Some engines, notably the old Fox Stunt 35, will run in an real 4-cycle when run a bit rich.  The engine will fire every other stroke.  The power is about 70% because the fuel/air charge is better from when the engine did not fire.  When the engine is loaded as in maneuvers, or leaned out, it will go into a normal 2-stroke mode.  Take off the load and it goes back to 4-stroke.   It is impressive to watch an airplane fly the stunt pattern when this works exactly right.  I just run in a rich 2-stroke and let it go at that.  More knowledgable people will clarify and correct my comments. 

Mike,

I'd only add to Jim's excellent description that you remember he said: "Some engines, ..."

Most mass produced modern engines could probably be worked around a bit, even if only with head gaskets, prop, fuel, plug, possibly some metal cutting inside... to yield this auto-switch from purr to shriek type of run. They aren't designed to do that out of the box, IOW.

For these engines, as Jim says, it is so much simpler to run in a low-2-cycle. That's more like what modern engines were designed to do, so they do it well, and last a nice long time that way. ...And they give more of the power they were designed to produce this way.

The Fox 35 is essentially unchanged since it first came out around 1950, and it still does well what it was designed to do. Odd fact: when the sound changes from purr to shriek, there is very little, if any RPM gain with the Fox 35 Stunt. ...just enough. Even the Fox 35 gains from certain practices that help it do its traditional thing, but those are well known. If you need to learn about them, ask! After you weather a storm of people asking you why you want a dinosaur to tow your plane around, you'll hear that a proper break-in is a great help, and (most often) that fuel with well over 25% all castor oil is a sure success with it.

As Jim said, when you hear a stunter going through a pattern in a 4/2-break setting, it is still impressive!

Ron,

Thanks, and excellent points, all.

I referred to the mass-produced, general (i.e., mostly RC) market engines availabe at bargain prices these days. They DO work, as I described it.  The purpose-designed/built CLPA engines cost a bit more and are a bit harder to locate for those just coming back into stunt.

With all the resources - like this forum - a new or retread flier can come up to whatever level of "speed" he/she likes.

It is a hobby, something we do because we wish to. That leaves the choices up to us, wherever our motivations and circumstances might lead                                                                                                                                                                                                                                                                                                                                                     

thanks for all the info fellas...dyin to find the time to get the ringmaster i got and the mustang back up to snuff and get to fly them again soon..i'm bidding on a couple of engines on ebay...couple of fox 35's and what seems to be a real nice os40 that is reworked..venturi, timing etc...all i have now is the merco 35 and merco 40...neither have nva's....and best i rmember the 35 has a bent crank..
(asphault is really hard on a ringmaster)..lol...the last time i ran the 40 was with the 35 nva...it never seamed to come up to power corectly...neither of these engines have alot of run time on em....maybe a dozen times each...i bought these in the late 80's..new in the box from hobby lobby....are these good stunt engines and worth having reworked and such??....wouldnt mind fixin em back up if it wouldnt cost a fortune...my cl budget is slim as my time to fly as we drag race harley davidsons on a full time basis.....i guess looking at the planes hanging on the wall everytime i go to the shop is givin me the itch....we use to fly the wings off cox planes and ringmasters when i was a kid....cant wait to do it again.....and thanx again for the replies.......Mike Mullis