Brett, when you say narrower, do you mean in top view?
Can’t get much narrower than the tank I’m using. It’s only about an inch wide, 1 1/2” tall and about 2 1/4” long.
That seems fine to me. But if you want to try to run a 9-6 (which it will do, no problem with running, big problem with flying), you will need a much smaller venturi, because you have to run it at much lower RPM, which also reduces the fuel draw dramatically. I would suggest instead that you use an APC 9-4. Set the engine by peaking it out lean in a 2-stroke, then backing off about 5-6 clicks until you get a *distinct* drop in RPM. It will probably be in the area of 11,500 rpm, but don't try to chase that setting, just peak it out and then back off 5-6 clicks. Fly it. You probably want something like 4.2-4.3 second laps assuming .015x60' lines. If it is too fast, or it seems to "sag" lean in maneuvers, particularly at the end of the flight, back it off by two clicks (you can observe the RPM but don't do anything based on the tach reading, just back off the needle and see what happens). If it ever drops into a 4-stroke, or it is too slow (more than 4.6 seconds/lap or so), then turn it in two clicks leaner, try again.
The likely issue is that it is too fast, rather than too slow. That's because the 20FP is too large/powerful for the airplane. There are several possible solutions to that, but the easiest is to put a few layers of panty hose nylon over the venturi inlet. Cut it into about 1.5x1.5 squares, and hold it on with a tight-fitting o-ring over the outside of the venturi. Then repeat, the same things should happen, but at a few hundred less RPM.
Note that this all presumes the engine is *completely stock* with all the original parts. I have flown and set up dozens of these engines and the setup and operation is remarkably consistent from engine to engine, if you follow the plan, it *will* work, or the problem is something outside the engine. If the engine is modified in any way, all bets are off, and my suggestion is to put it back to stock and try again, if possible.
Running a 9-6 loses a tremendous amount of performance even if you get it to work. This will make the flying much more difficult, and will expose the limitations of the airplane to a great degree. There's no reason to give up that kind of performance that I can see.
Brett