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Fuel trouble

Old 08-17-2008, 03:49 AM
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Capt. Yossarian
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Default Fuel trouble

I have a new os 46 fx engine on a Hobbico Superstar 40 with a fuel problem. I'm getting plenty of bubbles in the fuel line between the regulator and the carb. I just changed all the fuel lines in hopes that the problem will be corrected (I'll check tommorrow). If it's the tank it self, I'm really not sure how I can fix it without ripping some of the balsa out to make room to pull it out.

Here's my question. Is it possible to get air in the line from the reg to the carb from anywhere other than the tank or the fuel lines? It seems logical that that would be the only way to get air in that line, but let me know if I'm over looking something.

Thanks,

Dave
Old 08-17-2008, 06:19 AM
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gunfighterII
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Default RE: Fuel trouble

I'm assuming your talking about the remote needle when you say regulator.

Yes, the RN could be leaking at the needle o-ring. Or the seat of the needle housing. You could also be drawing in bubbles from the tank not being isolated from vibration. Did you balance your prop?
Old 08-17-2008, 08:58 AM
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Default RE: Fuel trouble

Does the FX run OK with the bubbles? I assume not. I had a similar problem with a tank on one of my planes. I had some debris in the pressure line which was blocking the pressure and when the engine was runnning, the vacuum in the supply line was cavitating the fuel and drawing bubbles. Check the lines by pulling off the supply and pressure lines and blowing into the pressure side to see if there is any blockage.
Good Luck,
Curtis
Old 08-17-2008, 10:58 AM
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TedMo
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Default RE: Fuel trouble

Some thoughts: If you are not seeing bubbles in line from tank to (regulater) needle valve then I would think all is well there. Bubbles are only between needle and carb, right? Pretty much isolates it I would think, to needle valve leakage. Ok, start engine open needle valve as many turns as it takes to get a rich run. If it takes more than 2 to 4 full turns needle is definitly leaking air. It should run peaked at about 1 1/2 turns and rich about 2 turns. One thing that at times works is to put a 1/4 inch length of fuel line on needle just above the threaded area. If this is a new engine this should not happen but if old then perhaps O ring is dried out, replace it. Also be sure the nipples that fuel line connects to at regulator and carb are tight.
Old 08-17-2008, 02:44 PM
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Capt. Yossarian
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Default RE: Fuel trouble

Great advice everyone, just what I needed. I'll bench test shorty to see if the new lines fixed it, if not, I'll start scrutinizing the O-ring on the needle valve.

I really hope it's not the tank it self, for some reason on this particular model, the fuel tank will be difficult to remove without tearing out some of the balsa in the fuselage.

Thanks again,

Dave
Old 08-17-2008, 09:39 PM
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Default RE: Fuel trouble

Just a comment : in the last season I've had 2 students with a simular problem and both of them were from a " cracked " remote needle valve. They cracked at the line coming out and going to the carb.. The needle valve is made of plastic and if you, bump, nudge, bang or what ever in the wrong way it will crack. Sometimes hard to find until it breakes completly. ENJOY !!! RED
Old 08-17-2008, 11:17 PM
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Capt. Yossarian
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Default RE: Fuel trouble

Thanks Red, good thought. I imagine that's not to terribly difficult to replace. Would I need to order the exact part for my particular engine?

Also, small update. Changed out all the fuel lines and something occurred to me. I have three ports coming from my tank as opposed to two, which seems more common. One line is just for fueling, the other two are self explanatory. On one hand, it does makes fueling slighty more convenient, but on the other hand, it's just another part that could go wrong.

The bench test with the new lines is a success. I witnessed no bubbles at all throttle settings! Woot! So of course that means the weather is going to be bad, scattered thunder and lightning and sprinkles. I'm going to fly anyway. I'm using a DX6 Spektrum radio with no antenna, should be ok. Check the news for "RC Airplane Pilot Found Next to Plane, Extra Crispy."
Old 08-18-2008, 05:46 PM
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Default RE: Fuel trouble

Sounds like you have the problem solved so it isn't the remote needle valve. That part can be ordered and it runs about $ 7.50 in case you ever need it, Tower has it. ENJOY !!! RED
Old 08-18-2008, 06:37 PM
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Default RE: Fuel trouble

Capt: Guess you have discovered this from this experience. Do NOT affix the tank in such a way that is diffecult to remove. Best way is place tank loosely in place. Hopefully with centerline of tank as close to center of needle valve as is possible,(vertically). Wedge some foam around it,firmly but not too tight to hold in place. That then holds securely but still enables it to be removed if need be. 3 lines are totally unnecassary unless engine is cowled since fueling can be done by removing supply line when cowl not preventing that. Tip: when fueling uncowled engine have a small plastic bottle with fuel line, plus a vent to plug into pressure line (remove from muffler). That will catch excess fuel in bottle rather than have it go into muffler. Saves fuel also prevents flooding motor. Don't forget to put pressure line back on muffler.
Old 08-18-2008, 08:26 PM
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Capt. Yossarian
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Default RE: Fuel trouble

THanks Red!

Ted, that was exactly my thought. The extra line just to fuel is totally unecessary on my OS 46 fx. If I don't put the screw which plugs that line on tightly, it's a guaranteed air leak.

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