Part of the Fuel-Powered RC Airplanes Center
A fuel tank looks simple enough, which is probably why it is so easy to overlook. One small problem with the stopper, clunk, vent, pressure line, tubing, or tank position can make a perfectly good engine seem impossible to tune. If the mixture changes every time the airplane points up, down, or rolls, check the fuel system before you start chasing the carburetor needles.
Glow and gasoline systems are not identical
Glow models commonly use a tank with a feed line to the carburetor, a pressure line from the muffler, and sometimes a separate fill line. Gasoline models may use a vented system without muffler pressure and require gasoline-compatible tubing, stoppers, filters, and fittings. Follow the engine and tank manufacturers’ layouts.
Tank position
As a starting point on many glow installations, the tank centerline should be close to the carburetor’s fuel inlet or spray-bar height. A tank mounted too high can encourage siphoning and a rich run. A tank too low can make fuel draw difficult, especially when the nose rises.
Gasoline engines with pump carburetors are often less sensitive to tank height, but the installation still needs correct venting, secure plumbing, and a freely moving clunk.
Build the tank carefully
- Use a stopper and hardware compatible with the fuel.
- Cut the feed tube so the clunk can reach the rear corners without jamming against the tank wall.
- Make sure the clunk cannot fold forward and become stuck after a nose-over.
- Arrange the vent or pressure tube according to the tank and engine instructions.
- Tighten the stopper enough to seal without distorting or splitting the tank neck.
- Mark the lines before installation so feed, fill, vent, and pressure cannot be confused later.
Choose the correct tubing
Use tubing specifically intended for the fuel in the model. Glow tubing and gasoline tubing are not automatically interchangeable. Replace tubing that has become hard, soft, swollen, cracked, discolored, or loose on its fittings.
Inside the tank, use tubing flexible enough for the clunk to move but not so long that it curls, pinches, or traps the clunk.
Mount and protect the tank
Secure the tank so it cannot shift, but isolate it from sharp edges and excessive vibration. Foam should support the tank without crushing it. Keep fuel tubing away from hot exhaust components, moving linkages, and places where the cowl or hatch can pinch it.
Pressure, vent, and fill lines
- Glow pressure line: Must remain open from the muffler fitting to the tank and resist exhaust heat.
- Gas vent: Must be routed as the manufacturer recommends and cannot be plugged during operation.
- Fill line: Needs a reliable plug or valve that seals completely after fueling.
- Carburetor feed: Keep it as direct as practical, with secure fittings and an appropriate filter if recommended.
Leak and flow test
Before installing the tank permanently, cap the appropriate lines and apply only gentle pressure using a suitable test method. Look for bubbles or pressure loss around the stopper, fittings, and tubing. Then confirm fuel can flow freely through the feed line and that the clunk moves through the required range.
Do not use excessive pressure. A damaged or over-pressurized tank can leak or fail.
Foaming and bubbles
Continuous bubbles in the feed line may come from a loose connection, split internal tube, poor stopper seal, excessive vibration, or fuel foaming. An occasional bubble after fueling is different from a steady stream while the engine runs.
When the engine changes mixture in flight
- Confirm the clunk remains submerged and free to move.
- Check tank height and fuel level effects.
- Inspect the muffler pressure line on glow systems.
- Look for pinholes, loose fittings, and cracked tubing.
- Verify the vent is open on gasoline systems.
- Check for heat affecting the fuel or tubing.
- Only retune the carburetor after the plumbing passes inspection.
Preflight fuel-system checklist
- Tank secure and dry
- Correct fuel-compatible tubing
- Clunk moving freely
- Feed, fill, pressure, and vent lines correctly identified
- No pinched, hardened, cracked, or loose tubing
- Fill plug or valve sealed
- No bubbles or leaks during the ground run
Continue with the glow-engine starting and tuning guide, the gas-airplane setup guide, or the engine troubleshooting guide. Return to the Fuel-Powered RC Airplane Center.