Recommended CO level/range for 2.7 RS Engine?

[73ess]

What is the recommended CO setting for a 2.7 RS spec engine with MFI? Mine is measuring 3.5% @ approximately 2500 rpm and 2.25% at idle. At these settings I'm getting some slight surging between 2000 rpm and 3200 rpm. I'm thinking I should adjust the part throttle fuel mixture setting on the MFI pump to richen the mixture more, but I have a CO meter, so thought I'd ask the experts what level I should set it at? Otherwise I guess I'll just keep adjusting it richer one click at a time until the surging goes away.

[jrbennett]

Here's the specs from the Porsche MFI check measure adjust manual
Looks like you might need to lean the part load and richen the idle

[edmayo]

My experience has been that although we used to use the factory C/O numbers, that they only worked because we had real gas at that time. We have found that with today's corn syrup idle C/O settings have to be in the 6% range.

[72targa]

Title corrected...interesting topic!

[73ess]

Thanks Ed. Good to know! So I will adjust the part throttle mixture screw setting to the 6% range. What should the CO setting be at idle speed? I'm assuming that the spring loaded idle mixture screw adjusts the CO setting at idle speed, but my experienced Porsche mechanic tells me that he usually doesn't touch the idle mixture screw on MFI pumps. In his experience it doesn't have much affect on CO values. He says the part throttle mixture screw will adjust/affect the CO setting both at idle AND part throttle.

On a somewhat related note. What is the proper CO setting(s) for a 73.5 911T with CIS injection? Do I measure at idle speed AND part throttle as well?

My experience has been that although we used to use the factory C/O numbers, that they only worked because we had real gas at that time. We have found that with today's corn syrup idle C/O settings have to be in the 6% range.

[edmayo]

The 6% number is the idle setting, adjusted with the spring loaded idle mixture screw. The load setting is another matter, a no-load setting for that range was usually in the 9% range, which would equate to around 3-4% under load (full throttle 3rd gear). While it's correct that adjusting the load screw (rack adjustment) will affect BOTH idle and load, and the idle screw will ONLY affect idle, as to which you adjust depends on which range you're trying to correct. Normally the procedure is drive and adjust the load setting, then finish up with the idle setting, since that won't affect the load setting.
73 CIS will need about the same idle percentage, but there is only ONE adjustment, which affects all ranges.

[Frank Beck]

6% idle CO for CIS?
Say what?!

Think you'll find HC off the chart with a number like that.
With all due respect to Ed, try half that.

[73ess]

Hmm...I'm down to my last "richer" click on the part throttle mixture set screw and my measured CO @2500rpm is only 3.75%. I can't get the mixture any richer than that. I'm guessing that's not normal. Haven't tried the idle mixture screw yet, so I'll see how rich I can get the idle mixture before I run out of clicks. I'm turning the part throttle mixture screw ccw for richer and I know the idle mixture spring loaded screw goes clockwise for richer.

The 6% number is the idle setting, adjusted with the spring loaded idle mixture screw. The load setting is another matter, a no-load setting for that range was usually in the 9% range, which would equate to around 3-4% under load (full throttle 3rd gear). While it's correct that adjusting the load screw (rack adjustment) will affect BOTH idle and load, and the idle screw will ONLY affect idle, as to which you adjust depends on which range you're trying to correct. Normally the procedure is drive and adjust the load setting, then finish up with the idle setting, since that won't affect the load setting.
73 CIS will need about the same idle percentage, but there is only ONE adjustment, which affects all ranges.