Thread: 4stoke question

A 4 stroke engine fires 1 time for every 2 crank revolutions..


Sent from my iPhone using Tapatalk

Intake stroke and compression stroke make one revolution of the crankshaft, power stroke and exhaust stroke make the second revolution. Your plug fires at the top of the compression stroke. A 4 cylinder 4 stroke would fire 2 times per crankshaft revolution, a 6 cylinder 3 times and so on. I think a 50hp outboard is 3 cylinders correct?


Sent from my iPhone using Tapatalk

Originally Posted by Tex_79

A 4 stroke engine fires 1 time for every 2 crank revolutions..


Sent from my iPhone using Tapatalk

Not always. Many have a waste spark system that fires every revolution, especially multi-cylinder engines which have companion cylinders, where one is at TDC exhaust, while other is at TDC compression, in which case both cylinders can be fired by one dual-plug coil.

Many small magneto engines fire once per revolution as well, the coil is energized every time the flywheel magnets go past it, whether the engine is on the compression stroke or not.

I have no idea if any outboards are set up that way, but I would not be at all surprised if it fires once per revolution, doing it that way is often much simpler.

I think they all use that waste spark now days. So it would be 1 spark each revolution.

Originally Posted by Atimm693

Not always. Many have a waste spark system that fires every revolution, especially multi-cylinder engines which have companion cylinders, where one is at TDC exhaust and the other at TDC compression at the same time, in which case both cylinders can be fired by one dual-plug coil.

Many small magneto engines fire once per revolution as well, the coil is energized every time the flywheel magnets go past it.

I have no idea if any outboards are set up that way, but I would not be at all surprised if it fires once per revolution, as doing it that way is often much simpler.

That is very true but I’m pretty sure 3 cylinder outboards don’t use a wasted spark system but I could be wrong.. an easy check would be to see what your tach reads at idle, should be +/- 800 rpms, if it’s reading +/- 1600 and obviously not idling that fast your spark per revolution setting is too low. If it’s reading +/- 400 and obviously not idling that slow your spark per revolution setting is too high.. it will for sure either be 1 spark per 2 revolutions or 1 spark per revolution it’s not going to be higher then that


Sent from my iPhone using Tapatalk

Well, no consensus but at least a starting point.
I'll see how it goes.

Well it must be 1 spark/revolution. That's the setting I had to use to get correct reading.

Is your motor a 3 cyl? I want to get a tach for my 25 hp 3 cyl suzuki but cannot find a tiny tach that indicates it can be used for a 3 cyl. Please advise if you get one to work on your motor if it is a 3 cyl. 4 stroke. Thanks