there is a breather vent on the engine crankcase - its hidden behind the upper engine bolt inside the chassis metal, if its dislodged it will make a "puffing"
This vent is also found on CG125 but its exposed, to get to the CD70E engine vent you need to remove the engine.
The really old CD70E models (first to about mid 80s) had the vent that shoots out into the magneto cover from a little hole.