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Help! SJ 50 won't run right

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Old 10-01-2006, 12:46 AM
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Airbike
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Default Help! SJ 50 won't run right

Hi Guys, I need a little help in trouble shooting a SJ50 to get it ready for Revver II.

Its mounted in a LR-1 plane and is using a bubbleless tank. It's had a dozen or more flights with no problems. Currently I'm running a 8.75x9 apc but I don't think the prop has anything to do with it.

The problem is the engine will not go to full throttle. When I get to 3/4 stick, the engine cuts out as if it ran out of fuel. I saw some air bubbles in the line going from the remote (backplate mounted) needle assembly to the carb so I changed out the line and zip tied the new tubing at both ends. I also zip tied the line from the tank to the needle valve. I still saw some small bubbles so I tightened the needle valve collar.

It still will not go to full throttle. What else may I be overlooking that can have air leaks? I tried it with and without a fuel filter (also zip tied) but still the same. Also tried new glow plug, no difference.

The fuel tank seems fine when filling or unfueling with the big syringe, no bubbles when unfueling.

Please dispense some of your engine tuning wisdom...Thanks.



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Old 10-01-2006, 01:40 AM
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Default RE: Help! SJ 50 won't run right

Try some different fuel.

What % nitro are you using? I had the same problem with some 35% nitro I was using, jumped down to new can of 25% & ran fine.

FWIW......
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Old 10-01-2006, 07:52 AM
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r-c-guy
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Default RE: Help! SJ 50 won't run right

Might need to pressurize the tank for better feed... Were the first runs ok at full throttle?

randy
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Old 10-01-2006, 09:55 AM
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Mike Connor
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Default RE: Help! SJ 50 won't run right

The bubbles must go to run right. Expose the lines all the way to the tank and where the bubbles start is the problem area. There may be a problem with the tank itself.
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Old 10-01-2006, 10:28 AM
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Default RE: Help! SJ 50 won't run right

I use Jett tanks on all my planes, I have had these same indications before and it was always a leaking/torn bladder. If you are still getting some air at the end of the syringe draw it is probaly the bladder.
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Old 10-01-2006, 10:39 AM
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Default RE: Help! SJ 50 won't run right

Plug? I mean if the darn things has been running fine and then one day it doesn't, check that tank and lines like Mike said. I doubt its the type of fuel.
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Old 10-01-2006, 11:29 AM
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daven
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Default RE: Help! SJ 50 won't run right

How are you filling the tank? If you pull everything out, there shouldn't be any air bubbles at all.
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Old 10-01-2006, 12:29 PM
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Default RE: Help! SJ 50 won't run right

One thing you might want to check is the remote needle valve assembly itself. Even if the locknut for the needle is tight,
you may still have a leak between the remote needle valve housing and the brass tube that goes all the way through it,
or the brass fitting threaded into the side. Check these points for tightness. Also, maybe you have a pinhole leak in the
fuel tubing going to the tank close to the remote needle valve assembly, that is closed off when you hook up the syringe
to fuel/de-fuel. If you can, cut off 1/4" off of the end of the tubing going to the tank; that would hopefully eliminate a
pinhole leak, or possibly the tubing may fit too loose from the stretching of putting it on and off the fuel nipple. One last
thing to check for, and it sounds crazy, but I have seen thin-walled tubing actually collapse and pinch off fuel flow under
high suction operation. I use and recommend the Prather medium red fuel tubing. One test, just to narrow it down to a
tank problem or engine problem, would be to hook an external tank up to the engine; if it runs fine, the engine and remote
needle are ok, if not, it will likely be a problem with the needle valve assembly or plumbing, and least likely the engine
itself. Good Luck! -Mike
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Old 10-01-2006, 06:38 PM
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Default RE: Help! SJ 50 won't run right

Thanks for all the response guys.

I'm using a new can of 15% powermaster fuel. The tank is a 8 oz Jett bubbleless tank and I'm using the Jett syringe to fuel/defuel it. I'm thinking along the lines of what Mike just posted. The tank fuels and defuels without any bubbles so I'm thinking there is a leak somewhere else. I'm running a pressure line to the tank but the engine won't go to full throttle even with a full tank, so i don't think that has much to do with it.

I'll pull the engine and check all the lines and try an external tank. It seems like there is a leak somewhere around the remote needle valve assembly as there are no bubbles before that, then somehow the bubbles show up before going into the carb.

I might as well change all the fuel lines just to eliminate that factor. Are the remote needle valve assemblies prone to leaks?

I'll disassemble the needle valve assembly and clean it to make sure there is no debris in it.

Won't get a chance to run it until later in the week.
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Old 10-01-2006, 07:20 PM
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Default RE: Help! SJ 50 won't run right

I hate remote needle valves. What engines I have that came with them got removed. I'd rather get my knuckles scraped than F with those things. You take a remote needle valve and add failsafe to it and kiss your plane goodbye!
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Old 10-01-2006, 09:43 PM
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Default RE: Help! SJ 50 won't run right

Well, I pulled the tank and it is the #1 suspect.

the pressure fitting was broken off so no pressure was getting to the bladder at all. The fuel pickup line looked a little loose at the connector also. I'm puting in a new line and sealing it all with some silicone sealant at the stopper.

I guess that without the muffler pressure, the fuel suction was strong enough to start sucking in air at locations where it normally wouldn't.

hope this is it. Will try to get some test flights before the Revver event.

thanks to all for the help.
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Old 10-02-2006, 07:56 AM
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Default RE: Help! SJ 50 won't run right

Sounds like you are doing things right.

A rule of thumb - when you have an engine that is running well, and then suddenly is acting weird, change the glow plug. Although in this case it was not the initial problem, its something to try first.

As you found the tank was the issues. Turns out that about 90% of the time, the fuels system is the problem. The Jett engines will not run without muffler pressure. And the tetra or jett tanks do not function without the muffler pressure.
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Old 10-02-2006, 08:10 AM
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Default RE: Help! SJ 50 won't run right


ORIGINAL: Cyclic Hardover

I hate remote needle valves. What engines I have that came with them got removed. I'd rather get my knuckles scraped than F with those things. You take a remote needle valve and add failsafe to it and kiss your plane goodbye!
The bubble he was seeing here are from the pressure drop at the needle. Instead of having positive pressure from the tank delivering fuel, the carb was creating suction pulling the fuel through the needle.

What occurs is similar to cavitation in a pump or on a ship's screw. The low pressure causes tiny gas bubbles within the fuel to come out of solution, which accumulate into larger bubbles in the fuel line.

In actuality, this is the situation with many of the remote-needle issues folks report. Its usually not an air leak. Evident, because (such in the case of the OS91 and OS160) when folks installed a pump feeding the inlet of the needle, the bubble problem went away. The pump feeds fuel through the needle restriction, rather than it being pulled through. Less delta-P. The engine/carb are doing their job, sucking real hard to draw fuel. So why does this not happen on a front needle carb? Well.. it does happen. But at that point, the fuel is already entering the carb body and mixes with air at the venturi. It can screw up the spraybar/fuel delivery, but it is rarely noticed and rarely a problem.
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