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something i strange, hey? afr actuals are way off, but probably due to the 'o2 sensor stuffed up the tailpipe' routine and huffing enough ambient o2 to make it read lean.
so, ignoring afr actuals and just looking at the shape of the curve, something is still way strange. afr should run 14.7 until 4500 rpm or so, then drop to around 12.5 as the dme adds a bit more gas to keep things cool. certainly not what is happening here. so, given that it's already pretty certain that the way they are measuring afr is hack, perhaps chuck afrs out alltogether and just look at dyno.
no escaping there is an odd power drop. that drop could be causing a lag in the exhaust which is causing the o2 sensors up the tailpipe to misread even worse. or, chicken/egg, the afr fluctuation is causing power drop.
so, what's going on? that's around cam rollover rpm, so perhaps a solenoid sticking and causing the lurch which is throwing afrs off - get your durametric out and log cam position.
or resonance flap sticking? again, might cause a bump in power delivery.
perhaps some crud in the fuel system that is plugging the filter once fuel flow hits a certain rate - this will lean out your afrs and cause a bump, but not necessarily throw a code.
bad throttle body with a flat spot or a loose/sticky flapper? flat spot in the pedal actuator - you are cable throttle, yes? perhaps log throttle position, or swap in a known-good throttle body.
did you try a known good-maf? more than one maf has been bad out of the box. perhaps log your maf readings.
trk out.
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