Well my answer came back to haunt me. Last year I bought a 4 oz metal tank to replace the stock Nobler Arf tank (and also my 4 oz clunk tank), and with it, my Brodak 40 just ran great. My only "problem" was that with a borrowed BY&O 11-4 prop (thanks Fred!), the combination had a tendency to wind up in the wind.
Early this year I decided to look for a new prop which didn't have this windup problem, starting with an 11-4 Black Grish Tornado. I immediately began to have run problems, which I thought was due to harmonics frothing up the fuel in my tank. I added some bamboo skewers to tie the two hardwood mounts together, but the problem persisted. I went to other props (APC, Zinger), and although I tend to like the behavior of the lower pitch, the engine runs became very strange. I attributed the problem to the fuel foaming, and engine heat up. I could get "ok" runs, and flew ok, but usually by the end of the pattern, the Brodak 40 would be in a rich 2 Stroke run. Fortunately the 2 stroke in a Brodak 40 isn't much stronger than the 4 stroke, so I could still fly the combo.
Anyway the problem kept getting worse. I was convinced this was a fuel/heating problem and not a fuel supply problem because I convinced myself that I could hear the uniflow vent coming uncovered near the end of the run, and the engine tended to go richer at that point. Finally this Monday I became fed up and realized that the run really resembled what you would have with a standard (non-uniflow) vented tank. So out came the tank. What I found was that on the top surface of the sheet metal, near the vent tube, a little pitted hole, which I believed was due to a blop of acid flux (from the original soldering of the tank), slowly etching its way thru the side. Since it was on the top surface, near the vent tube, I didn't notice any fuel leakage at all. After I fixed this, I also found just a tiny little leak through the uniflow vent tube/front endplate, but when I went to close that tiny leak, the uniflow vent dropped into the tank! (meaning the other end of the tube was not held internally to anything). This meant I had to open up the end of the tank @$)$_@---and I thought of Fred!
Well It has been a long story, but suffice to say, the engine run is now back to where it was last year, a nice solid growl around the circle, and my search for prop nirvana can begin anew. And yes, I think I'll just make my own tank next time.
