hi ,
looking as your description of problem sound like coil problem or a air leak ?
check both cyls firing, pull 1 lead off at a time and note what happens , if pull r/h side lead and engine stops it is only running on the left hand cylinder
, but if pull l/h lead off and it stops then its running on r/h cyl, a engine will run on one cylinder ok until the load demand is more than the power output for the unit running on 1 cylinder , the unit then stops , just be sure fuel supply and fuel pump are in good working order, as is compression and also check for leaking inlet manifold gasket @ cyl head ( if its broken / leaking sucking air ) it will run on 1 cyl as well have seen this happen even though compression and spark and fuel were good
if this is good then coil is suspect on the side that is not firing - dismantle and replace coil , reassemble and recheck
hope this helps
pauly50