The screw on the tuned pipe blocks a hole where a fitting can be installed to provide pressure to the fuel tank. Use it in non-pumped installations. With a pumped engine, you don't use pressure to the tank. The tank just gets vented to the atmosphere.

To set up a tuned pipe, you need to have it too long to start with. You then install the desired propeller and fill the tank with the type of fuel you wish to use. These cannot change once you've tuned the pipe to the engine. If you change prop size (in pitch, diameter, or both) or fuel, you'll have to re-tune the pipe.

Start the engine, lean it to maximum RPM, and then tach it. Shorten the pipe system by 1/4". Repeat the process: start, lean, and tach. If the RPM increases each time you shorten the pipe, keep going. When the RPM remains the same or decreases, leave it there and then start flying the model. Listen to it very carefully. There is a very distinct sound when the engine is "on the pipe".

Don't use the silicon joiner to "trombone" the pipe system shorter while the engine's running. You'll find that you won't be able to keep a setting that way. You have to lean the engine at a given pipe length and see how it runs.

Some other things: A too-short pipe system will cause the engine to trend lean as it's run. A too-long system will cause the engine to trend rich. Pipes have to have time to warm up before they work well, so you can't just "start and go". You'll need time to get everything warmed-up. More RPM needs a shorter pipe, so if you have a slow-turning engine, you'll go with a longer pipe.

As far as a non-pumped installation is concerned, you need to keep the fuel tank as close as possible to the engine. Much farther aft than behind the firewall will result in fuel-draw proplems. If you want the tank at the CG of the model, you'll need a pump.