Is a Stihl 4-mix a true four cycle?

[ANewSawyer]

My brother and I were talking about four vs two cycle engines, mainly me because he doesn't like machines. He was postulating that a small, flywheel driven ignition system sparks every revolution even during the exhaust stroke. That got me thinking about a pet thought, wouldn't both 4-mix valves open during each revolution? It has the single piece cam gear, so in theory it should be opening both valves per revolution. And since, I believe that a 4-mix is set up to spark every rev, I get confused. Wouldn't that make it a two stroke if it had a power stroke every rev? Am I tired and thinking to much, YES!

 

[AVB]

First the 4 mix is a true four stroker with crank to cam ratio of 2 to 1; otherwords, for every crank revolution the cam turns a half turn. And yes it uses wasted spark setup as does most magneto based single and twin 4 cyclers. Even the v-twins that are using two magneto ignition coils are wasted spark systems. A good example of the system is the Briggs single cylinder engines.

Wouldn't that make it a two stroke if it had a power stroke every rev?

Can't produce power on the exhaust stroke when (1) fuel is already burnt (2) exhaust valve is open so no compression other than the pressure pushing out the spent fuel/air.

 

[ANewSawyer]

Ok, if it is two to one on the cam shaft, then yes it would be a true four stroke. I didn't know that the cam was geared 2 to 1. So my tach set to one spark per revolution would be correct.

 

[AVB]

Yes that is because it fires every time the flywheel magnet passes the ignition coil.

One thing to remember on these 4 Mix unit is that the valves do need adjusting every so often just like any other 4 cycle engine and you might want to get a few valve cover gaskets as they are cork if you work on several of these during the year. Also you may want to get the Stihl valve gauge for these as it very narrow so it will fit in due to the design of the rocker arm contact with the valve stem. Personally I had to cut down a gauge to do it here.

 

[ANewSawyer]

I did a valve adjust on my KM-130 when I got it. Thanks for the info!