One airframe that I've always found to be impressive is the Baby Flite Streak. How many in regards to this post actually flew a match of combat using period airframes and Cox Bees? If you think back on it, you probably would have more fun smashing yourself in the crotch with a sledge hammer. Integral tanked bees are plagued with air leak issues yielding not so successful back to back runs. I'm not saying one can't work their way through the issues because I do it all the time. The difference is when you do go up for a match, NOTHING works. Engines want to run backwards, engines that typically yield one to two flip starts don't want to start etc.
While not combat related, I've tried and tried to successfully do a full stunt pattern which flies just like the big counterparts. This is quite difficult. I mean a judged pattern with definition and all maneuvers easily recognized. This is not possible with a Bee of any kind due to capacity so I've utilized external tanks and product backplates. Applying all that I learned in doing so, I've found this to be very useful on combat planes . However, many designs such as the Satan and Kitten are tight for space.
I did put a tank into my Kitten and it was the worst thing ever. For starters, the engine was constantly fuel starved and without running rich, fuel draw was always a issue. Running rich on a combat plane is pretty unproductive in terms of what one is trying to achieve. Hard maneuvering would make the engine quit. Therefore, it was quite obvious early on that bladder pressure resolves this problem. It's not that a tank won't work but certain factors assist in fuel draw.
Old timey upright engine combat planes utilize their fuel tanks directly behind the engine and as close to the back of engine as possible. This alone is a key factor in successful engine runs. I've found square tanks to feed exceptionally well when mounted directly behind the engine whereas profiles like a coffin shape or pinched back outside corner. Many of the designs however don't allow for a tank installation directly behind the engine. Usually, the bellcrank platform is in the way resulting in a redesign if necessary. Another factor which also helps in fuel draw is uniflow. I know of no commercially available 1/2A tank. Therefore, making your tank uniflow is important and understanding design, cleanliness, technique is critical to success.
If this isn't your thing and your hard pressed to use integral tanked bees, which I still do, there's a few things to watch for. Obtain a 5 oz's luer lock syringe and use small fuel tubing on the tip. Take the tanked engine apart, hook the fuel tubing to the back of the tank with the syringe retracted. Press the syringe and it should go down easily, however your shouldn't be able to retract it backwards AT ALL!!! If you can, the reed is leaking and it's not going to work successfully. Before going any further, place a drop of oil on the reed and insure it wicks under it and try again to see if it passes the suck blow test.
Assemble the engine and now carefully tighten down the needlevalve enough to block it off. Using the syringe, place the fuel tubing on one of the filler pipes and with a spare finger hold it firmly against the overflow filler and press down on the syringe. Lend a ear to the engine, if the plunger immediately depresses, listen to where the air is leaking from. If you hear air directly from the rear of the engine such as the intake screen, the venturi gasket is leaking. The stock Cox gasket is a POS!. Obtain throwing dart o-rings, they're cheap, they're actually round and they work great for this application.
Other air leaks can occur at the screws, place a drop of oil on each screw and repressurize t and check for bubbles.These are a bit difficult to fix. Some use sealers, I prefer a cotton swab. I tear off a piece and use the cotton similar to Teflon tape using it the same way as to not unscrew it when tightened. Place a drop of oil on the tank to tank back connection and let it wick around the tank while keeping pressure on the tank. Air leaks can develop here as well which can be remedied by inserting dental floss into the v-groove of the tank. This is a pain in the rear to do but it does fix the problem. A drop of oil can hold the dental floss while laying it in there. I've found nitrile gloves good for this because the floss right when I get it all the way around sticks to my fingers. Only overlap a minimal amount. Also with oil check for bubbles at the case to tank joint where the gasket is, I prefer to lap the back of the case with 400 paper on a piece of glass using a little oil . The screws tend to pull material and raise the surface which I like to make absolutely flat prior to installing.
Also check your needle valve assembly, this can leak where it's installed into the backplate itself and also around the threads where the needle screws in. I've successfully removed the threaded body out of the die cast tank back and carefully reassembled once clean with JB weld if the leaks were present at the assembly to tank back. The needle leakage can be remedied by using small tubing on the threads with a washer on the needle.
Most importantly, is the plug. While it might glow and it looks bright, this always doesn't equate to the fact that when you fly it , it becomes unpredictable. These things are nearing $15 a piece and more from others. It's hard to not use the old ones.New plugs just seem to have a very overall positive impact on engine run. Look for the warning signs, rpm drops after igniter is removed, Roughness when running and inconsistent rpm's. Many times I've discovered the Cox engines when mounted in the traditional laid over profile fashion with cylinder to outboard tip can cause burping in outside maneuvers similar to the Fox.35. While it might some non traditional, I tried the inboard mounted cylinder like the Hyper Viper and it really seemed to me to improve the run.