1998 Johnson 50 2 cylinder 2 stroke - why two sparks per rotation?

[TMW123]

Why does my tiny tach knock off need to be set to 2 sparks per rotation on this engine to be accurate? Is it wasted spark?

 

[flyingscott]

That tach has to be set at 2 to get a accurate reading. Since a 2 strk fires twice as many times as a 4 strk the tach would read wrong. A 4 strk running at 4000 rpms will fire 1000 times. A 2 strk running at 4000 rpms will fire 2000 times. So that # needs to be translated with that setting the tach knows it is a 2 strk.

 

[racerone]

No-----Not wasted spark on a 2 stroke.------Each plug fires once per revolution.----Perhaps read the instructions again.-----If they are a Chinese translation all bets are off.

 

[tomhath]

I suspect your instructions are for a connection at a distributor when there's only one coil in the system.

A 2-stroke fires each cylinder once per rotation, a 4-stroke fires each cylinder once per two rotations. More cylinders = more firings per rotation.

However...your motor has two coils that each fire every rotation, so you need to lie to the tachometer and pretend it's connected to the distributor of a four cylinder/four stroke motor, i.e. two firings per rotation.