15 hp 4 stroke johnson,2005

[ritchie]

my engine starts but wont idle even after a reasonable warm up time,also if you increase the throttle there appears to be excessive vibration with the occasional backfire. this is only the second time Ive tried it since I bought it 4 weeks ago. the first time it was reluctant to start, eventually started and idled ok after a lenghthy warmup period, however when you tried to increase the revs again it was erratic and wouldnt rev freely , any ideas?

[JUST-IN-TIME]

check for spark on both cylinders