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What year,what mileage?
If you have a real oldun with c/b points and condenser then could be dirty /badly gapped/worn/burnt-out points or faulty condenser(which is essentially a tight wad of rolled-up tin foil which absorbs excess current not used by the ignition,thereby preventing excess wear/arcing at the points)
The odds are you have electronic ignition so next thing to check is all the wiring for loose connections at the junctions or corrosion.
Then check the tappets,an easy job on all stepthru models,Haynes manual will show you how.With high mileage,tight valve clearances can occur and will cause back-firing,clearance does reduce as valve faces/valve seats wear....talking from experience on this model.
There is a screw at the bottom of the carb float-bowl which when slackened drains petrol through a plastic pipe you should have attached.Use a clean jar to collect contents of float-bowl and leave petrol tap on to flush through a little before re-tightening the screw.If a lot of crud comes out you may have some muck in a jet.
Lastly(or maybe firstly!) give your silencer a thump with your fist(preferably when cool :) )If it rattles then your baffles are collapsing.You`ll`ve noticed a noisy exhaust note if this is the case...only likely with high mileage.If the silencer is falling to bits internally then it will affect both exit of gasses and mixture.
Good luck and let me know what you find.
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