Fuel System Checkup for Glow and Gas Models

There are few things more frustrating than carrying your airplane out to the flight line, starting the engine, getting everything adjusted just right—and then hearing the engine suddenly change tune halfway through the flight.

Maybe it goes lean. Maybe it starts sagging at full throttle. Or maybe it simply quits.

The engine usually gets blamed first.

But sometimes there is absolutely nothing wrong with the engine.

A glow or gasoline engine depends on a surprisingly simple fuel system: a tank, some tubing, a pickup or clunk, vents, fittings, and possibly a filter. Yet a tiny leak or restriction anywhere in that system can cause an otherwise perfectly good engine to run poorly.

A periodic fuel-system checkup can save a lot of frustration at the field—and possibly save an airplane from a dead-stick landing.

Start With the Fuel Tank

The fuel tank is hidden inside the airplane on many models, which makes it easy to forget about.

If you can access it, inspect the tank itself for cracks, damage, or signs of leakage. Pay particular attention around the stopper and fittings.

Check that the stopper is tight and hasn’t deteriorated. Rubber components can harden, swell, or degrade over time, especially when exposed repeatedly to fuel.

Also examine the metal or plastic fittings passing through the stopper. They should be secure and properly sealed.

If you see fuel residue around the tank or smell fuel somewhere you normally shouldn’t, don’t ignore it. Something is leaking.

Check the Clunk

Inside most fuel tanks is a flexible pickup line with a weighted fitting commonly called the clunk.

Its job is simple: follow the fuel.

As the airplane climbs, dives, banks, or flies inverted, the clunk should move toward the fuel so the engine continues receiving a steady supply.

Before installing a tank—or whenever you have access to one—move it around and listen for the clunk.

You should be able to hear or feel it moving freely.

The internal fuel line needs to be long enough for the clunk to reach the important areas of the tank but not so long that the clunk becomes trapped against the rear wall.

A clunk stuck in one position may allow an engine to run perfectly on the ground and then quit when the airplane changes attitude.

That’s not the kind of surprise anyone wants after takeoff.

Inspect Every Inch of Fuel Line

Fuel tubing doesn’t last forever.

Over time it can become hard, brittle, cracked, swollen, discolored, or loose around fittings.

Check every piece of accessible tubing.

Gently bend it and look for small cracks, especially near fittings where the tubing has been stretched. Make sure each connection is secure.

Don’t forget that the problem may be extremely small.

A pinhole that barely leaks fuel may still allow air into the system. That air leak can create bubbles in the fuel supply and cause an engine to run lean or inconsistently.

If tubing looks questionable, replace it.

Fuel tubing is inexpensive. Airplanes aren’t.

Use the Right Tubing for the Fuel

Glow fuel and gasoline are different chemicals, and fuel tubing needs to be compatible with whichever one you’re using.

Traditional silicone fuel tubing works very well with glow fuel, but ordinary silicone tubing should not be assumed suitable for gasoline.

Gasoline models require tubing specifically designed for gasoline or petroleum-based fuels.

Using incompatible tubing can cause it to soften, swell, harden, or deteriorate. Eventually it may leak or separate from a fitting.

Whenever replacing fuel line, make sure you’re installing the correct type for the fuel your engine uses.

Check the Vent System

Fuel has to leave the tank, which means air has to enter it.

That’s the job of the vent system.

A blocked or incorrectly routed vent can create a vacuum inside the tank as fuel is consumed. Eventually the engine may begin running lean or stop completely because fuel can no longer flow properly.

Inspect the vent tubing for kinks, restrictions, or damage.

Make sure the vent opening isn’t plugged with dirt, fuel residue, or debris.

Gas-powered models may use different vent arrangements depending on the tank and engine installation, so follow the manufacturer’s recommended setup.

Whatever the arrangement, the important point is the same:

The tank needs to breathe.

Don’t Forget Muffler Pressure on Glow Engines

Many glow-engine installations use muffler pressure to help provide consistent fuel delivery.

A line runs from the muffler pressure fitting back to the fuel tank.

That little piece of tubing can create a surprisingly big problem when something goes wrong.

Inspect it for cracks, loose connections, burns, or deterioration from heat and exhaust residue.

Also inspect the pressure fitting on the muffler.

If the fitting becomes plugged with exhaust residue, tank pressure may be reduced or lost.

An engine that suddenly becomes difficult to tune or changes mixture between idle and full throttle may have a fuel-pressure problem rather than a carburetor problem.

Before reaching for the needle valve again, check the pressure line.

Inspect Fuel Filters

If your model has an inline fuel filter, inspect it periodically.

Filters can catch dirt and debris before they reach the carburetor, but eventually that material can restrict fuel flow.

Make sure the filter is clean, securely connected, and installed correctly if it has a specified direction of flow.

Also check the filter housing for leaks.

A filter that prevents debris from reaching the carburetor is useful.

A filter that prevents enough fuel from reaching the carburetor is considerably less useful.

