Fuel System Question
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Fuel System Question

  1. #1
    Senior Member 78Southwind's Avatar
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    Default Fuel System Question

    The Spectra currently has a fuel system I am suspect of since it's not running a regulator and the fuel pressure seems to be a little low for a blower engine.

    It's a loop type of set-up that pumps fuel through a -8 line to a fitting that splits it to the carbs and the return line that is -6. The return line loops right back in front of the fuel pump. When I tested the boat the gauge was saying 9 PSI the next time I tested it the gauge was going right to 15 PSI. So the sender is bad and will be replaced but I just tested the fuel pump with a manual gauge and it's reading 5-5.5 PSI on the trailer (without the engine running). So it looks like the pump may have been modified some how (different spring?). What do you think rebuild the pump back to 12 PSI and run a regulator?

    Last edited by 78Southwind; 03-24-2016 at 06:04 PM.

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  3. #2
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    If the engine isn't running, the regulator is probably not getting 12 volts and would have low fuel pressure.
    The generator will probably put 14 volt to the pump, but at least 12-14V.

    Hook a battery charger to the battery or run the motor and try again.
    It might be OK.

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    Senior Member 78Southwind's Avatar
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    Quote Originally Posted by jimsplace View Post
    If the engine isn't running, the regulator is probably not getting 12 volts and would have low fuel pressure.
    The generator will probably put 14 volt to the pump, but at least 12-14V.

    Hook a battery charger to the battery or run the motor and try again.
    It might be OK.
    Yeah, the test was with the battery charger connected to the battery (13.65 volts). But I was wondering if that last .35 volts would make that much of a difference? When I get the new sending unit back I am going to test it again (engine running).

    What is a good fuel pressure setting for a 572 with a 10-71 blower running about 5.5 to 6.5 of boost (about 1,000 HP). I was thinking 6.5 PSI of fuel pressure but I was reading maybe even higher when it's over driven.
    Last edited by 78Southwind; 03-24-2016 at 07:05 PM.

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    My system is similar to yours except I use a bi pass regulator and my pump is a Clay Smith mechanical.
    Tried to post a picture but it didn't work.
    Last edited by VD Cruiser; 03-24-2016 at 07:21 PM.

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    Senior Member 78Southwind's Avatar
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    Quote Originally Posted by VD Cruiser View Post
    My system is similar to yours except I use a bi pass regulator and my pump is a Clay Smith mechanical.
    That's what I am use to, my other boat has a Clay Smith mechanical and it's been running strong for over 12 years. I don't really want to make any changes if I don't have to but on the other hand I want it right. What pressure do you set your pressure regulator at?
    Last edited by 78Southwind; 03-24-2016 at 07:24 PM.

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    Senior Member 78Southwind's Avatar
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    I think I am going to clean it to make sure there isn't something in the pump causing the low pressure.

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    Same here with the Clay Smith, been running it for a long time with good results. My deal is a 548 ci with a 10:71 @ 12 lbs of boost and fuel pressure set at 7 psi.
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  10. #8
    Senior Member 78Southwind's Avatar
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    Quote Originally Posted by VD Cruiser View Post
    Same here with the Clay Smith, been running it for a long time with good results. My deal is a 548 ci with a 10:71 @ 12 lbs of boost and fuel pressure set at 7 psi.
    What carbs you running and do they have power valves?

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    Carbs are 1050's with no power valves. Jets are 98's all the way around.

    Just noticed my bypass goes back to the inlet side of the fuel filter, instead of the fuel pump, like yours appears too. Don't know if that matters or not.
    Last edited by VD Cruiser; 03-26-2016 at 06:53 AM.

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    Quote Originally Posted by VD Cruiser View Post
    Same here with the Clay Smith, been running it for a long time with good results. My deal is a 548 ci with a 10:71 @ 12 lbs of boost and fuel pressure set at 7 psi.
    Very nice installation. Clean!

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    6.0 To 7.0 psi is a good place to be. The important and maybe the most important pressure is when it's running wide open, and if it's falling below 4.5 and 5.0, your fuel system is not keeping up and the carbs. will run low on fuel.
    Your idle pressure will generally be slightly higher than your running pressure.

    I can't say the 4.5 - 5.0 psi is an absolute low number, but if it's falling lower, there is a problem.
    As far as the need to increase fuel pressure for higher blower pressures, I would say no unless you're running a blow thru system. For a roots system, it's still no.
    The carburetor just needs to maintain a certain level of fuel, and unless the carburetor is pressurized itself, let the fuel system do its job for you.

    Others may have different opinions, if so I'd like to know also.

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    Senior Member 78Southwind's Avatar
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    Quote Originally Posted by VD Cruiser View Post
    Carbs are 1050's with no power valves. Jets are 98's all the way around.

    Just noticed my bypass goes back to the inlet side of the fuel filter, instead of the fuel pump, like yours appears too. Don't know if that matters or not.
    Where do you have your idle set and how much surge do you get?

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    Senior Member andyc's Avatar
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    Looks clean but always try to reduce 90's, The least amount of sharp bends the better.

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    Quote Originally Posted by 78Southwind View Post
    Where do you have your idle set and how much surge do you get?
    Idle is about 1000/1100 in neutral and it surges a little. ln high gear it will come down to 800 rpm. In second gear it surges a lot.

    Thanks Maxey
    Newer picture with updated headers.
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    Last edited by VD Cruiser; 03-26-2016 at 01:28 PM.

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