'74 CIS Mixture Adjustment

I've got a 1974 911 Targa with the 2.7 CIS engine. It's running too lean at highway speeds. I've got the Haynes manual, but it's not to clear on how to adjust the mixture. Could someone help me out with this?

Thanks!

Mike

Here is an address for a real good explanation. the tool is a 3mm Allen about 5" long. It's a tight fit so length is critical. It's an explanation. Let me know if it doesn't work.
http://www.rpm.com.au/911/html/tech_f.html



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'83 SC