Could be that your valves are not properly adjusted and are pumping up and
hanging the valves open.  I would zero out the valve adjustment (let the
adjust screw just touch the valve) and see how you make out.
Who did the rebuild?  (Just curious).
PS Oxygen sensor is $110 (per your phone call).

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
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