Cabin Fever -

[JK McDonald]

Well Hell - we've been rocking back and forth with all the discussions about the reopening of our economy, but my wife and I did finally decide to canx our trip up through the Chattahooche National Forest and on down along the Georgia coast. I had Miss Purdy buffed, vacuumed, prepped, A/C charged and all ready to go too......

Our planned historic mansion row visits, civil war site battleground loops and hotel reservations were all made back before January. Hopefully our next trip in June wont result in another disappointment.

Here's a small family owned vineyard around the Kennesaw Overlook from last year. Wishing we were there -

How is everyone holding up ?
Michael

[Danglerb]

928 goals .... much more focused on digging in style of wrenching that could easily put off driving for a year. Things like really sorting out all the minor annoying bits.

Wife does not like the 928, loves her MB convertible, so its the trip car. 928 will be 100% just for me car.

[foggy]

This comes under the heading of Cabin Fever. I was emailing a friend about what I was doing during this quiet time. He has done a lot of work on engine management but none on CIS so I sent him the following:


In the meantime I've been playing with the Porsche and trying to set up the CIS fuel injection system with the help of the wide-range air/fuel system I installed over the winter.

Because I was house-bound on the weekend, I pulled the injectors and blew Sea Foam through them before I started the test runs and while the spray patterns on some of the injectors weren't great, they were OK.

My first runs showed high numbers (15-16) and the engine would miss at anything over 15.5, so I adjusted the little set screw and got it to run, not well, but it did run. It was inconsistent across the range which I broke down into idle, light acceleration, heavy acceleration, cruise at medium speed and coast.

The set screw changes the physical relationship between the air measuring disc and the fuel metering plunger so I think of it as an offset. The actual ratio is set by the control pressure counter force on top of the metering plunger in the gizmo Bosch calls the warm up regulator. There is a function in this thing that reduces the control pressure during cold starts and warm up but that is out of play once the engine is hot so I haven't even looked at that and she starts so no problem.

The way you adjust the running control pressure is you take it apart and whack it with a hammer. Well a little more sophisticated than that but the pressure regulator is a small steel module pressed into a die-cast housing and you punch it out to decrease the control pressure while its apart, then reassemble on the car and carefully tap it back in from the outside to get the setting you really want.

I didn't set the control pressure with a gauge but instead took the car for short runs and checked the air/fuel ratio. It took 4 runs, each followed by careful tapping to get the engine running well.
Interesting that the current position is almost back where it started, about 0.16 mm out as far as I can measure with my vernier depth gauge.

Each one of these test cycles was accompanied by adjusting the set screw which some people call the idle ratio adjustment but I think its effect is across the entire range. I speculate that it is called the idle ratio set screw because you normally set the CO gases at idle from under the hood. Hard to do when driving!

There is one more complication. My control pressure regulator has a vacuum connection, in fact two. There are several different versions of this thing and it creates a lot of confusion and I've seen technical articles picture one version and then present a section drawing of a different version as if it was the same thing. It had me so confused I took mine apart and hooked up a vacuum pump to make sure I had it hooked up right.

In any event the vacuum connection operates a small diaphragm with a second spring to make the mixture leaner at light loads and allow richer for heavier and acceleration and in fact, it does that, maybe a little too well.

When I started this project I was working on the assumption that the target would be 14.6 to 14.7. Now as I read more I'm wondering if it should be set a little richer.

Here's what I'm getting at the moment. Idle; 14.5, modest acceleration; 14.2, heavy acceleration; < 14, cruise; 14.6, and deceleration; 15.4 When the engine load is light such as smooth pavement with a long down slope the number is around 15.2 and I get some misses from the engine. Overall the engine runs great, the best I can remember in recent history, of course my memory isn't great.

So this is a long-winded way of asking the question. Should the target be a little richer overall or should I look for ways to reduce the effect of the vacuum module?

I haven't been able to find any information on adjusting the vacuum module and I don't know if hitting it with a hammer will work. I think I could try to make the spring softer by cutting a little off.

Any ideas?


Since I wrote the above, my friend suggested making it a little richer.

For background, The car is a 79, 5 speed, I think US version and I have had her 25 years, Mostly I just drive it a little each summer in Nova Scotia and fix or replace what needs doing to keep her going. All of the original pollution control features such as catalytic converter are either gone or inoperative.

Now that I have extra time on my hands, I'm digging a little deeper.

[Reanimotion]

Yes, idle target for best running should be around 13.5

79 USA spec for CO is between 2 and 4 with the air injection disconnected and plugged

CO of 2.0% equates to an AFR ratio of 13.77
CO of 4.0% equates to an AFR ratio of 13.00

[Reanimotion]

https://www.mgexp.com/article/co-to-air-fuel-ratio-afr-table.149