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Unread 03-20-2007, 08:56 PM   #1
1989 Islander
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Why do you need a return fuel line

Once you put on a MC2100 what purpose does a line going back to the tank serve?

I understand that on the orig system, Carter carb BBD feedback, it was part of a grander scheme but now that one has a 2100 could you get by with just the one line from the pump to filter to carb?

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Unread 03-20-2007, 09:01 PM   #2
Richmond88yj
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Because of the valves in the carb only let so much gas in. The rest has to be bled off and not back up pressure to the mechanical pump.

This is called an open loop system

Essentially look at your fuel filter. The drain line is on top, carb feed on bottom. Any "over pressure" condition bleeds out through the top and back to the tanks to be cycled again with little or no pressure.

It extends the life of the pump and keeps the pressure regulated at the carb.
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Unread 03-20-2007, 10:06 PM   #3
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Quote:
Originally Posted by Richmond 88 YJ
Because of the valves in the carb only let so much gas in. The rest has to be bled off and not back up pressure to the mechanical pump.

This is called an open loop system

Essentially look at your fuel filter. The drain line is on top, carb feed on bottom. Any "over pressure" condition bleeds out through the top and back to the tanks to be cycled again with little or no pressure.

It extends the life of the pump and keeps the pressure regulated at the carb.
i couldn't have said it any better myself
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Unread 03-20-2007, 10:29 PM   #4
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Just to give a visual aid, the carb has a little bowl in it that it draws fuel from, there is a needle which rises and falls with a float in the fuel bowl, when the float raises up to a specified point the flow from the feed line is cut off - there's no smarts in the mechanical fuel pump so it just keeps on pumping and that extra fuel has to go somewhere.
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Unread 03-20-2007, 10:46 PM   #5
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Two years ago my fuel return line became clogged. Normally I'm supposed to have 60 pounds of pressure on my fuel rail (94, has injectors).... but when my return line became clogged I was getting close to 90.

I thought I had a spark problem at first because it seemed sluggish. To test I'd start off in 3rd gear up a slight hill. It would keep running and not stall, so thats when I figured it had to be a fuel problem.

PS: On a side note, after fixing my fuel return line.... about 4 months later my fuel pump went. I had to open up my damn gas tank twice that year.
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Unread 03-21-2007, 08:36 AM   #6
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Thanks. I hope this is as informative to others as it is to me. It gives me several thing to check on the carb set up.

This is one I'm glad I asked.
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Unread 03-21-2007, 11:04 AM   #7
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OOOO I like the picture!
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Unread 04-05-2007, 07:09 PM   #8
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Axel Jack

Have yj 91 4.0 fuel pressure 92 lbs

How/where does the return fuel line get plugged??
Is there a valve at the pump???
Looking at removing tank like to clarify firdt, need pump?????

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Unread 03-25-2010, 11:33 AM   #9
lidlitalian
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on the filter setup, does the return line go above the inline or below? Think mine is upside down. when I pinch the return line, my holley fpr goes up to the psi (3.5) that it should be. when its not pinched, it just bounces around 0-1...
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