It's often not the engine. This is especially the case on profiles. At least that has been my experience. When the nose fell off the ARF Streak as they do if left stock, I epoxied it back on using carbon fiber arrow shafts, even JB weld to fill in the hollow areas around the arrow shafts. Nose appeared absolutely solid. Couldn't get an engine to run consistently after that. Put the LA25 back on, inconsistent run, same with an FP25 and an FP20. Figured it was a bad profile harmonic. Went on to other planes. Dan, Mike , myself and other folks in our club have run into many profiles that set up a bad vibe which causes inconsistency and runaways. Many different solutions have been tried. No one solution fits every bird/engine combo. These days I build the noses of profiles as rigid as possible using the common methods. 5 ply 1/8", motor mounts back near to the leading edge, asymmetrical in length. Aluminum pads under the engine mounts. I've even used an aluminum plate on the inside of profiles, full engine width, engine bolts going through and cinching at the corners. Danny went to steel engine pads on his Foxberg. Bolts are cinched down tight as possible. FPs like rigid mountings, sometimes FOX 35s work better with a bit of flex, short engine bearers, thinner ply, the old time Sterling construction.
Then of course there is the issue of getting enough fuel to flow through an FP40 or 35 to keep it cool. This often means a tongue muffler with a lot of holes and a big venturi. At times the stock muffler can work once the back pressure is relieved by opening the exit hole as much as possible. Then there is the plug, the fuel, the prop, head gaskets and so on. We run FP25s like this too. A few new folks are into the 25 profiles like the Skyray. These engines have worked out well. I know this differs from the Brett Buck idea. I think the key tho is a low pitch prop, 9x4, something like that.
So many variables. Each plane/engine combo tuning differently.
In our experience LA25s are one of the easier engines to get to work. Usually we swap in a stock FP needle and aluminum back plate. Many times we only swap in the needle valve. The back plates are pricey or hard to find. The plastic stock back plate can benefit from silicon seal. Don't over tighten the screws. Low pitch prop of course.
Lately I've been overpowering Flite Streak size birds with FP40s and Towers. Have a lot of those engines. Like flying sport planes fast as I can do it. So. I haven't done much with FP25s or 20s. My reliable LA25 also has been ignored. Sometime in the future I plan to do one of those 25 Brett style stunters. Just for the heck of it. I have a few pristine examples of the 20/25 FP engines. I certainly will experiment with the West Coast idea. Curious to do so.
For Beginners we aim for engine consistency. Are not concerned about break. Often we tell folks to forget about the break when first getting into doing the figures. That's not to say that an FP25,20 or LA25 cannot be tuned to break by varying head gaskets, prop, fuel and other factors. We think that fliers new to stunt benefit from the simplification of approach.