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  1. #1

    Fuel Return Line. Do you really need it??? 787 / 96 Challenger

    Is the fuel return line really necessary? I'm putting in a fuel flow meter and not sure if enough fuel goes through the fuel return line to make the readings off. I think the fuel return line outlet is very small. Think I read it was 0.4mm. In any case, I planned to put a fuel shutoff valve in the fuel return line so I can shut it off to have accurate fuel flow readings. I figured if it did vapor lock I could open up the shutoff valve and have it back to stock running. Anything I need to be concerned about with blocking the fuel return line on the Mikuni Super BN carbs?
    Last edited by PilotSmith; 02-19-2008 at 11:15 PM.


  2. #2
    xxx2's Avatar
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    You would be jacking your fuel pressure up= way richer fuel conditions

  3. #3
    Do you know that for a fact or is it an educated guess?

    Another option I considered was to run the fuel return line into a graduated measuring bottle that would collect the fuel so after a days run I could measure how much fuel was bypassed to adjust the calibration accordingly. Basically bypassed fuel would go into bottle and a line from the bottle would return to the tank. However, only air would go back into the tank unless bottle filled up to top and then fuel would also go back into the tank. That would keep the system with stock pressure, function, etc. This should work if there is only a small amount of fuel that flows through the fuel return line. At this time I have no idea what the volume is...
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  4. #4
    cheatin' piston popper addicted's Avatar
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    The return line is critical to controlling fuel pressure. Most of what is pumped into the carbs is returned to the tank. DO NOT add any restrictions to the return line.

  5. #5
    Quote Originally Posted by addicted View Post
    Most of what is pumped into the carbs is returned to the tank.
    Yikes!!! Guess my bottle idea won't work then. I was thinking maybe it was a small amount of fuel that went through the return line.

    Perhaps the only solution is to add a fuel flow sensor to the return line where you subtract one from the other to get an accurate reading. That is getting to be more than what I want to do...

  6. #6
    Spoke to factory Mikuni guy. He said at very low throttle setting you can have a moderate amount of fuel coming through the fuel return. However, there is a restrictor and that will only let so much fuel go through. He felt that at higher throttle settings there may be minimal to no fuel going through the fuel return.

    That kind of makes sense in that I read that on some tricked out motors that they could suck more fuel than the stock fuel pumps in the carbs could supply at WOT. With that being the case, on a stock motor at high throttle settings I would guess that there is not a lot of excess pumping capacity available.

    I will hook up a measuring bottle between the fuel return and tank next time I go out and post the info here. Will be a little bit of trivia to add to our collective knowledge bank.

    One thing that was clear was not to plug the return line. He said going from high to low throttle would cause excess fuel to dump into the motor.

  7. #7
    Please consider a few things like coast guard approved devices that keep you and your family safe while boating! You are about to tamper with the most volatile system on the boat! I'm all about mods but only when they actually improve something without blowing myself up.

    The money you will spend on flow meters would in my eyes be a huge waist and you will never see it back in savings! If saving fuel is your goal then let this free thought save you the bucks! "the more you push the throttle forward the more fuel you use"

    You want to dead head the fuel system! What happens when the fuel pressure is now way too much for the seats in the carbs and you are constantly flooding out and or dealing with vapor lock?

    I'm so sorry not trying to be a dick just telling it how I see it and your quest for fuel savings in a boat!(of all things) Sorry and I'm going out on a limb of being banned with this next statement but you may be a better candidate for a blow boat
    If anything I hope you will consider the safety issues involved with your fuel modifications.

  8. #8
    Quote Originally Posted by PilotSmith View Post
    Spoke to factory Mikuni guy. He said at very low throttle setting you can have a moderate amount of fuel coming through the fuel return. However, there is a restrictor and that will only let so much fuel go through. He felt that at higher throttle settings there may be minimal to no fuel going through the fuel return.

    That kind of makes sense in that I read that on some tricked out motors that they could suck more fuel than the stock fuel pumps in the carbs could supply at WOT. With that being the case, on a stock motor at high throttle settings I would guess that there is not a lot of excess pumping capacity available.

    I will hook up a measuring bottle between the fuel return and tank next time I go out and post the info here. Will be a little bit of trivia to add to our collective knowledge bank.

    One thing that was clear was not to plug the return line. He said going from high to low throttle would cause excess fuel to dump into the motor.
    You typed this while I was replying to your other post.

    So you want to drive around with a non aproved container of fuel in your bilge so you can see it fill up higher and faster as you give it more throttle.

  9. #9
    I work in a capacity that what we do is defend lawsuits from people that do stupid stuff that injures others. I know how accidents happen and the importance of doing things right. Hence, I did not simply go out and get a 2 liter bottle of Coke and a straw from 7-11 to rig up a bottle. If you notice I posted comments for feedback from people more experienced with boats. I also spoke with the manufacturer of the carb to get data. The mods I've made to far have been to enhance safety. (ie. lamp that does on to indicate heat problem if buzzer fails, temp gauge, etc.) Using a proper fuel bottle with proper fuel line and connectors does not make the boat unsafe. You could do it in a fashion to make it unsafe but that would not be very wise. I like data and to know what is going on all the time. My philosophy is to avoid problems before they happen. If you follow proper maintenance, and have good monitoring tools, you can avoid many problems BEFORE they happen. I have a lot of questions now as boating is fairly new to me. Don't assume I plan on doing something stupid if I ask a stupid question. I tend to gather a lot of information before doing something to make sure it will work and work right the first time around. With that said, when I was shopping around for a boat I was amazed at the unsafe and crappy conditions of many of them. So I can understand your concern as there are many out there that rig up some pretty bad stuff.
    I also like to tinker so you guys may hear me ask some weird stuff. Something I have planned is to put an electronic device in the RAVE plastic valve diaphragm covers that will tell me how far open they are. Not expensive to do and not a lot of point to it other than to tell if one is sticking or when stetting up the pressure. But I like doing weird crap like that. The kids have fun racing around and I have fun with my gadgets.
    Last edited by PilotSmith; 02-20-2008 at 02:08 PM.

  10. #10
    Moderator RX951's Avatar
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    Quote Originally Posted by addicted View Post
    The return line is critical to controlling fuel pressure. Most of what is pumped into the carbs is returned to the tank. DO NOT add any restrictions to the return line.
    +1 , ditto on dat

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