Hotter plugs help at idle but can cause pre-detination at higher RPM.

Hotter plugs maintain their heat for a longer period of time, so they are usually needed for 4-strokes and will help some 2-strokes idle and transition. So, yes to what KitBuilder asked.

As for being in trouble with the A3? No, not yet. You have the plug, put it in and run it and see. If the engine doesn't idle or transition well, or doesn't produce the RPM it used to, or you hear a frying egg sound while it's running, and the engine didn't do any of that before, then maybe the plug is the issue.

I don't know why OS makes their plugs shorter than just about everyone else. Some Evolution engines I've run into have been helped by a hotter plug, but I actually run mine on a K&B 7300, which is a colder plug. But I'm using 15% nitro, took the baffle out of the muffler, and run an APC 11x5 prop, so I need a colder plug.

The stock Evolution plug appears to be on the hotter side, so if you want to use an OS plug, I'd start with the A3, but plenty of guys here use the #8 just fine in their Evolutions.

Frankly, unless you're either trying to maximize power output or running high nitro or really low nitro, or have played with the compression ratio of the engine, most of the medium to medium-hot plugs will usually work fine.

It does sometimes vary with the engine though. I've seen two brand new Evolution .61s on Alpha 60s actually seem to like differnet plugs. I think one was particuarlly tight in compression, but I'm not sure (didn't spend that much time playing with it, just got it running and went back to getting the student into the air).