Scott,
You have been given some good suggestions by folks who are much better flyers than I am. I am not going to contradict what they have told you. But I will add the extent of my personal experience with and observations of the two additional Hyper Vipers I've seen fly in my area, and you can decide which path to follow. They are fun to fly planes that now cost $43.00± at our local hobby shop. As you know they are complete with everything necessary for flight except the fuel and batteries.
Since you state that the only reason that you succumbed to that moment of insanity was to get you in the air to relearn inverted flight, I would advise you to do only a few things to get ready to fly.
1. Before assembly, test the tank in the manner that Nick suggested. Get a replacement, if necessary, directly from Cox.
2. Two of the three HV's flown here required a new tank for the very problem he describes. Assemble the plane exactly as described in the instructions, including (heresy!) using the supplied Dacron lines and handle.
3. Push a nickel into the "tip weight box" (the hole on the bottom of the outboard wing) for the extra tip weight recommended.
4. Use adhesive or clear tape to fasten the front of the canopy to keep it on the plane during flight.
5. Start and break in the engine exactly as shown in the instructions.
6. Go fly, preferably over grass until you feel comfortable with your progress with inverted flight.
While all the other suggestions have merit and will undoubtedly increase performance, the extra effort is not necessary to obtain a plane that will capably fly reverse wing overs, inside loops, lazy eights, inverted flight, and outside loops when built stock except for the items I’ve previously mentioned.
All three of the local planes were purchased at the same time and from the same source, along with three sets of SIG .008" × 35' lines. We got a wild hair to fly the planes one afternoon before the lines were made up. rjbob, one of the other flyers, had a set of .008" lines in his box, so he used those, but the two of us remaining, CR and I, were without, so we used the full length of the Dacron included with the plane, as well as the Cox handle. We broke in our engines at the field that day.
After three recommended break in runs, CR was first up and his plane flew well, doing the maneuvers I’ve described effortlessly, even though the engine was a touch rich. Upon landing, he restarted the engine peaked it out and flew again. This flight was great, as was his third, which immediately followed the second.
rjbob was next, and he had his needle set properly, but his plane appeared much faster than CR’s had been. After a few very fast, tight maneuvers, he flew the remainder of the flight pretty much level, getting dizzier and stumbling around the circle more and more as he flew. The engine finally quit and, to his relief, he landed. It was then that we discovered that those .008" lines of his were only 27’ long instead of the 35’ he thought they were. That extra 7' of line that CR used certainly made a big difference.
At that point, rjbob had to get back to his office, CR had some errands to run and I needed to be somewhere else, too, so we packed it in. The maiden flight of my own Hyper Viper was a few days later, but my experience was identical to CR’s. I, too, used the Dacron lines even though the .008" × 35’ lines were made up by then. We will probably switch to the .008" × 35’ lines for next summer’s flying (I know that rjbob will!) but will probably not make any of the other changes recommended by others unless we decide that we need to get to the Nth degree of performance out of our engines.
I should also mention that all the flights flown so far have been with less than ideal fuel (15% Nitro, 22% lube). Cox, as you’re aware, recommends a minimum of 25% nitro and a maximum of 18% castor fuel. We now have the Byron’s 1/2A fuel, which has that composition, but have not used it yet. This alone should improve performance quite a bit, even though we haven’t yet suffered from lack of performance.
For the price, I don’t think that one can’t beat the Hyper Viper, even stock, for its good, fun flying characteristics. In light of my experience, I would not hesitate to advise you to fly it stock at first and decide for yourself whether the extra mods are necessary for your flying style.