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924RACR · Posted: Wed Jul 16, 2003 11:48 pm Post subject: 931 mixture adjustment again

I'm hoping to get my hands on a wide-band O2 sensor setup from work next week, to try and really dial this thing in - I'm still not 100% happy with the way the car drives. I will be sure to take plenty of notes and document what I end up with, and post back. Stay tuned...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

positivesale · Joined: 12 Nov 2002 Posts: 126

What is yours doing.. stalling on you occasionally, and idling up and down? I managed to cure mine up pretty good, but every once in a while when i turn at low speed and hit a parking lot ramp or what not, it can stall.
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82 931

924RACR · Posted: Fri Jul 18, 2003 9:31 pm Post subject:

No, it's idling OK, though not totally smooth. Of course my O2 sensor wire is broken, needs fixing. It's still got some odd-running hesitation off boost that I'd like to totally get rid of.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

wdb · Joined: 02 Nov 2002 Posts: 2024

is it possible to disconnect all the computer control stuff, to try and isolate whats causing the problem . if it runs better its must be a sensor,connection,or a control box . if it doesnt ,it could be an injector ,ignition system , etc . maybe configure the TPS to be fixed at WOT to bypass the computer and see if anything changes . I read on a previous post you were turning the air/fuel adjustment screw 1/4 turn at a time , maybe its a bit more sensitive than that, and your over shooting the correct setting . it seems your overlooking something , maybe get 1 of those multimeters with a thermocouple probe and check the exhaust gas temp , to check your o2 sensor ,when they get old the lag increases and you get a constanly moving target you can pin down . I read on this site that some dynos have a wide band 02 sensor , It says there really exspensive , but it couldnt hurt to look around for 1 .

http://www.sficc.net/features/feature4.html

positivesale · Joined: 12 Nov 2002 Posts: 126

Well I have no idea what the problem is, but would like to know if people feel a sweetspot when adjusting the mixture

When I adjust my mixture, there is always this one point in which the engine idles the fastest - a peak spot almost like a climax. It's a bit of a flat climax though, there's a bit of leeway once you find that highest RPM. I'm thinknig about an 1/8 of a turn leeway, before it suddenly drops the RPM and goes rich, clockwise, and the engine starts sputtering. Have you ever noticed that? There's about 1/8 of a turn area where the engine idles it's fastest. The engine will keep going up in RPMS for a while and then suddenly STOP at this 1/8 area and start going down immediately after the 1/8 area.That's where mine seems to run best - within the flat part of the climax (this reminds me of those short story diagrams the teacher wrote on the board, or sex). I back it off a tiny bit from that spot to go a bit lean. Some person with a C0 meter (Charles Ditmore, from way back when) said that's where the C0 meter he was comparing to looked best - in that high rpm spot.

If you are getting a C0 meter please show us the sweetspot in words some how. I'd like to get my hands on a C0 meter to see whate really goes on. Of course I guess there is always a power sweetspot, and a mileage sweetspot though.
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82 931

undrprzur · Joined: 27 Apr 2003 Posts: 18 Location: Eastern Washington

I have been a big fan of this site and learned to this here a few years ago.
In fact I have just rebuilt the engine and head, also put a new waste gate diaphram and 1 bar spring, on my 81 931 this spring, used the following to set mixture and tune.

1. Make sure all hoses and vacuum lines are secure and in good shape.
2. Clean the throttle body, espesialy the holes around the idle by pass. Don't forget to take the idle screw out and make sure it is clean.
3. check , clean and adjust ( should be even with the cone)the sensor plate at the fuel distributor.
4. No use adjusting anything unless you know for sure that the WUR is working correctly and that the O2 sensor is working.
5. Now start the engine, (A)check timing 1st, (B) let it warm up, (C) connect a dwell meter at the terminal by the Master Cyclinder( it is a round black electrical terminal with 3 wires(blk,red and brown). Connect the red to the positive side of the dwell meter and the black to the negative. Now you can see the needle moving, this because the O2 sensor is adjusting and trying to maintain the mixture. I have always set mine at 45 degrees (+/- 2 degrees)on the scale set to 4 cyl on the metere.

I run my car competively in autocross, and this method has always worked for me. In fact the car won the season in 2002 in E stock SCCA Autocross. I work on my own car because there are no porsche tech here in my part of the state.

Hope this works and helps.
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81 931 Always on Boost.

924RACR · Posted: Mon Jul 21, 2003 10:09 pm Post subject:

Yeah, I have access to a dwell meter - but it didn't work, didn't register.

Tried using an oscilloscope, but no real definitive good reading.

I will not be using a CO meter; I will be using a wideband O2 sensor that replaces the stock O2 sensor.

I haven't found (for sure) that sweet spot yet, though it's not bad.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype