Best fuel and plug, not sure. I use K&B 7300 (HP) plugs, and 15% to 20% nitro on the fuel, depending.

If it's running good for 2min, then sagging, what happens if you launch with half a tank of fuel? If you run for another 2min and then sag, you probably have a tank setup issue, or you're too lean. also check you're muffler pressure if using a conventional tank. If it just won't run right at half a tank or so, then I'd suspect a problem with the tank pickup. If you find you have to richen at half a tank vs a full tank to run right (or get the same RPM on a tach), then your tank is probably too low, or you have something else with the fuel draw.

Fwiw, I never thought of the Webra as hard to needle or touchy engines at all, mine were always easy and smooth running, and had surprisingly good idles for having such a huge carb opening. But then I always made sure my tanks were mounted rather high with these engines, fuel draw with high performance engines is just so critical.