Help with Dwell Meter CIS Mixture Adjustment

[Nickshu]

I have the Actron analog dwell meter hooked up on my 82 911SC, trying to adjust the mixture properly. I have a brand new O2 sensor installed in SSI's (no cat). What should I be seeing???

The needle fluctuates between 5 and 10 degrees on the 8-cyl scale for dwell. According to what I have read here it should be more like 20-25 degrees on the 8-cyl scale, however no matter what I do w/ the mixture screw it does not seem to change much. Am I not turning the mixture screw enough? I am turning it back and forth (clockwise for rich/counter for lean) about one full turn either direction and the engine speed changes, but the fluctation range on the dwell meter seems to change very little.

Any help is appreciated.

[Nickshu]

[edgarcia737]

What does the dwell meter have to do with the mixture? Dont you need a exhaust gas analyzer to measure CO. Good luck

[Nickshu]

Originally Posted by edgarcia737

What does the dwell meter have to do with the mixture? Dont you need a exhaust gas analyzer to measure CO. Good luck

On CIS-Lambda cars you hook a dwell meter to the test port and use the duty cycle of the O2 sensor to set the mixture. CO can be used if the car has no O2 sensor (ie: early CIS w/o Lambda or Euro models).

[mholbrook38]

This may be one of the first tips I have heard that might make a later SC in some way easier to tune than the earlier ones. I do know that messing around randomly with that allen wrench can really mess things up. Especially if the car has a couple bad or weak plug wires and one or two plugged or partially plugged injectors and has been sitting for 8 or 9 months not running. Ask me how I know.....

[81 SC Ray]

Hi Nick,

I have been down this road. I bought the damn dwell meter and did the whole thing. Got the needle to swing. After that, my gas mileage went down to 11 and 15 respectively. So, I brought it to my resident pro, Abacus Racing, and they informed me I had messed up my mixture to the point that the lambda could not compensate. So, they adjusted it using a exhaust gas analyzer. Now all is well. Just admitting my failures to try and prevent you from doing the same

Ray

[Nickshu]

Originally Posted by 81 SC Ray

Hi Nick,

I have been down this road. I bought the damn dwell meter and did the whole thing. Got the needle to swing. After that, my gas mileage went down to 11 and 15 respectively. So, I brought it to my resident pro, Abacus Racing, and they informed me I had messed up my mixture to the point that the lambda could not compensate. So, they adjusted it using a exhaust gas analyzer. Now all is well. Just admitting my failures to try and prevent you from doing the same

Ray

Interesting. Mine seems to cycle properly, so the O2 and Freq valve are working. It just seems to be in a different range on the scale than what others have reported.

[Neel]

Originally Posted by Nickshu

I have the Actron analog dwell meter hooked up on my 82 911SC, trying to adjust the mixture properly. I have a brand new O2 sensor installed in SSI's (no cat). What should I be seeing???

The needle fluctuates between 5 and 10 degrees on the 8-cyl scale for dwell. According to what I have read here it should be more like 20-25 degrees on the 8-cyl scale, however no matter what I do w/ the mixture screw it does not seem to change much. Am I not turning the mixture screw enough? I am turning it back and forth (clockwise for rich/counter for lean) about one full turn either direction and the engine speed changes, but the fluctation range on the dwell meter seems to change very little.

Any help is appreciated.

You can use a digital dwell meter, set on the 4cyl. scale to check the duty cycle, usually 40-50 degrees on a normal operating temp engine or roughly 50% duty cycle.. I use a labscope with a duty cycle function, looking for ~50%.. This result tells you that the frequency valve is functioning correctly.. You could use this to set your mixture, I guess.. However, you must be sure that EVERYTHING else is working properly, first.. Compression, igniton, timing, vacuum leaks etc. all affect mixture.. If you start adjusting the mixture without everything else in place, you are indeed 'chasing your tail'.. Back to basics first, always..

[Nickshu]

True. At least I know that the O2 sensor is working b/c the Dwell meter was cycling from 5-10 degrees very regularly and I can hear the Frequency valve running. The car runs great, I'll check it again after I pull the engine and button up all possible vacuum leakes etc (when I get around to it....). I know CIS can be very sensitive to lots of minor issues that culminate into driveability problems, mine have just not become major enough yet to effect how the car runs.

[Bob Kontak]

Are you dropping he 3mm wrench down the hole or turning the air bypass screw? The air bypass screw makes the engine run faster or slower without adjusting the af ratio.

[Neel]

Originally Posted by Nickshu

True. At least I know that the O2 sensor is working b/c the Dwell meter was cycling from 5-10 degrees very regularly and I can hear the Frequency valve running. The car runs great, I'll check it again after I pull the engine and button up all possible vacuum leakes etc (when I get around to it....). I know CIS can be very sensitive to lots of minor issues that culminate into driveability problems, mine have just not become major enough yet to effect how the car runs.

Correct you are!!! If you have access to a smoke machine or a reliable tech that knows how to use one, you can find vacuum leaks and repair, maybe without pulling the engine.. If it 'runs great' leave it alone.. Don't start chasing ghosts.. My $.02 as always...

[psalt]

Nick,

It sounds like the meter or the hook up is not right. The needle should change its range with as little as 1/8 or 1/4 turn of the mixture screw. I would try a different meter.