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Os-fs 1.20 four stroke question

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Os-fs 1.20 four stroke question

Old 08-23-2016, 04:05 PM
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chucksaw812
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Default Os-fs 1.20 four stroke question

I just bought a nice used OS-FS 1.20 four stroke engine. It is pumped but the muffler also has a nipple for pressurizing the fuel tank. Do I need to pressurize the tank if the engine has a pump? If not, what would happen if I did?
Also, I found an engine manual online and it says to vent the tank. Is that absolutely necessary ?
I've never has a four stroke before but I've had a lot of 2 strokes. I don't really know much about four strokes..Any advice about pressurizing the tank would be much appreciated.

thanks in advance!
Chuck
Old 08-23-2016, 04:55 PM
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RBACONS
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Ignore the pressure tap on the muffler. You don't use it for that engine. And yes, you have to use a vent line. The tank needs to be able to pull in air from somewhere as the fuel level decreases otherwise you'll create a vacuum in the tank and the engine won't be able to pull anymore fuel out.
Old 08-23-2016, 05:27 PM
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chucksaw812
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Thank you!..I noticed when I first started the engine that fuel was just pouring out of the vent. I guess I'll have to raise the vent higher or attach a short length of hose to get the opening up higher.
Old 08-24-2016, 04:17 AM
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Inside the tank, the vent line should just touch the top of the tank. When the line exits the tank, run the vent line in a loop across the top of the tank before running the line out the bottom of the fuse or cowl (up to top of tank, back across the top, forward across the top, and then down and out).
Old 08-24-2016, 02:11 PM
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chucksaw812
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Thank you! That's a good idea..ill try that!
Old 08-25-2016, 12:05 PM
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Bacons has it right.
i have something that may help you.
A fuel tank is never empty, it either has air or fuel or a mix of the 2 in it.
To take fuel out you must let air in, to put fuel in you must let air out.
Old 08-25-2016, 03:50 PM
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chucksaw812
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Thank you. That makes sense, fuel,or air...but it seems like it would be better to connect a hose from the muffler to the tank. Could it mess up the pump if I do that? Mess up the mixture?
Old 08-26-2016, 05:58 AM
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From the tap on the muffler you would be putting positive pressure on the top of the fuel (head pressure). It would force fuel to the input of the pump. I doubt you could supply enough pressure to interfere with the pump
Old 08-26-2016, 12:13 PM
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chucksaw812
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The reason I ask is because when I started the engine after venting the tank I had fuel shooting out of the vent everywhere!..probably lost a third of it but since it was just a session to tune the needle valve I did t worry about it much at that point.. But I wouldnt want to fly like that!
Old 08-26-2016, 01:25 PM
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Nothing should be coming out of the vent line, with or without the engine running. One of your lines must be hooked up wrong. Particularly with the muffler pressure tap not feeding the tank, there is nothing that should push the fuel to the top of the tank and out the vent line. Any chance the tank is installed and accessed from the bottom of the plane (i.e., with the plane upside down)? If so, when you install the tank, it has to go in upside down so that when you turn the plane right side up, the inside vent tube is now at the top of the tank and the external loop is also on top of the tank.
Old 08-26-2016, 02:51 PM
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chucksaw812
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The more I think about it the more I think that it could've been vibration shaking up the tank and forcing fuel out the vent because I had the tank really full. I'm 99% sure that the hoses were hooked up correctly and the tank access is from the top of the plane and not the bottom. So, what I will do next is connect some fuel tubing the way you suggested and see if that helps but here is a picture of the tank as it currently is. I thought maybe the vents wasn't high enough but looking at the picture I think it is high enough.
Lol...it posts the picture upside down but when you click on it , it is right side up..not sure what's up with that..
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Old 08-26-2016, 04:10 PM
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Should not be due to vibration, especially a third of a tank. Maybe a couple of drops. Gas engines vibrate way worse than glow four strokes and don't have that problem. Are you sure the vent tube inside the tank is basically touching the top of the tank? That is, you have a short section of fuel tubing on the end inside that brings the vent tube right up to the top? BTW, no need to bend the vent tube up outside of the tank. It can come straight out like the fill and carb lines.
Old 08-28-2016, 08:39 AM
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Your vent line should be out of the tank then looped up on top of the tank before exiting out the bottom of the airplane. Any positive pressure would force fuel out the vent. I guess we tired and it didn't work. Remove the pressure line and cap both ends
Old 08-28-2016, 01:58 PM
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chucksaw812
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I think it,will,work>..thank,you very much!
Old 08-29-2016, 08:00 AM
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Normal tank venting has the vent at the inside-top of the tank and exit through the bottom of the cowl. That prevents siphoning, but allows free flow of air into the tank as fuel is consumed. You also fill through the fuel line and then the vent indicates a full tank.

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