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55 ax: Bubbles? Help Diagnose, Please!

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Old 07-23-2012, 11:16 AM
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Deandome
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Default 55 ax: Bubbles? Help Diagnose, Please!

Engine is running in Goldberg Sr. Falcon ARF. I'm a newbie, and have only taken the controls for a few minutes once it's airborne, otherwise my experienced buddy is flying it (and loving it!). I bought the engine used from an experienced flyer who broke it in on a bench & then flew only 2-3 times on a plane; it was pristine when I got it, but not brand-new.

Starts great, runs great on ground, and sounds great in air....for about 3-5 min.

Then you start to hear a bit of sputtering, brief dropouts to the point where you don't want to push it any more & probby want to bring it in. Haven't lost power fully in flight, but of course, that's the big worry. This same 'behavior' has occured several times on several flights, not just once.

My buddy was thinking it might be air bubbles in the tank; it flies fine until the cumulative vibrations start making bubbles, in theory. We're thinking that it might be because of the way I have the tank set up without any real padding It's tight in the space behind the firewall, so my 11oz tank sits directly atop the Rx battery (which is velcroed to the fuse), it's held firmly on the sides by support/strengthening 'beams' on the fuse, and then I have maybe 1/8" or less of padding on the lid of the fuel-hatch cover. In a nutshell, the tank is defacto 'connected' to the fuselage vs. having padding on sides/bottom/top.

I could see how that could cause bubbles but I want to ask if

[ul][*] it might be something else, like a carb-gasket or carb-o-ring that needs replacing[*] could a dying plug cause this, or does the fact it happens only after a few minutes of flying negate that theory[*] would the OS 'bubbleless clunk' help out? Is there a better bubbleless klunk to be had?
[/ul]

I might go to a smaller tank, but I'd rather not, as the one in there now is the recommended size for this engine to get an 8 min flight or so (but I'm running 1/2 throttle most of the time...)

What do you think; should I assume it's bubbles in the tank from excessive vibration, or should I look to other causes/solutions? If so...what?

Thanks!
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Old 07-23-2012, 04:44 PM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

I've flown a lot of planes without padding around the tank for just the same reason that you spoke of - no room. Have you actually had an eyeball on the tank? Is the clunk free, or has it flipped to the front of the tank. Is there possibly a slit in the fuel pickup tubing that is letting air in when the tank gets down to half full?

Of course, on the engine you can check the usual things about leaks around the carb base and the needle valve, but those would show up regardless of the fuel level in the tank.

Just my $.02

Bob
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Old 07-24-2012, 06:05 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

Any number of things can cause the engine to quit after a few minutes. Since the engine is out in the breeze, we don't have to worry about cowling problems. That leaves us with fuel flow, air leakage, and needle settings.

If there's any obstruction at all in the fuel flow, the engine will lean and die. Very common is a blocked tank vent. That prevents air from coming into the tank to replace the fuel pulled out by the engine. A vacuum builds up and prevents fuel draw. Make sure the tap in the muffler and the pressure line are clean and allow free flow. You never use a check valve in a muffler pressure line.

If there's any kind of air leak in the line, it will be caused by cracks or holes. This adds air to the fuel, which is the same as turning the needle a bit leaner.

Please make sure that the high-speed needle is set on the rich side of peak RPM. Richer is better, especially with a newer engine.

Air being agitated into the fuel inside the tank adds air to the mixture, which leans the engine. It will overheat and quit.

If all else fails, you need to put the engine on a test fixture and run it extensively with a known-good fuel system. If the engine will run "all day" on the test stand, then the problem is in the airframe/fuel system/engine combination.
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Old 07-24-2012, 07:15 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

Thanks!

Again, it has never quit, it just skips a few beats now & then, which is cause enough for concern. All the fuel lines are new, it's a Hayes 11 oz tank (which only uses pressure to seal the stopper; no expansion-screw stuff. But I have the original 8 oz tank that came w/the kit (dubro, I think), so I think I'll drop that in (with some padding, as it's smaller) & see if I get the same effect.

I do remember that the gasket (or was it an O-ring) sealing the carb onto the engine was really small. And from my experience with car engines, I think some people added a SMALL bead of RTV gasket-sealant to that connection. Do you recommend something like that?

Thanks again!


