Sensor Plate Adjustment Screw

thunderroad · 12-02-2016, 02:36 PM

Hello all: I have a 1983 911sc that is running very rich at idle causing the engine to surge. Upon our inspection, we determined that the sensor plate adjustment screw located in between the plenum boot and the fuel distribution head is gone. Therefore, when we attempted to adjust using a 3mm allen wrench, the wrench dropped through the hole. However, when we slightly depress the sensor plate using our finger, the idle smooths out.

How do we go about locating the replacement adjustment crew? Anyone have any input? Thanks.

gazzerr · 12-02-2016, 03:18 PM

Is it the adjusting key on this page?

1982 Porsche 911 SC Targa - Intake System - Page 2

Kinda expensive. Contact boyt911sc I bet he has one he can sell you.

thunderroad · 12-02-2016, 03:48 PM

I actually don't know. Perhaps it's possible we just missed it with the allen wrench. However, it seems like there should not be an open hole there. Whatever it is, the car is running very rich and surging at idle. When we slightly depress the sensor valve, everything straightens out. Any help would be appreciated.

thunderroad · 12-02-2016, 04:03 PM

The symptoms are identical to this video.

https://www.youtube.com/watch?v=8TcAzqKxWgY

boyt911sc · boyt911sc's Garage

Thunder,

If I understood your post correctly, the 3-mm Allen fuel mixture adjuster screw is missing? Could you poke through the hole and pass through the arm/lever? I am having a difficult time visualizing your problem. Even with the absence of the 3-mm Allen screw, there is a plug at the bottom side of the lever unless this was also removed. Anyway, to install this 3-mm Allen screw, you need to disassemble the sensor plate assembly's lever or arm and the air flow metering unit has to come off the engine.

Could you test it by threading a straight piece of wire to the hole all the way to the bottom of the airbox? I doubt the the screw is missing (?). Keep us posted.

Tony

thunderroad · 12-02-2016, 08:15 PM

Tony: do you know if the hole located between the plenum boot and the fuel distribution head is supposed to be plugged? This really seems to be the issue. I found this diagram which (at part 14) appears to require a plug. This is certainly not present in my vehicle. Thanks. 911 Sc Mixture Control Unit

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by thunderroad

Tony: do you know if the hole located between the plenum boot and the fuel distribution head is supposed to be plugged? This really seems to be the issue. I found this diagram which (at part 14) appears to require a plug. This is certainly not present in my vehicle. Thanks. 911 Sc Mixture Control Unit

Thunder,

Part number 14 in the diagram is the rubber plug to the access hole. Most mixture units I've seen have missing plugs. The missing plug is not critical because it does not affect the operation of the fuel mixture unit. Find something to cover the hole if you want it plugged. BTW, what other part # is missing in your air flow meter unit?

Tony

thunderroad · 12-03-2016, 03:51 AM

Part no. 14 appear to connect to the Allen wrench which in turn would allow me to adjust the fuel/air intake with the Allen wrench. Is this not correct?

75 911s · 75 911s's Garage

Part 14 is only a plug. It's a wire with a loop at the top for your finger and a rubber cone shaped plug at the bottom. It doesn't contact the adjustment screw. For that you need a long 3mm Allen wrench as stated earlier.

boyt911sc · boyt911sc's Garage

Thunder,

The fuel mixture set screw is not shown in the diagram (?). It is mounted right below the hole where the plug (#14) is shown. The fuel mixture screw is actually mounted on part #8. I have some disassembled AFM that I could take pictures and show the actual fuel mixture screw. If it is in fact missing, you need to remove the AFM and do some inspection. With the AFM removed, it is not easy to have a good view of the 3-mm Allen screw until you remove the lever or arm.

To remove the AFM, you will need to remove a lot of things. But that's the only way you could test and confirm if the 3-mm screw is present or not. Honestly, I think it is there. You are just not familiar how to locate it.

Tony

75 911s · 75 911s's Garage

New post for better pics.

I think Mixture control lever is sold containing the set screw:

thunderroad · 12-03-2016, 08:06 AM

Quote:

Originally Posted by boyt911sc

Thunder,

Honestly, I think it is there. You are just not familiar how to locate it.

Tony

I think you're correct. Went up to my local auto parts store. Grabbed a 3mm metric allen wrench, 5 inches long. See attached. Is this correct? Any other pointers on how to locate?

Many thanks for your help. Gotta about two days left or porsche driving here until the 911 goes down for its winter nap. Want to make them count!

Bob Kontak · Bob Kontak's Garage

3mm is the size you need. The handle on your wrench may hit the metal fuel lines on your 83. Some "crafting" may be in order to allow it to clear.

Can't say for sure but if it does interfere, you could cut the handle off to leave about 1/2" after it makes that first right hand turn from the main shaft of the tool.

May also work perfectly as is. Good luck.

thunderroad · 12-03-2016, 08:37 AM

Thanks. So to confirm, the elusive screw I'm trying to contact is directly adjacent to this hole in between the plenum boot and fuel distribution head? Correct?

Thanks again.

gazzerr · 12-03-2016, 10:32 AM

Yep it's under the hole between the plenum boot and fuel head. It does always take a bit of fiddling to get the allen wrench seated. You can tell when it's mated with the allen head. You'll get a feel for it. Like Bob said you need the correct length otherwise you'll have all sorts of trouble getting it in place between the engine blanket and the fuel lines. The big blue handle on yours will probably foul with the plenum boot.

rwest · rwest's Garage

Keep in mind that a little goes a long way on adjustment- 1/16 to an 1/8 of a turn should be all you do each time. Best to use an air fuel ratio meter for getting proper mixture.

The factory manual has a process for setting the baseline

thunderroad · 12-03-2016, 12:10 PM

Took some work and some manipulating but got it running good and holding steady idle...I am amazed. Thanks all for your help.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by thunderroad

Took some work and some manipulating but got it running good and holding steady idle...I am amazed. Thanks all for your help.

Outstanding.

Word on the street is you have to end the adjustment with a slight clockwise turn to set the threads so they won't have slop.

Do you have a dwell meter? With the later SC's you can dial in AF ratio pretty close for idle. Once off idle, you depend on the integrity of the system to keep the ratio where it should be.

There is some beauty to the system. However it's a mild can of worms (comprehending) at first blush once off idle.

Advise if interested.

thunderroad · 12-03-2016, 06:48 PM

I'm interested in a dwell meter...can you explain more? Would really like to fine tune it while it is asleep for the winter. Thank you all.

gazzerr · 12-04-2016, 07:47 AM

I ended up using a Craftsman multimeter with a duty cycle function. On this meter it reads the duty cycle on a different segment of the square wave on the test port so the numbers were 100 minus what's in the spec book but I think I've got it tuned in pretty good.

Here's a good thread:

Help with lambda/freq valve test please