Check the Carburetor Connection

Follow the fuel line all the way to the carburetor.

Make sure the tubing fits securely over the fuel inlet and isn’t cracked or stretched.

This area is exposed to engine heat and vibration, so it deserves regular attention.

Also inspect the carburetor and surrounding area for unusual wetness or fuel residue.

A loose connection here can introduce air directly into the fuel supply just before the fuel reaches the engine.

When an Engine Problem Isn’t an Engine Problem

This may be the most important lesson in the entire fuel-system inspection.

Pilots naturally start adjusting the carburetor when an engine isn’t running properly.

The engine runs lean, so we adjust the needle.

It sags at full throttle, so we adjust it again.

Then we turn the airplane nose-up and the engine changes tune, so we make another adjustment.

Pretty soon we’ve adjusted a perfectly good carburetor completely out of adjustment while the real problem is still sitting somewhere between the fuel tank and the engine.

If an engine that previously ran well suddenly becomes difficult to tune, check the fuel system before making major carburetor adjustments.

Look for:

• Air bubbles in the fuel line
• Cracked or hardened tubing
• Loose connections
• A leaking tank stopper
• A blocked vent
• A damaged clunk line
• A restricted fuel filter
• A leaking or blocked muffler pressure line on a glow engine

Finding one of those problems can explain an afternoon’s worth of mysterious engine behavior.

Pay Attention to Fuel-Line Routing

How the fuel line is routed through the airplane matters.

Keep tubing away from sharp edges, moving linkages, hot exhaust components, and areas where vibration may cause it to rub against the airframe.

Avoid unnecessary loops and sharp bends.

A line that looks perfectly fine while the airplane is sitting still may kink when the tank is installed, the cowling is fitted, or another component presses against it.

When possible, inspect the complete routing from the tank to the engine.

Check the Tank After a Hard Landing

Hard landings don’t just bend landing gear.

The sudden impact can shift a fuel tank, pull tubing from fittings, damage the internal pickup line, or move the clunk into an unusual position.

If the airplane experiences a particularly hard arrival, give the fuel system a quick inspection before flying again.

If the engine suddenly starts behaving differently after a hard landing, that’s another reason to look at the fuel system before changing the carburetor settings.

Watch the Fuel While Filling and Draining

You can learn quite a bit about the condition of the system while fueling the airplane.

Pay attention to how easily fuel flows into and out of the tank.

Watch the vent line and make sure the tank is venting normally.

Look for leaks around fittings and connections.

If you’re using transparent or translucent tubing, watch for unusual air bubbles.

Fuel appearing somewhere it shouldn’t—or failing to appear somewhere it should—is telling you something.

Don’t Forget the Fuel Container and Pump

The airplane isn’t the only part of the fuel system that deserves attention.

Inspect the tubing on your field fuel container and fuel pump as well.

Old tubing can deteriorate and introduce debris into the fuel you’re putting into the airplane.

Keep fuel containers clean and properly sealed when not in use.

Gasoline should be stored only in an appropriate approved container, and glow fuel should be kept tightly sealed because some of its components readily absorb moisture from the air.

Clean fuel going into the airplane gives the rest of the system a much better chance of staying clean.

Do a Final Engine Check

After inspecting the fuel system, run the engine normally.

Let it warm up and check the transition from idle through full throttle.

For a glow model, perform the usual full-power checks recommended for your particular engine and installation.

For gasoline engines, follow the engine manufacturer’s recommended tuning and operating procedures.

Listen carefully.

A properly adjusted engine with a healthy fuel system should transition predictably and maintain a consistent mixture.

If something doesn’t sound right, don’t convince yourself that it’ll clear up once the airplane gets airborne.

The sky is a terrible place to troubleshoot a fuel problem.

A Simple Fuel-System Routine

You don’t need to disassemble the entire airplane before every flight.

Develop a simple routine:

Tank. Tubing. Clunk. Vent. Pressure line. Filter. Carburetor connection. Leak check. Engine run.

Some of those items can be checked visually before each flying session, while a more thorough inspection can be performed periodically or whenever engine performance changes.

The important thing is remembering that the fuel system is part of the engine installation.

When one isn’t working properly, neither is the other.

A Few Minutes Now Can Save an Airplane Later

Glow and gas engines remain one of the great parts of traditional RC flying.

There’s something satisfying about fueling an airplane, starting the engine, hearing it settle into a good idle, and then opening the throttle for takeoff.

Keeping that engine reliable doesn’t end with adjusting the carburetor.

Fuel tanks age. Tubing deteriorates. Connections loosen. Filters become restricted. Clunks get stuck. Vents become blocked.

Most of those problems are inexpensive and relatively easy to fix—if you find them while the airplane is still sitting on the ground.

Spend a few minutes checking the fuel system before it gives you a reason to wish you had.

Because while practicing dead-stick landings is a good idea, there’s no reason to let a five-dollar piece of fuel tubing schedule one for you.

Happy flying, York RC Club.