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Thread: Dual STI build log

  1. #681

    Join Date
    Sep 2015
    Location
    Cave Creek, AZ
    Posts
    164

    Default Re: Dual STI build log

    Awesome! Glad you got it figured out. Terry had his pistons swapped out and is running fine. I don't think he's reached his first oil change yet. It will be interesting to see if he has the same results.

    Clark

  2. #682

    Join Date
    Sep 2019
    Location
    Cody
    Posts
    65

    Default Re: Dual STI build log

    Thanks for the info.
    Will be swapping pistons out.
    Attached Images Attached Images

  3. #683
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    940

    Default Re: Dual STI build log

    Jcard highly recommend it!

    These are such fun planes to fly! Made a trip down to Mexican Mountain. Awesome place to fly. Took Cory down to Hidden Splendor. On the way back had the fuel burn imbalance happen again. I thought the new fuel cap gaskets took care of problem but it showed up again. Its bad enough that when it happens almost all fuel of one tank will push into the other tank. I honestly want to go fly over a lake bed when this happens and keep flying because I think that I am going to stop getting fuel to the header tank and engine will quit. As I was flying I un-velcroed my breather line and it was full of air. Had fuel in it but only on the bottom portion of it? Its very annoying when it does this because one wing is heavy and I can fill it when I fly. Problem is I think I have the problem fixed but then on a longer flight, it randomly shows up again. Still trying to come up with theories on what is going on. Still trying to decided some solutions. Decided to take a dip out of one of my fuel lines and switched fuel caps to see if it changes side? Also checked the vent tube on the fuel caps and one was a couple degrees off the other one. Not sure that would effect it? For some reason one tank is pressurizing and the other tank is venting the pressure? Left tank was full and right tank was empty. Read through the forum and understand this has been a problem but I think when it happens to me it more severe because I will have all the fuel push from one tank to the other tank. Something weird is going on? May just need to put inline full shut offs? I tried to pinch the full lines with my fingers to see if I could force it to draw fuel out of the other tank but fingers hurt after 30 seconds. Thoughts?








    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  4. #684

    Join Date
    May 2020
    Location
    Boise, ID
    Posts
    51

    Default Re: Dual STI build log

    Quote Originally Posted by Shadowrider View Post

    Still trying to come up with theories on what is going on. Still trying to decided some solutions. Decided to take a dip out of one of my fuel lines and switched fuel caps to see if it changes side? Also checked the vent tube on the fuel caps and one was a couple degrees off the other one. Not sure that would effect it? For some reason one tank is pressurizing and the other tank is venting the pressure? Left tank was full and right tank was empty.
    Try making the right fuel supply line *exactly* as long as the left line (well, within a half-inch or so).

    A shorter line will drain that tank first.

    HTH
    Building N137DF
    STi, Rotax 915is, Airmaster

  5. #685

    Join Date
    Dec 2009
    Location
    Lawrenceville, GA
    Posts
    155

    Default Re: Dual STI build log

    Seems like there is a big pressure difference from the right to left tank. The left one doesn't want to feed. Leaky tank seal on the left side?

    One thing to note is that these pressure differences are pretty small. Pitot pressure at 100 mph is a little less than 5 inches of water column or a bit less than 7 inches of gasoline. But that's enough to push gas right up and out the top of the other tank in level flight.

    Belts and braces: shut off valve from each tank. Also a one-way valve in a horizontal position behind the seat in the left tank hose. That valve is just a free floating ball and doesn't add any resistance. But if it is pointed downhill it would keep opening.

    That's what I'm planning for my fuel system. It would let me burn from the left tank 1st: close the right valve and open the left valve. The complication is that vent line back up to the right tank. If so and there is a pressure difference then there are two cases:

    1. Right tank has more pressure than left tank. The right tank will feed the header tank through the vent line until the right tank fuel level drops below the vent line port. The vent line gas level will drop less than 7 inches below the left tank level and then the one-way valve will open. The left tank now feeds the header tank until empty. So this case works.

    2. Left tank has more pressure than the right. The left tank will feed the header. And fuel will go up the right vent line into the right tank. If there is a leak around the right tank seal gas will be leaking out. If the pressure difference is big enough some fuel will be coming out the fuel cap vent. It's hard to see how there could be enough pressure difference to give the 4" of gasoline head to do that unless one cap was just not sealed at all. So I think this case works mostly too.


    And obviously if the left tank valve is closed then it's just the right tank fuel/vent line system which works fine.

    FWIW,

    Mark Napier
    Building a KF IV Classic

  6. #686
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    940

    Default Re: Dual STI build log

    Thanks Mark and Yocum137 for your thoughts. I think the EFI fuel pump is also playing into this somehow? Fuel pump provides over 50gph to engine so it is returning a bunch of fuel. It somehow is pushing fuel into one tank possibly in certain situations? I can be at 1/2 tanks and one tank will start filling and one will start draining? Its only happened a few times but still annoying.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

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