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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
    66

    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
    960

    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
    57

    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
    159

    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
    960

    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!

  7. #687
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    960

    Default Re: Dual STI build log

    Update: First oil change since 915 pistons. (other than breakin oil) Cut the filter open and filter is clean. I found a couple tiny specs but I imagine they are left over from old pistons. Also had some black, not sure if its paint from cutting the filter or some carbon? I have 40 hours on the oil and didn't burn any oil either! Cp pistons where getting blow by the rings.

    For the cold weather guys, I installed the Thermo-bob. It is a huge improvement for cabin heat. Before I was taping over the radiator to try and get the temps up. Now CHT's come up to 180 and stay at 180 entire flight. Even on long descents the temps stay right at 180. Downside is if the thermostat ever gets stuck closed. Because I have a valve to shutoff heat in the summer to heater, I hooked up a second bypass. So now fluid will still bypass the thermostat with the valve closed. Works great. I just plan on changing out the thermostat every couple years.

    Couple pictures of flying. Love the cold weather as climb rating is pretty good!







    Dustin Dickerson

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

  8. #688
    Senior Member Kitfox Pilot's Avatar
    Join Date
    Aug 2019
    Location
    Spring Garden Illinois
    Posts
    534

    Default Re: Dual STI build log

    Do you have any pics of where you installed the Thermo-bob?
    Harlan and Susan Payne
    Flying Piper Archer
    Building STI Kitfox N61HP
    Rotax 915is , Airmaster prop
    https://www.youtube.com/channel/UC5A...oCVUP15G0uB-Yw

  9. #689
    SomeDay7's Avatar
    Join Date
    Jan 2012
    Location
    Puyallup,WA
    Posts
    180

    Default Re: Dual STI build log

    I knew a long time ago I will have a bob on mine.
    Starfox

  10. #690

    Join Date
    May 2020
    Location
    Boise, ID
    Posts
    57

    Default Re: Dual STI build log

    That's about 29 degree climb angle! What were your VSI and ASI reading?
    Building N137DF
    STi, Rotax 915is, Airmaster

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