This regards air in the fuel line. Before I tell about the symptoms, I'll preface is by telling you that this is for my brother, who purchased a 4* ARF and a new Saito 56. The plane had these symptoms and before he could find the problem, he crashed the plane. He then bought another one, and this plane has the same problem: This is what I know:

1. When running at any speed, there is a constant stream of air bubbles in the pickup line.
2. Even with the bubbles, the engine sounds very good.
3. Holding the plane upright, the bubbles remain.
4. In both cases, the tanks were new and built correctly. The clunk moves freely, new lines, filed tubing tips
5. Because of the Saito carb location, he turned the tank stopper over so that the lines are coming from the bottom side of the stopper. The vent tube is close to the top of the tank. The lines coming from the tank are about 1-1.25 inches lower that the inlet tube on the carb.

I'm a little ahead of him on the learning curve but I'm out of things to check. Any good things to check? Thanks in advance.

Vibration? Does the airplane vibrate excessively? Has the prop been balanced? Is the tank surrounded by foam rubber?

Two suggestions:

1. Excessive vibration
2. A pinhole or crack in the line or tank tubing.

Dr.1

After checking what Mode One suggested, also confirm that when your brother assembled the tank he didn't cut a hole in the pickup line. You shouldn't be getting a constant stream of bubbles in the fuel line and one should NEVER FLY with this condition. Out of curiosity, what brand and size of fuel line is being used from the tank to the carb? If it's not getting a tight fit at the tank end, the suction could pull in some air. Also confirm that the lines are hooked up correctly. If you can't get it working right in the plane, remove the tank and engine and try the same setup on a test stand.

These guys are on the right track

Vibrations (They don't even have to be excessive) can cause the fuel in the tank to foam - so your engine may be sucking foam instead of liquid. Wrapping the tank in Foam Rubber should cure this.

A pin hole in one of the lines (Inside or outside the tank) can also let air bubbles in.

An easy way to see which problem you have would be to remove the tank, hook it up to the engine, and run the engine while holding the tank in yor hand. If you still get bubbles, you've got an air leak.

Thank you all so much for your suggestions....jusdt love this forum.

Or really bad nerves!

Whenever you suspect a leak, try something simple to find it. It takes almost no time to do the tests.
If you have a fuel bulb or one of those big plastic syringes you're good to go.

Unhook the fuel tubes from their ends. Unhook the muffler pressure line from the muffler, the delivery line from the needle valve, and the fill/vent line from it's fuel dot or pull it's plug. After all, you want to test the lines and their connections as well as the tank, right.

If you have a three line setup, the connect two lines to each other and connect your bulb or syringe to the 3rd line. Put some pressure on the bulb/syringe and hold it. While waiting to see if the pressure is leaking, listen carefully. Most leaks will show up under this simple test, but some don't respond to pressure. Those will often respond to "vacuum". So introduce vacuum into the test. If you have a syringe, pull it's plunger until you've got a decent "vacuum" in the system and hold that. If you have a bulb, then squeeze it flat and connect it to the 3rd line and it'll "vacuum" for you.

If you've got a two line setup.... plug one line and blow/suck on the other one.

It's dead simple to test stuff like this. Testing beats discussion every time for finding a problem.

BTW, if you're using muffler pressure, then the system already is seeing an introduction of "extra air" into the tank. For the bubbles to be "air" coming from somewhere else would suggest that there is suction greater than the muffler pressure that's available at the point of the leak. It's worthwhile to check your muffler pressure into the tank to make sure it's got a clear path. Muffler pressure feed lines sometime plug and that causes problems. Check it out.

Also, a bunch of today's engines are really designed to run with muffler pressure. If you're not using it, it's never a bad idea to start. Some of the more powerful engines can run without it, but won't do as well since their venturi areas aren't optimised for "suction".

You can do darock's test while holding the fuel system under water. You'll see bubbles from any external leak when under pressure. You can pull the fitting out of the tank to test the internal portions. A hole in the vent line won't cause bubbles but may cause you to lose pressure.

I am new to this hobby but just finished putting a 4* 60 ARF together and the instructions say to put silicon around the neck of the fuel tank and then insert it into the firewall. If you follow these instructions wouldn't foam around the fuel tank be a waist since you will pick up any vibration from the firewall? Should we put our tanks in differently? This could be part of his problem and my future problem. Thanks

Matt

I suspect the problem is vibration as both of his 4*s had the same problem, both with a new tank and lines. The 4* has large openings in the bulkheads for the tank to slide into, very tightly I might add. Its not designed to have foam packed in. The only way to get the foam in would be to cut the bulkhead openings bigger than the tank. Its also my understanding that the 4 stroke vibrates more than a 2 stroke. I gave my brother a copy of this thread and he's going to mess with it some more. I liked the idea of attaching another tank to it and seeing if it still bubbles...he can do this without even removing the engine. No bubbles will mean that its indeed a vibration problem. Nothing to do then but to pack the foam in and/or balance the prop. Thank you all so much for your ideas.

Do your selfs a favor and ditch the stock tanks that comes with the AFR! Ive had 3 different 4star tanks that had air leaks around the stoppers. Get a Dubro, Sullivan...ect. And install them in foam!