With a 1 wire o2 sensor it needs to be within a certain distance from the cylinder head or else it won't register correctly because it is too hot or too cold. I'd be willing to bet because of the proximity of the sensor to the open collector that it is not getting warm enough. If you can I would put a cheep muffler on and see if that helps, I'm thinking the muffler will hold enough heat to be adequate for the sensor.
Something else to point to the o2 sensor is that I don't believe the o2 sensor comes into play untill it comes out of hi idle **I might be wrong about this**
Last edited by Wehan; July 8th, 2014 at 09:46 PM.