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Author Topic:   Estimating diameter of a fuel hose

Frank O posted 03-24-2012 03:13 PM ET (US)
Today my son and I went down to my 1995 Outrage 21 to conduct a pressure test of the 119-gallon aluminum fuel tank. Originally my thought was to connect the gauge and pressure source at the fuel hose between the primer bulb and outboard (with the fuel tank/s fill and vent hoses plugged). This, however, didn't work out as the primer bulb appears to act as a check valve.
We then disconnected the hose that runs from the tank to the fuel-water separator filter. This had a larger diameter than the fuel hose at the outboard, so we couldn't get our test hose onto the barb at the fuel tank.
The hose from the fuel-water filter to the outboard has an inner diameter of 5/16". I didn't have an accurate measuring device, but made a sketch of the width of the flared parts of the barb fitting. The widest diameter of the flared parts of the barb are about 1/2".
So now I need to buy a hose to fit the barb on the fuel tank, and an adapter to connect it to 5/6" hose. If the barb on the fuel tank is 1/2" at the widest, I'm thinking the inner diameter of the hose that fits it would be a little smaller. What would be a typical size in the boat rigging world? 7/16"? 3/8"?

contender posted 03-24-2012 05:23 PM ET (US)
This is a mistake from who ever put the tank together, change out the fitting for a 3/8 size and run a 3/8 fuel line. Do not forget you will need a 3/8 primer bulb as well, best to use a factory bulb, for some reason after market ones do not last.
Frank O posted 03-24-2012 05:41 PM ET (US)
What's the downside of the way it's rigged? Does it stress the engine to pull fuel through smaller-diameter hose?
I could put a 3/8" fitting on the fuel-water filter holder and run 3/8" hose from there to the outboard, but it looked from a quick glance as though it goes down to a smaller-diameter line there anyway.
jimh posted 03-24-2012 07:11 PM ET (US)
Most manufacturers of outboard engines will specify the minimum inside diameter of the fuel supply hose required to properly feed fuel to their engine, and the size typically varies with the engine horsepower. Larger horsepower engines usually specify an ID of 3/8-inch for the fuel line. Check the installation instruction manual for your outboard engine if you have that literature.
Chuck Tribolet posted 03-24-2012 10:29 PM ET (US)
And yes the primer bulb has TWO check valves, one on
each side. Why not just turn the primer bulb around?

Chuck
contender posted 03-25-2012 05:57 PM ET (US)
Frank what size engine do you run?
BQUICK posted 03-26-2012 08:39 AM ET (US)
I like to use 3/8 on anything over 100 hp. Can't hurt.
I've seen rigs with 1/4 in hose on 200 hp motors.....very scary....must be on the edge of running lean....especially on ethanol fuel!
Frank O posted 03-26-2012 11:26 PM ET (US)
New engine is a 2011 OptiMax 250. On the one hand the rated power is higher than the old 1994 Merc Offshore 225 that was on the boat when I bought it. On the other hand, the new outboard sips gas by comparison.

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Author	Topic: Estimating diameter of a fuel hose
Frank O	posted 03-24-2012 03:13 PM ET (US) Today my son and I went down to my 1995 Outrage 21 to conduct a pressure test of the 119-gallon aluminum fuel tank. Originally my thought was to connect the gauge and pressure source at the fuel hose between the primer bulb and outboard (with the fuel tank/s fill and vent hoses plugged). This, however, didn't work out as the primer bulb appears to act as a check valve. We then disconnected the hose that runs from the tank to the fuel-water separator filter. This had a larger diameter than the fuel hose at the outboard, so we couldn't get our test hose onto the barb at the fuel tank. The hose from the fuel-water filter to the outboard has an inner diameter of 5/16". I didn't have an accurate measuring device, but made a sketch of the width of the flared parts of the barb fitting. The widest diameter of the flared parts of the barb are about 1/2". So now I need to buy a hose to fit the barb on the fuel tank, and an adapter to connect it to 5/6" hose. If the barb on the fuel tank is 1/2" at the widest, I'm thinking the inner diameter of the hose that fits it would be a little smaller. What would be a typical size in the boat rigging world? 7/16"? 3/8"?
contender	posted 03-24-2012 05:23 PM ET (US) This is a mistake from who ever put the tank together, change out the fitting for a 3/8 size and run a 3/8 fuel line. Do not forget you will need a 3/8 primer bulb as well, best to use a factory bulb, for some reason after market ones do not last.
Frank O	posted 03-24-2012 05:41 PM ET (US) What's the downside of the way it's rigged? Does it stress the engine to pull fuel through smaller-diameter hose? I could put a 3/8" fitting on the fuel-water filter holder and run 3/8" hose from there to the outboard, but it looked from a quick glance as though it goes down to a smaller-diameter line there anyway.
jimh	posted 03-24-2012 07:11 PM ET (US) Most manufacturers of outboard engines will specify the minimum inside diameter of the fuel supply hose required to properly feed fuel to their engine, and the size typically varies with the engine horsepower. Larger horsepower engines usually specify an ID of 3/8-inch for the fuel line. Check the installation instruction manual for your outboard engine if you have that literature.
Chuck Tribolet	posted 03-24-2012 10:29 PM ET (US) And yes the primer bulb has TWO check valves, one on each side. Why not just turn the primer bulb around? Chuck
contender	posted 03-25-2012 05:57 PM ET (US) Frank what size engine do you run?
BQUICK	posted 03-26-2012 08:39 AM ET (US) I like to use 3/8 on anything over 100 hp. Can't hurt. I've seen rigs with 1/4 in hose on 200 hp motors.....very scary....must be on the edge of running lean....especially on ethanol fuel!
Frank O	posted 03-26-2012 11:26 PM ET (US) New engine is a 2011 OptiMax 250. On the one hand the rated power is higher than the old 1994 Merc Offshore 225 that was on the boat when I bought it. On the other hand, the new outboard sips gas by comparison.