The afterfire (commonly called backfire, but backfire is thru the carb) is caused by the epa tuned altra lean carb. A pop upon closing throttle can be tuned out by readjusting the pilot screw on the right side of the carb. The screw is hidden as it comes from the factory by a plug. Most likely you'll have to drill it out to adjust it. Give it about a 1/4 out (ccw), that fixed it for me.
Other sources of the pop are loose header bolts, torque them with the engine dead cold to 15 to 20 ft-lbs.
Or the slip on muffler clamp can be loose.
Also check the carb duct clamps, a leak thru there can cause it too.
p.s. sorry about the late reply. we must've been deep into another discussion. Give us a shout if we don't reply.