1983 Yamaha 30EN Outboards Service Manual, Page 170Get this manual
ENGINE MISFIRES
First check for spark to find cause
Fuel tank Fuel is deteriorated or dirty Fuel filter .Clogged Carburetor Clogged
Spark plug 1ncorrect gap, Carbon deposits, Dirty, Oily or burned Plug cap Broken or not tightly fitted gnition coil Defective, Check resistance COl unit Defective, Check resistance Ignition timing Check timing