Different size fuel lines throughout the boat.. what size do I use???

Robert104

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Nov 23, 2015
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I own a 2002 Aquasport 190 Osprey. It has a '02 Johnson 115 Outboard (2 stroke) on it as well.... I bought the boat a few months back and from what I know, I've only been the third owner. Apparently its had some modifications along the way and I've been slowly replacing some parts that probably shouldn't have be there in the first place. Whoever worked on the boat in the past apparently did things half a** and I'm just now finding some of these things out. One in which I'm replacing the fuel lines, fuel filter and primer bulb. My dilemma is that there are about 4 spliced lines on this boat. Two are running from the tank to the fuel filter and two more running from the filter to the motor... All in which are different sized fuel lines. The line running from the primer bulb to the motor in 3/8 line so I thought "OK" I'll use 3/8 throughout the whole boat. I'm not for sure, but after looking at the line that came off the tank, its definitely not 3/8 line.. its a lot bigger. Maybe even bigger than 5/16... so this brings me to my question.
What size line am I supposed to use? Why would there be so many different sized lines in the boat? Is this common to have a bigger line from the tank to the filter.. then a smaller line from the filter to the motor? I'm not a mechanic, so please be patient with me when it comes to understanding this stuff. The main thing is that I want to make sure I'm using the right sized fittings on this boat.

Thanks in advance on whatever advise you can give me...
 
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Oct 22, 2007
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3/8" should be fine for a 115. If you have an exceptionally long run you might want to go to a larger size, but it doesn't sound like that's the case. Fuel line size affects the pressure drop - basically how hard the fuel pump has to work to get fuel from the tank to the engine. One thing that's important is that all fittings are properly sized for the fuel lines, problems here could lead to fuel leaks.
 

F_R

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Jul 7, 2006
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The boat builder probably installed the tank and hose off it. Maybe the filter also. Then somebody else installed the motor, probably the dealer. That would explain the differing hose sizes. Use 3/8" ID or larger, letting the fitting sizes be your guide.
 

oldboat1

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Fuel line size affects the pressure drop - basically how hard the fuel pump has to work to get fuel from the tank to the engine.

If the line from the tank was larger than the one(s) in the engine, wouldn't the pressure in the engine plumbing be decreased -- maybe the Venturi effect? That might be easier on the pump. Don't think I've intentionally put together that setup, but pretty sure I've sometimes grabbed a tank with 3/8 lines and used it on a motor with smaller 5/16 lines. Maybe that's actually a better way to go?? Lower pressure and higher velocity through the engine lines??
 
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Nope, 'venturi effect' has nothing to do with this. When you're pumping a fluid you just want to keep pressure drop (friction) to a minimum.
 

Robert104

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Hey guys.. thanks for the input. I think I might have had the lines backwards on my post. It is going from bigger to smaller from the tank to the motor. I tried installing the 3/8 line originally starting from the tank, but the line wouldn't fit. I think it's to small. This is really confusing since the line (as mentioned above) going to the motor from bulb is 3/8's... A friend of mine said it would be tough and to force it on. Maybe a little oil around the connection to help it slide on? Let me know what you think.
 

fhhuber

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Get a brass barbed adapter with the correct 2 sizes to make the transition.
Any decent auto parts store should have them or be able to get them within 2 days.

Larger on tank than the motor calls for is fine. No fuel line should be smaller than what the motor calls for.

Ddo not force a fuel line onto a larger barb than what is correct because you are breaking the reinforcing weave in the line. That fuel line will fail right beside the barb.
 

emdsapmgr

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Dec 9, 2005
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A 115 should be able to run very well with a 5/16" fuel line. It's when you went up to a V6- that the larger 3/8" line was required. So, either will work, and it's not a problem if it's 3/8" line from the tank up to the engine.
 

Robert104

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Thanks for the input guys,,, I was finally able to get everything to fit right. The 3/8's line is what it was calling for and I got the hose to fit. (Little oil did the trick) I'm replacing the line all the way through and getting it to feed through the transom was a B**** .. lol... but was able to complete it. The only thing now is I'm trying to replace the fuel filter that goes directly into the motor. For some reason though, this part is absolutely hard to find for some reason. The model on my motor is difficult to locate parts for, but don't know why.
 

Vic.S

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Thanks for the input guys,,, I was finally able to get everything to fit right. The 3/8's line is what it was calling for and I got the hose to fit. (Little oil did the trick) I'm replacing the line all the way through and getting it to feed through the transom was a B**** .. lol... but was able to complete it. The only thing now is I'm trying to replace the fuel filter that goes directly into the motor. For some reason though, this part is absolutely hard to find for some reason. The model on my motor is difficult to locate parts for, but don't know why.

If that filter is part number 0433190 it is available from iboats, and other parts suppliers, as is the Sierra equivalent 18-79781

It is used on dozens of models

#27 in this diagram from the BRP parts website ?
convert
 
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