explosions sounds fun. dynamite or just backfire? I´d check the idle mixture. sounds like its runnning lean as you shut off and fuel is still burning as it goes through the exhaust. could be dirt in the jet or just needs setting properly. start by adjusting the mixture screw (not the idle speed screw!)
1) warm engine thoroughly
2) count number of turns to gently wind screw all the way in (don´t force it cos it´ll break)
3) wind screw in till idle becomes rough
4) wind screw out till idle becomes rough
5) set screw to mid way between these two "positions"
6) re-set idle speed to correct value using idle speed screw
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