Johnson Evinrude Outboard Motors 1956-1970 1.5-40hp repair manual., Page 41Get this manual
3 POWERHEAD 3-1 INTRODUCTION The carburetion and ignition principles of two-cycle engine operation MUST be understood in order to perform proper tuneup on an outboard motorTherefore, it would be well worth the time to study the principles of two-cycle engines as outlined in this sectionA Polaroid, or equivalent instant-type camera, is an extremely useful item providing the means of accurately recording the arrangement of parts and wire connections BEFORE the disassembly work beginsSuch record is most valuable during the assembly workTags are handy to identify wires after they are disconnected to ensure they will be connected to the same terminal from which they were removedThese tags may also be used for parts where marks or other means of identification are not possibleTHEORY OF OPERA nON The two-cycle engine differs in several ways from conventional four-cycle (automobile) engine1- The method by which the fuel-air mixture is delivered to the combustion chamber2- The complete lubrication system3- In most cases, the ignition system4- The frequency of the power strokeThese differences will be discussed briefly and compared with four-cycle engine operationIntakeExhaust Two-cycle engines utilize an arrangement of port openings to admit fuel to the combustion chamber and to purge the exhaust gases after burning has been completedThe ports are located in precise pattern in order for them to be open and closed off at an exact moment by the piston as it moves up and down in the cylinderThe exhaust port is located slightly higher than the fuel intake portThis arrangement opens the exhaust port first, as the piston starts downward, and therefore, the exhaust phase begins fraction of second before the intake phaseActually, the intake and exhaust ports are spaced so closely together that both open almost simultaneouslyFor this reason, the pistons of most two-cycle engines have deflector-type topThis design of the piston top serves two purposes very effectively