The best answer to this question that I have seen to date is that a good stunt engine must have the ability to accelerate rapidly from a loaded speed condition back to a non loaded one.

Now the 'how' of doing this is bound to vary.

"I have an engine that was altered to only run in a 4 cycle mode at all times"

Must have been a four stroke then.

     Hello All:

     What is needed to make an engine run well for Stunt to give a 4-2-4 engine run? 

 A 4-2 break engine hasn't been a mainstream stunt engine in the US for close to *30 years*. The gold standard of stunt runs is a completely unmodified RC pattern engine - the OS 40VF. you *do not want* to try to modify most current engines for a 4-2 break run, all that does is lose all the power.

Record (NATS Open Championships since 1990)
20 piped (PA/OS VF/RO-Jett)
3 4-2 break (1 ST60, 2 Discovery Retro 60)
2 electric
1 4-stroke  (Saito 72)

  And in the cases where the 4-break engine won, it was the only one among the contenders. If you want a good stunt engine, get a PA, RO-Jett, or OS VF.

     Brett
   

Hey Frank:

Prognostication is always iffy, but I think that IC engines are going to be around for a while, just not in as great of numbers.  The 46LA is gone because from the RC'ers perspective it's a gutless wonder -- it was designed to be easy to handle and inexpensive, but as electrics got cheap enough it was simply overtaken.  The 46AX has almost twice the power of a 46LA, makes more money for OS, and will probably be around for a while.

If you're going to fly a 40-sized RC plane you need a special place to do it, whether it's IC or electric powered.  I think this is going to keep the space open for slimers, at least for the time being.

     Hello Again:

     Yesterday, I spent an afternoon flying Stunt and Sport.   I came away with some new ideas as to what makes a good Stunt engine.   will attempt to elaborate about my opinions below.

     There was a LA .40 and a LA .46 engines in use that impressed me greatly.  They were instant starters that ran flawlessly.  To get them to be a "perfect" Stunt engine three things were done to them:

    1. The prop was an 11x5 on the .40 and the prop on the .46 the prop was an 12x6.

    2. The needle valves were set to run in a wet 2 cycle.

    3. The airplanes were large enough to provide the desired airspeed at all times.

     These were box stock engines with no modifications that provided perfect Stunt runs! It could not get any easier!

     I got sucked into thinking that I needed a modified engine that would be a perfect Stunt engine but have since learned that my FOX .40 Stunt circa 1972, my OS .35 Stunt, and OS .40 S give what used to be a nearly perfect  4-2-4 stunt run of days gone by.  I would guess that there are many other engines that perform as well as the OS LA series without any modifications by engine tuning experts.  What engines did you fine that worked well without any modifications?

     Lesson learned was to FIRST try your existing engine to determine if you are able to extract the optimum performance from it by experimenting with the fuel, plugs, props, needle valve settings, line length etc before investing in a custom tuned or custom engine. Are you satisfied with your findings? My next project is to take some of my RC engines and wire the carbs open and see if they will work as a suitable Stunt engine.  I may be pleasantly surprised. Lol

     I think that I have ben making a mountain out of a molehill and overthinking my Stunt engine needs.  Has anybody else made this discovery?

                                                                                                            Aint this hobby interesting?

                                                                                                            Frank McCune