Bayou Talker

My Feedback: (1)

What can you possibly do wrong in the plumbing of a fuel tank to make the engine use about twice as much fuel as it should? I have a Seagull Spacewalker with an OS 46AX that runs out of fuel after approximately 5 minutes at less than full throttle. The tank size is supposed to be 11 oz. I have tried to measure it and the best I can tell is that is correct. I have also flown it once with an OS 40LA with the same results. I bought this plane from a guy that assembled it as his first plane and never flew it so he could have made a mistake. The fuel tank does fill with fuel to the top and it is very near empty when it shuts down. The remaining fuel is easily pumped out. I would assume from this that the clunk is working correctly. The tank is glued in very securely and I haven't tried to remove it yet to check it out. I just can't imagine what could be wrong to cause this problem. There are three lines coming from the tank. One goes to the a GP Fuel Filler and then to the engine, one to the muffler and the other is plugged off.

I also have a Nexstar with the OS 46FX and a 9.6 oz tank and it will fly for 10 minutes and still land with fuel in the tank. In fact, I have never run it out while flying. From the OS specs, I would expect the same from the 46AX.

Any ideas would be appreciated before I pull the tank.

RCKen

My Feedback: (9)

This does sound a bit odd. Have you ran it on the ground (secured of course) where you can watch what is happening? Like you said, you should get 10-12 minutes flight with that motor/tank combo. If the fuel isn't burning in the engine, and it's not left in the tank, then it has to go somewhere. My guess it that it's either coming back out in a fuel leak somewhere. Inspect your interior frame COMPLETELY to ensure that the leak isn't inside the fuselage. This could be very bad as the wood will weaken and fail if you fly. Run it on the ground and watch of any leaks in any of the tubing. You can also check for leaks by emptying the tank and then plug the line to the carb and blow into the muffler pressure line, then pinch off that line. Look to see if there are any air leaks. If you can get the fuel tank out of the plane then you can do the same check in the sink under water.

Check these out and see what you find.

Hope this helps

Ken

Bayou Talker

My Feedback: (1)

RCKen,

Yes, I have run it on the test stand for the first tank in the new 46AX. I was running it rich and the only fuel on the table was from the exhaust. The next tank was run in the air with the engine a little rich for more break-in, but not that rich. Third tank was a little leaner, maybe a bit leaner than it should have been for a new engine, it ran great and I landed after 5 minutes with just a little fuel left in the tank. The inside of the fuel tank compartment and the engine compartment are completely dry. I will try the pressure test you suggest after I remove the engine cowl, but I do not expect to find anything there. The tank will be a real pain to remove as it is glued in. but if necessary, I will get it out. Can you think of any way that the lines inside the tank can be done incorrectly to make it pull fuel out of the tank through the exhaust? I can't figure out how that can be, but I don't know what else to check.

Thanks for the reply

RCKen

My Feedback: (9)

You can run the line incorrectly and still possibly get it to run. But to the best of knowledge I don't know of anyway to make it use more fuel that way. This is a good head scratcher. Maybe somebody out there more experience has an idea about this one, because I'm stumped at the moment.

We have a guy at our flying field that has been flying since the Wright brothers were on a buddy box who should be at the field tomorrow. He's also our field engine guru. I'll run this past him tomorrow and see what he thinks about it. I'll let you know what he thinks.

Sorry I couldn't be more help right now

Ken

bruce88123

Are you absolutely sure the tank is full? Are you verifying visually or just seeing overflow from the vent tube? If the latter, then the vent tube end may be too low inside the tank. Have you used a graduated cylinder such as a large fuel syringe to make certain how much fuel you are putting in? You could also fill a tank of known quantity outside the plane and then make sure it can all be transferred into the plane. In your case, most of 2 x 6 oz tanks would be appropraite also. I am just real suspicious of you having a full tank.

Bayou Talker

My Feedback: (1)

The tank is visibly full. I have filled it with the wing removed just so I could see the tank. That was the first thing I thought of was that the vent was not letting it fill properly. The second thing was the clunk line installed incorrectly. I filled it while watching and it was full to within 1/8" of the top and then pumped it dry and there was just enough left to see a few drops.

I checked with Horizon as to the size of the tank supplied with the model and they said it was 11 oz. I tried to measure it, but it is difficult with the way it is positioned. The best I could measure it worked out to 10.2 oz. but I could not get exact measurements.

Your other idea was the next thing to try. Measuring how much fuel is actually going into the tank and back out. I might get to try that later today, but I don't think that is the answer.

