Yes, the duty-cycle represents the correction necessary to achieve the mixture required. When it is different it means the effective mixture is different.

In your case the mixture is richer at higher speed. That actually translates (usually) to the mixture being leaner at idle. This is the most common way it will be off and usually represents small airleaks or injector flows that are poor at small flow rates. One must remember that the mixture required is barely that which is necessary for good running.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician