Yes, the secondary butterfly opens rather late and that is normal.
For the lack of power and backfiring, I believe the ignition timing is too low. You would need a timing light to get the correct ignition timing. Anyway you can try this.
At the distributor there is a holding bolt that sits on a slotted hole. Mark the initial position of the distributor with a point where it sits onto the cylinder. Loosen the holding bolt and turn the distributor lightly counterclockwise.
Retighten slightly just to hold it and start engine. Retest the response. If it worsens, with engine running, loosens the bolt slightly so that you can twist the distributor and slowly turn it until you get the best idling at the highest rpm. It can be colockwise or counterwise depending on condition. Can't remember if the rotor turns clockwise or counterclockwise so one way advances the ignition timing where countering it would retard the timing. Tighten bolt and retest.
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Saturday, June 16th, 2012 AT 2:11 PM