Discussing dwell meter/Air fuel mixture & cooler running engines.

[Damon88]

I have been searching old posts looking for more information about using a dwell meter to fine tune the Air fuel mixture. Also looking into trying to make my not emissions 83 911 run cooler in these hot summer months. I came across an old post from user Psalt from back in 2015. He mentions using a Dwell meter and setting the Air/fuel between 25-35 rather than 45-55 on idle will reach the goal of running rich at WOT ?( currently I have mine set at 55 fluctuating) He also mentions disconnecting the vacuum retard and plugging it, then resetting timing and idle. This will allow to make the engine run smoother when idling. here is a screen shot of his recommendations. Would love to hear from others on their thoughts. Cheers

Here is the original response to this topic by Psalt back in 2015

The best way to set the mixture on a lambda 911SC is with a dwell meter. Idle mixture is larqely irrelevant outside of emission testina. You goal should be a rich WOT AFR in the 13's and this usually occurs when the idle dwell fluctuates between 25-35 rather than 45-55. Most of these cars run best with the vacuum retard disconnected and pluaged. the hot breather air out of the intake and as much timina advance the best fuel vou can find can subbort. These cars had a fluctuating idle when new and the goal of a rock stead idle is misguided fantasy.
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[AndrewCologne]

For correctly setting up the mixture following emission restrictions you unplug the sensor and set the mixture screw until a CO of 0.4-0.8 results. After replugging the sensor the lambda control shifts the duty cycle up to its ballpark between 50-60%.

But this results in a too lean acceleration or WOT mixture as the ECU shifts the Duty Cycle to only "static" 65% when accelerating. In many many tests I came to the result that a jump to +30% added to the current Lambda 1 duty cycle is needed to reach Lambda 0.85 means best for acceleration.

So as Psalt described, you should set the mixture that a lower Duty cycle results at idle when lambda control is active.
Leave the sensor connected and watch the resulting Duty Cycle using a DMM with Duty Cycle option. Then turn the CO mixture screw clockwise till you obtain a duty cycle of approx. 35% at idle.
So ... by this at accelleration or WOT when the ECU sets the Duty cycle to 65% this results in a mas o menos Lanbda 0.85 state.

Everything is explained here:
https://nineelevenheaven.wordpress.com
Just use an online translator as till now I did not have the time for translating my site

[Damon88]

Andrew thank you for responding. This now makes sense. I currently had my duty cycle at idle set around 55% with sensor plugged in. I will adjust and seesaw it runs. lastly, you Wordpress site is amazing. Thank you for doing what you do. Very much appreciated!

[David Inc.]

Originally Posted by AndrewCologne

For correctly setting up the mixture following emission restrictions you unplug the sensor and set the mixture screw until a CO of 0.4-0.8 results. After replugging the sensor the lambda control shifts the duty cycle up to its ballpark between 50-60%.

But this results in a too lean acceleration or WOT mixture as the ECU shifts the Duty Cycle to only "static" 65% when accelerating. In many many tests I came to the result that a jump to +30% added to the current Lambda 1 duty cycle is needed to reach Lambda 0.85 means best for acceleration.

So as Psalt described, you should set the mixture that a lower Duty cycle results at idle when lambda control is active.
Leave the sensor connected and watch the resulting Duty Cycle using a DMM with Duty Cycle option. Then turn the CO mixture screw clockwise till you obtain a duty cycle of approx. 35% at idle.
So ... by this at accelleration or WOT when the ECU sets the Duty cycle to 65% this results in a mas o menos Lanbda 0.85 state.

Everything is explained here:
https://nineelevenheaven.wordpress.com
Just use an online translator as till now I did not have the time for translating my site

In other words you're adjusting the mechanical mix rich, letting the lambda lean it back out, such that when it jumps to 65% at WOT it's actually running richer?

I've read a bunch about lambda control but honestly I think it's one of those things that I won't really internalize unless I adjust it myself some day.

[ahh911]

Hi Andrew,

Good to read.

One question, when I set my 02 duty below 40%, the first few minutes of drive can be a little jerky, even sometimes later in the drive. Is this just me?

Also, there is a 75% option that we could re-wire on the "ecu", it has very little use otherwise, have you invested any time into substituting that 75% for the 65% full throttle mode by modifying the "ecu"?


Thanks,
Phil

[AndrewCologne]

Originally Posted by david inc.

in other words you're adjusting the mechanical mix rich, letting the lambda lean it back out, such that when it jumps to 65% at wot it's actually running richer?

e x a c t l y

[AndrewCologne]

Originally Posted by ahh911

Hi Andrew,

Good to read.

One question, when I set my 02 duty below 40%, the first few minutes of drive can be a little jerky, even sometimes later in the drive. Is this just me?
l

That's cause this also effects the mixture at cold running with also fixed 65% duty cycle.