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Old 27-06-2008, 20:00   #1
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Fuel Line size

How do you determine what size fuel line to use. I have a Yanmar 4JH4E 56 hp naturally aspirated diesel.. The fuel tank pick up is about 24” to 30” below the lift pump. There are a set of Racor 300 fuel filters between the engine and the tank. There is also an Algae X fuel cleaner that I am going to remove. The fuel tanks are lead to a “Tee” and then thru the Algae X and then to the fuel filters. There are atleas two different size fuel lines. Ό” and 5/16”. I have it on expert advice (Never Monday) that the engine maybe starved for fuel. She won’t go up to 3600 rpms even at idle and there is no black smoke when I try to get her up to 3600 rpms when under load. What size would be best? Oh yeah the runis about 60” from fuel tank to lift pump.
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Old 27-06-2008, 21:24   #2
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I was just looking at the 4JH4E spec sheet on the Yanmar site and it shows data only up to 3000 rpm. Are you sure you are supposed to get 3600 rpm. Or is the spec. sheet wrong.

http://www.yanmarmarine.com/uploads/...E_TechData.pdf
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Old 27-06-2008, 21:29   #3
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Forgive me if I am wrong. I don't believe the naturally aspirated Yanmar 4JH 56hp is supposed to rev as high as 3600 rpm. I think you should be in the range of around 3000 rpm tops.
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Old 27-06-2008, 21:32   #4
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Darn DeepFrz beat me to it.
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Old 27-06-2008, 21:42   #5
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I'm not sure where I got the 3600 rpm number. I don't have my manual it is on the boat but I think that I got it from the manual. Perhaps I was looking at a different engine that the manual covers. Since it seems like I am incorrect there is a good reason why the engine will not get up to 3600 RPM. Ok now what size fuel line is best for the engine?
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Old 27-06-2008, 21:56   #6
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The turbo version would bring those rpm's up to the 3600 rpm mark. I believe you want to use 5/16's fuel line.
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Old 28-06-2008, 12:28   #7
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I think 1/4 inch is to small, not because of the fuel flow but because it is more likely to plug up with crud. I would use 5/16 and make sure there are no sharp corners in the line if possible. If you have to make sharp 90 deg. bends in the run then 3/8 would be an even better size. Maximum flow for your engine is only just over 3 gph. so 5/16 should do if you can keep the bends gentle. I don't have any specs. Just seat of the pants engineering...<gr>
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Old 28-06-2008, 13:04   #8
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I just looked up some fuel line sizing and the site I found (for gasoline engines) does not recommend any fuel line smaller than 5/16. That is up to 250 hp. Same with return line, min. size is 5/16, however I would think that for a 56 hp. engine return line could be 1/4 inch.

Edit: all sizes are ID.

Century Performance Center, Inc. :: Fuelish Tendencies - Understanding Fuel Pressure and Volume

I realize this is for performance engines but was about the only guideline I could find.

Does your manual say anything about max. lift for your fuel pump?
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Old 28-06-2008, 13:18   #9
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Thanks DeepFrz:

My manual does not mention the lift pump height. I am considering getting a day tank but first want to get the fuel line sized up and led properly. I assume that 5/16 should be big enough if it will handle a 250 HP engine. I don't know what size the petticocks are for the fuel filters. That is something that I am going to have to look over on Monday when I go down to the boat.
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