Today at the field a friend of mine had what appeared to be (and he said it was) a "new" OS 40 4-stroke. In this case I believe he meant it was "newly" acquired by him, but it also appeared to be in like-new condition. At this writing I do not know if it was a very early model (back in the 80s) that had never been used, or if it is a newer model like you can buy from Tower Hobbies or other vendors. He bought it from someone else supposedly "new in the box" and sure enough, it appeared to have never been run. We double-checked the valve settings which were fine (by today's OS recommendations), and he is using 15% Wildcat with synthetic lube and a good "F" plug. We set the needle valve 2 1/2 - 3 turns and it started right up and sounded great. We made sure there was a good smoke trail (running rich) and the throttle was fine from low to about midrange. This was all done with the engine on a plane and the plane on the table........no flights yet.
Now here's the problem......and I have never seen this happen before: As we slowly (or quickly) advanced the throttle past midrange, a "popping" sound developed rapidly which soulded like pre-detonation or backfire....and the engine would instantly change from running in the proper direction to running backward and would then quit!!!
I could not believe an engine running in the correct direction and so smoothly would suddenly "pop...pop...pop...pop" and instantly change direction. This must be horrific on the internal parts whose inertia is headed in one direction to suddenly have everything change to the opposite direction.
In any case, we have no clue as to what is causing this....and it is readily repeatable.
I told my friend not to run it anymore until we could learn more about what might be causing the problem.
I also told hime that my guess (if it is an old model engine) is that the cam timing may be out of whack (possibly from the factory or poorer quality control during the early years of manufacturing)....or possibly the person he bought it from "new in the box" may have taken some of the engine appart and not reassembled it correctly...but this is just guesswork on my part.
(a) how can we tell if this is a new or old model engine?
(b) what could be causing an engine to run fine at low to mid throttle...and then suddenly (and I mean INSTANTLY) reverse direction and stop?

Thanks for your assistance.....
ob1