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ORIGINAL: Bax

Any number of things can cause the engine to quit after a few minutes. Since the engine is out in the breeze, we don't have to worry about cowling problems. That leaves us with fuel flow, air leakage, and needle settings.

If there's any obstruction at all in the fuel flow, the engine will lean and die. Very common is a blocked tank vent. That prevents air from coming into the tank to replace the fuel pulled out by the engine. A vacuum builds up and prevents fuel draw. Make sure the tap in the muffler and the pressure line are clean and allow free flow. You never use a check valve in a muffler pressure line.

If there's any kind of air leak in the line, it will be caused by cracks or holes. This adds air to the fuel, which is the same as turning the needle a bit leaner.

Please make sure that the high-speed needle is set on the rich side of peak RPM. Richer is better, especially with a newer engine.

Air being agitated into the fuel inside the tank adds air to the mixture, which leans the engine. It will overheat and quit.

If all else fails, you need to put the engine on a test fixture and run it extensively with a known-good fuel system. If the engine will run ''all day'' on the test stand, then the problem is in the airframe/fuel system/engine combination.
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Old 08-02-2012, 05:45 PM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

i am having the same troubles wih a 55ax on a saratoga by hangar nine except i have had 15 did stick landings it starts to spuder just barley noticable and then it just stops no crashes yet always make it back to the strip but i fly very high altitude and ive been very lucky only been flyin 6 months and the they call me dead man flying i have changed fule lines several times and changed clunks and have gone from two lines to three lines with two clunks plane sounds great fly great for two to five minutes and the dead i have even gone from 12x8 prop to 11x6 prop and back to the 12x8 im going to replace the tank now if that doesnt fix it i bought two of these engines from tower so i have another new one to put in and that will be might last attempt to fix it if that doesnt i give upall my other os engines run without any problems i have a 46ax o a next star a 120ax on a pulse 125 and a 95v on a texan trainer no problems i have noticed that the fuel bar and idle needle on the 55ax isoff set in the carb and all the other os engines are in the very midle of the venturie both my 55ax are off set and the fact that the saratoga by hangar nine is turned 90 or more might be causing the problems but i see on rc universe and hangar nine website many people are using the 46 ax and the 55 ax so im not sure whats up ?
got any ideals


lost Rick NUNES
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Old 08-03-2012, 06:37 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

When an engine consistently quits at the same point...when the fuel level reaches a certain point, your fuel tank isn't vented properly or you have a heat difficulty.

When a fuel tank isn't properly-vented, a vacuum builds in the tank and eventually prevents fuel from flowing and the engine quits. If your engine isn't getting good cooling, then it will gradually overheat and quit.

Everything needs to be checked. If all else fails, you must remove the engine from the model and run it on a test fixture to see what's going on.

An occasional "burp" or "hesitation" in an engine's running usually means an air bubble getting generated somewhere in the system. Look for a pinhole somewhere in the fuel lines. If it happens after a few minutes, it may mean that the pinhole is becoming exposed to air as the fuel level in the tank goes down...the clunk line is usually the culprit here.
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Old 08-03-2012, 08:24 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

thanks bill i have changed the lineson the inside of the tank along with all new clunk and hard lines in the plug but i havent replaced the tank it self just because it is a specific size as to where it fits in the plane but im ordering one today from hangar nine at least that way i have replace everything once and in some cases a few times each.


also could you tell me if when you have the back off of a ft 120 gemini is there a set of timing marks to set timing between the cam and crank gears i have cleaned this engine with a branosonic cleaner tank and it looks new but i would like to get it timed and run by os if i need to i am hesitant to try to start it because of a setting that might not be right and damage the engine it has great copression and now looks brand new
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Old 08-06-2012, 06:21 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

To set the timing on the twin cylinder OS engines, you just need to set the gears so that the timing marks are together in the middle between the gear centers, and in a straight line through those centers.
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Old 08-07-2012, 01:48 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

Bill do i need to bring one of the pistons up to top dead center and if so which one and do the valves have to be in any particular closed or open order before i set the timing marks on the cam gear to the marks on the crank gear
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Old 08-07-2012, 06:12 AM
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Default RE: 55 ax: Bubbles? Help Diagnose, Please!

When you turn the crank so that the timing mark is between the centers of the crank and camshaft, the pistons will take care of themselves.
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