1987 Johnson J15RCUD with Racor Filter and Longer Fuel Line Run

I am using this motor on a rigid inflatable boat (RIB) that will be used on a cruising sailboat. I am interested in adding a Racor 120R-RAC-01 fuel filter and water separator to clean fuel purchased in the Caribbean islands. I am also moving the gas tank to the bow locker of the RIB. My questions are:

Given the estimated 9’ total fuel line length, should I use a 3/8” diameter hose instead of a 5/16” diameter hose to make it easier for the fuel pump to pull fuel through the line?

Is there any concern with the addition of the Racor 120R-RAC-01 fuel filter and water separator with this motor and planned 9’ fuel line? Racor only makes these screw on filter solutions available with a 10-micron filter (no 30-micron option), which is more than I need for a carbureted two stroke.

Looking for constructive input. Thanks

Terrapin said:

I am using this motor on a rigid inflatable boat (RIB) that will be used on a cruising sailboat. I am interested in adding a Racor 120R-RAC-01 fuel filter and water separator to clean fuel purchased in the Caribbean islands. I am also moving the gas tank to the bow locker of the RIB. My questions are:

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The Racor SNAPP series would probably be a better choice given the application

Terrapin said:

Given the estimated 9’ total fuel line length, should I use a 3/8” diameter hose instead of a 5/16” diameter hose to make it easier for the fuel pump to pull fuel through the line?

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The filter is rated 15 gallons per hour.
Your 15 Hp motor is pulling 2 gallons or less per hour.
There is negligible pressure drop (~0.09 psi/ft.) at that flow

Terrapin said:

Is there any concern with the addition of the Racor 120R-RAC-01 fuel filter and water separator with this motor and planned 9’ fuel line?

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Your flow rate is in the "noise band" of the pressure drop graphs

Terrapin said:

Racor only makes these screw on filter solutions available with a 10-micron filter (no 30-micron option), which is more than I need for a carbureted two stroke.

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I call 30 micron filters stick and rock catchers......
Anything the filter doesn't catch ultimately end up in the bowl of the carb

Thanks for your thorough reply to my questions. I was not aware of the Racor SNAPP filter series. At first glance it looks like a lower cost option, but the overall size seems larger. I'll also stick with the 10 micron filter given your comments.
My main concern is not to have a fuel delivery system that exceeds the maximum allowable vacuum pressure for this engine. I was unable to find maximum vacuum pressure readings in my OEM Service Manual for this engine, which is what led to my fuel hose I.D. question. Can you point me to the pressure drop graphs you reference?

In the early 1960's the 5.5 / 10 / 18 HP had a little glass bowl filter / separator.---Find one and install it.

sucking gas 9 feet isn't so bad compared to LIFTING . lots of inline filters you can install under the cowl .

Terrapin said:

My main concern is not to have a fuel delivery system that exceeds the maximum allowable vacuum pressure for this engine.

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Unless it uses an electric pump, the fuel pump is driven by crank case or intake manifold pressures.

As noted in the attached, a 2 stroke may generate pressures in excess of 8 psi

Terrapin said:

I was unable to find maximum vacuum pressure readings in my OEM Service Manual for this engine, which is what led to my fuel hose I.D. question. Can you point me to the pressure drop graphs you reference?

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This chart gives pressure drop per 100 ft of hose at a given flow.
Note: This chart is for water, fuel will be less restrictive

As you can see at 0.25 gallons per minute, you’re pretty much off the chart. If you draw in a 5/16” line, at worst case you’re looking at 1 psi per 100 ft of hose.

This works out to 1/11 psi = 0.09 psi for your 9 feet of hose. Roughly the same as the filter you proposing

dingbat, racerone and Crosbyman, thank you for the detailed information and continuing education. It's greatly appreciated!