel distributor.

Kuroki924

And forgive my ignorance here, but I have to admit I always thought it was carbureted. I thought it was odd that it would have a carb and a computer (K-Jetronic box under the dash on driver's side) but never really considered that it wasn't carbureted, for whatever reason. With that out of the way, where could I begin looking to sort this out? And if there wasn't a carb to flood, what might've happened with the battery on that would have made the car do what it did?

Kuroki924

Sorry for the bombardment of replies, but this new (to me) information really has me curious now. If you take the rubber boot off of the intake just behind the fuel distributor, there is a big plate under that, and we have noticed while adjusting with that 3mm Allen key, which is very close to that plate, any vertical movement of the Allen key whatsoever considerably affects the engine's performance, even so much as getting the key in and out of the hole that's there for it. Is this the plate you speak of? If so, I think I can piece together why it would be so severely sensitive to any pressure. I've also heard that this 3mm Allen key adjustment is actually called a warm up regulator? If this is true, what specifically does this adjustment, adjust? I'm also not aware of what specifically the adjustment on the front of the throttle body does, my best guess is that it can regulate how much air passes through the throttle body, maybe, but I honestly don't know. My dad and I just adjust them both accordingly to get the proper performance when it behaves like it is out of tune, according to what our maintenance manual suggests, but don't really know exactly what's happening with them.

V2Rocket

this should help somewhat....
at one end of the lever is the big plate you see under the boot.
at the other end is a plunger that restricts the through-flow of fuel in the distribution block part.
the idle screw adjusts the relation of the plunger vs the plate in closed position...

for really good help, i suggest you check 924board.org...rennlist doesn't have that many 924 folks, its mostly 944 on this side which is totally different engine-wise/fuel injection-wise.
those guys are CIS-wizards...

Kuroki924

Thanks for that, that does shed some light. So using the diagram's terms, it looks like idle mixture control screw adjusts the resting place for the sensor plate lever, thus adjusting the resting place for the sensor plate itself? And in terms of the idle bypass screw, should that be tinkered with? From our observations, it's necessary to adjust that accordingly with the idle mixture control screw, but I just want to be sure we're not going to mess anything up by adjusting it. Also, any ideas on why the system would go out of tune, having a tendency to begin to run rich after a couple of weeks?

Kuroki924

Also, I suppose I should ask. Being an '82 model, it has the electronic idle stabilizer. However, presumably due to it's age, it stopped working properly, being incredibly sensitive to any level of shock, to the point where tapping on the top of it with your finger would cause all kinds of running issues, on top of making the car incapable of driving over any kind of bump without making the engine stutter and backfire. While I do baby it, living in Michigan makes a car that can't take any bumps very impractical, so we bypassed it, plugging the two plugs in the bottom of it into each other. I know doing this will make the car require a substantial amount of extra warm up time, which I always give it. But could this be contributing to the issue at hand?

mel_t_vin

At this point, neither of you are qualified to tune this CIS-based 924...not for lack of ability, but for lack of knowledge.

It would benefit you, and this 924, to acquire a few technical books on Bosch CIS fuel injection. Strongly suggest you do not mess with the Warm Up Regulator [aka Control Pressure Regulator] unless you know what you are doing and have a set of CIS gauges on hand...it is not an inexpensive component.

The WUR is the heart of CIS and responsible for cold, hot, and reference fuel system pressures. Without a set of CIS gauges and an exhaust gas analyzer, you will never get this 924 running properly...period.

russmw

Hi

I have a 1982 2.0 924 (UK version) but suffered the same issue last summer. I have pressure gauges etc to set WUR pressure\timing\mixture but every week or so the mixture would change a little bit and need tweaking.

I discovered it was the head gasket starting to go which showed itself with a occasional drop in the exhaust note, sounded like it was missing every 30 seconds or so then I noticed the exhaust manifold heating up quicker than usual

Went through a HG change and all good since then with no change in mixture and its running great. Make sure its set up 100% before presuming you have the same issue I had.

Good luck

Kuroki924

​​​​​​It actually wasn't so major, it turned out the intake boot wasn't completely seated over the mafs, cis engine sucking more air than usual=using more fuel than usual. Popped it on all the way and clamped it and she's been running just fine ever since😁