Magnum76

I have a .15 which is new to me and a .40 (both OS) both which run fine.

But at about a half tank they quit.

The .15 is in a SSC SPAD and the .40 is in a SPAD. The one thing they have in common is new gas tanks. I don't remember the manufacturer of each tank but they are brand new, the .15 has a 4oz and the .40 has a 6oz. I followed all the directions and seemed to assemble them correctly. I deburred the brass tubes and checked for a cut or nicked fuel line and it's all good.

One of the guys at the field said that maybe the .15 can't make enough back pressure to pressurize the fuel tank. Is this possible? What is a solution?

flipstart

Using a 4 oz tank on .15 and 6 oz tank on a .40 size engine, there should be no problem. Assuming you have no leaks or pin holes in the fuel tubing, then only culprit I can think of is that the clunk is not going to bottom of tank. On small tanks very flexible fuel line needs to be used for the clunk line. Check to make sure the clunk is resting on bottom of tank and is free to move around.

mike109

G'day Are the tanks taller than they are wide? This can cause significant differences in fuel delivery from the top to the bottom of the tank.

A friend of mine has an ASP 120 (rather larger than your engines I know) in an Ultimate and he was tuning it slightly rich taking off and then having it cut at about half a tank. So we did some experiments.

Firstly we put only about one quarter of a tank of fuel in the tank and started the engine and tuned it so it was slightly rich. (Saito actually advise tuning their engines at the bottom of the tank.)

Then we stopped the engine and filled the tank to the top and restarted it. The engine now ran very rich, so rich that when he tried to fly it, it was almost useless but he persisted and as the tank emptied, the tune improved and by the bottom of the tank it was running perfectly.

It was running with the standard muffler and muffler pressure.

I suspect you are suffering the same problem.

Solutions -

1. A Cline or Ironbay regulator (though I have no direct experience of them)

2. A "flatter" tank like a DuBro.

3. A Uniflow setup in the tank. This uses two clunks, one is connected as usual and the second is connected to the vent that usually goes to the top of the tank. The two are connected together though the inlet for the fuel to the engine and the outlet from the muffler pressure are separated by about 3/8 inch (muffler pressure tube is slightly shorter). Full descriptions of this setup are elsewhere on this site. This setup is used on most control line tanks and it really works well. You do need to use a third pipe into the tank to act as a vent for filling which you then plug when running the engine. Give it a go. It will only cost one piece of brass pipe, one piece of fuel tube and one clunk and a cable tie or piece of heat shrink tubing to hold the two tubes together.

MrGreenSpeed

Make sure the clunk line is not too long, if it is, the clunk will be stuck at the back of the tank.

I have also noticed the some fuel line is too stiff for the clunk to move freely to the bottom of the tank.

I added a nut to one clunk for extra weight, the other I replaced the clunk with brass tubing it worked great.

Jeff

downunder

Are the fuel feed lines to the carbs connected to the clunk line inside the tank?

TedMo

One thing is your tanks are too small. 8 OZ. is common for 46. Perhaps if it fits put the 6 Oz on the 15 and get 8 Oz for 40. Important that the brass tube inside of tank not be too long, about 1/2" is good if too long fuel line will not be able to flex and keep clunk on bottom. Could also help especially for small tank to use the smaller diameter fuel line which is more flexible. Keep length of supply line with clunk just short of back of tank. One more thing is perhaps fuel is foaming? This could happen if tank is solidly mounted in fuse.

jaka

Hi!
Gas tank size is important! Do not use too large tanks!
A 4oz tank for the .15 and a 6-8oz tank for the .40 engine. Uni-flow (two clunks) is good to have but the ultimate tank is the Tettra "Bubbleless" tank. These tanks are used by pylon racers worldwide and give more consistent engine runs.

BarracudaHockey

My Feedback: (11)

Pull the tank and run a fuel line from the clunk line to the pressure line.

Put the whole thing in a bucket of water and squeeze softly and look for leaks, especially at the neck/stopper area.

Most people dont need uniflow etc for sport flying, check the simple stuff first.

jeffie8696

If it always dies at half tank then it sounds like the lines are switched.

freeair

check to make sure these fuel tanks are set at the correct height levels in the fuse. normally a tank thats too high will flood the engine and a tank thats too low will cause the engine to run lean as it empties ,resulting in a stop. make sure the clunk weight is on the fuel line in tank and its sitting on the bottom of tank .