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Old 28 March 2010, 17:19   #1
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help with fuel lines please!

hi all im new here and looking for a little help, last year i bought a johnson v4 90 hp engine as a do uper the engine is early eighties non vro i spent most of last summer sorting the engine out and it seemed like every time i took it out it let me down with fuel problems, this year ive bought a hull and built it up to the stage of fitting the engine again but i was wondering what size fuel line should i be using i was using 8mm but should i be going for 10mm? the take is no more than 1meter from the engine any help or thoughts would be much appriated thanks steve
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Old 28 March 2010, 17:37   #2
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I'm no expert but when it comes to fuel feeds I always believe more is better as the engine will only ever use what it needs. Also check pick ups in fuel tank for obstructions as this has been a PITA in the past.

Ian
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Old 28 March 2010, 17:45   #3
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10mm is what most modern engines call for so I would go with that. Doubt it would have caused your previous issues though.
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Old 28 March 2010, 17:55   #4
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i think i had a mixture of problems last year including blocked jets and fuel fittings which were only rated for smaller engines! would going too large on the fuel lines cause a loss of pressure?
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Old 28 March 2010, 19:01   #5
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Quote:
Originally Posted by steveplumber View Post
i think i had a mixture of problems last year including blocked jets and fuel fittings which were only rated for smaller engines! would going too large on the fuel lines cause a loss of pressure?
There is no pressure in the fuel line to the engine, so no 10mm will be fine
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Old 29 March 2010, 10:54   #6
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Quote:
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and fuel fittings which were only rated for smaller engines! would going too large on the fuel lines cause a loss of pressure?
Too large won't but too small will.....

I carried my my portable tankage from my old Suz 25 to a Yam55. Used the 1/4" hose / barbs / small primer etc, which ran fine with the Suz. I couldn't keep the Yam at WOT for more than about a minute before it konked out.

One upsizing to 3/8" later and it ran fine.

going from 8 to 10mm gives approx 50% more cross section for te fuel to flow through.

At a high level, think of it that a bigger engine will drink more fuel per second. A bigger diameter pipe means the fuel doesn't need to travel so fast through it to get a given volume through per second. Same goes for fittings. It's an unfortuinate law of physics that essentially says "as speed goes up, so does drag". As the drag increases, your fuel pump spends more of it's effort overcoming the drag than moving fuel, and you see the result as fuel starvation.
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Old 29 March 2010, 13:29   #7
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Up to a point increasing size is ok. However, if you go too large then the fuel pump will have trouble pulling the fuel up the pipe. Imagine drinking a lemonade up a straw, then replace the straw for a piece of downpipe off your house gutters. Doubt you'd have enough 'suck' to draw any lemonade up a 68mm pipe. An extreme example I know!
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Old 30 March 2010, 04:43   #8
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Yeah, but your fuel pump is essentially a positive displacement machine. It will move X ml per pulse regardless of the (excess) diameter of the pipe.

And the reason you couldnt drink through a downpipe is more down to you not having enough "sooking" capacity to empty the pipe of air above the lemonade. Try 1, 2, 3, 4 straws - in the limitations of your sooking capacity, you'll get a LOT more lemoade through 4 straws (or a short length of garden hose) ,

The fuel pump has "infinite" capacity, and the fuel line is a connected continuous lump of fluid (no air), so no problem.
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Old 30 March 2010, 04:46   #9
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lemonade???
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Old 30 March 2010, 07:20   #10
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It's a family forum!

....if you drink beer through a straw I suggest you may not want to join CAMRA!
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