Is it possible that the fuel fill valve is letting air into the line and I am having to run the needle open more to keep the engine from running lean? I really don't know where the needle setting ended up. I need to go and check it but I would think that it would need to be extremely off to cause this problem. Does anyone know what the OS 46AX usually requires on the needle?

RCKen

My Feedback: (9)

Mine runs at approximately 1-1/2 to 2 turns out. That varies with temp and humidity but that's close enough to start with.

Ken

Jim Thomerson

This is probably no help, but the only time I have had that problem is with a sidemounted Tower 40. Thus the muffler is below the fuel tank. I fill through the uniflow with the vent cap off. then I replace the vent cap and hook the muffler line to the uniflow. Go fly, no problem.

Got a couple of half time flights, realized fuel was coming out the muffler. When I put the vent cap on a little squirt of fuel comes out the uniflow. If I put the muffler line on first, then the vent cap, fuel siphons into the muffler like gangbusters. I had been careless and put the muffler line back on first.

Flyboy Dave

My Feedback: (21)

I wonder if you are losing fuel out the fuel valve at speed in the air ?

FBD.

Bayou Talker

My Feedback: (1)

Jim

What brand of valve are you using? The one I have (Great Planes) has only 2 lines going to it. The clunk line goes to the valve and the other side of the valve continues to the engine. The second line from the tank goes to the muffler. The third line from the tank is plugged and unused. I was thinking that the fuel could be going out the exhaust but could not figure out how with the pressure from the muffler. The engine is mounted sideways in the plane with the muffler down and I stop filling when it runs out of the muffler. That could be the problem, but only a small amount of fuel siphons out after filling. Maybe it is siphoning when it is running. It will be very difficult to disconnect the pressure line for filling as it is hidden under the cowl but I may have to find a way or a different filler valve.

Thanks for the ideas guys. Any more?

Bayou Talker

My Feedback: (1)

OK guys here goes. I finally did the smart thing and removed the tank from the plane. The first thing I noticed was that the tank was noticeably smaller than Horizon said that it should be. After taking an accurate measurement, it worked out to just over 8 oz if it was full to the top. Add to that the clunk line was just a little short and the vent was just a little low in the tank and I would say that the useable fuel in the tank was somewhere closer to 7 oz. than 8. That would account for the low flight time with running the engine rich for break in.

I though I had the answer in hand with a Dubro 12 oz tank that was just laying around. After slight modification to the width of the fuel compartment, I tried to slip the tank in and guess what.....about 1/4" too long to fit. The story of my life. As it is Sunday afternoon and the nearest hobby shop is 50 miles away...and closed..., I guess I will have to wait until tomorrow night to go and get a 10 oz tank for it. That should get me the flight time I need to have some fun with this thing.

The moral of the story is don't take a tech support answer for Gospel, always check it out for yourself. Also, no good deed goes unpunished!

Thanks for all of the help. I will let you know how it goes on the next flight (hopefully next weekend).

bruce88123

At least it is making sense now and you have an action plan to correct the situation. Check online and see if you can tell if Dubro, Sullivan or one of the other mfg. offer the best tank soze for your plane. They don't all make the same sizes ya know, some are taller and shorter etc for the same volume.

BillS

Before you go to a great deal of effort you might want to consider running the engine at a normal needle setting.

You may find that a rich setting on the needle will have considerable affect on fuel consumption. On a twin we set one engine slightly rich for break in and ran out of fuel and broke the airplane. When we arrived at the crash site the other tank was maybe 40% full. None of us anticipated the additional fuel consumption.

Bill

Bayou Talker

My Feedback: (1)

Dubro makes a 10 oz tank that will fit in just fine. It is the same dimensions in height and width as the 12 oz that I have but is about 3/8" shorter. It is 4 7/8" long and I have 5" to work width. I think that is the largest that will fit but I will need a shoehorn to get it in. With any luck, I will be able to pick one up this afternoon and get it installed. That should get me sufficient fuel for a decent flight time.

I realize that running it rich will use more fuel, but after a couple of more flights I will lean it out a bit more. I expect that it will be fine.

Paternguy

My Feedback: (1)

Why can't you leave that 1/4 hang out of the compartment ?

bruce88123

He may have a solid bulkhead?

Paternguy

My Feedback: (1)

Just let it stick out into the radio compartment, I have been doing this for years with 60 sized pattern planes.

Bayou Talker

My Feedback: (1)

Bruce is exactly correct. The tank fits against the firewall and down in a hole. There is no way to make the 12 oz fit. Believe me I tried. I really wanted the extra fuel on board.

This is a really nice flying aircraft and is a lot of fun to fly. If I had it to do over again, I would probably have spent the extra money and put in a 52 four stroke for the extra low end power but I think this 46 will do fine.