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912ULS Sputtering Intermittent

in 912 / 914 Technical Questions 6 years ago
Problem : Engine sputtering 2-4 seconds as if it wasn't getting fuel. This happened 3 times while flying for an hour along the Hatteras National Seashore to the Hatteras airport. My two tanks were 3/4 full and I had checked the fuel on pre-flight to make sure it had no water present. OAT was in the low 80's, and humidity was also in the low 80's. I had made several 360 degree turns 5-10 minutes prior to the problem. Immediately after the problem I checked, and my fuel pressure was 5psi and my fuel flow was 4.6-5..6 gph with slight throttle changes between about 4000-5400 rpm. After 5-10 minutes on the ground, I made the return flight to my home airport in Manteo, NC without any problems, but made only flight pattern turns. I had the sputtering just once during a similar flight 2 weeks earlier.

History : My TSN is only 200 hrs over 5 years. Within the past 2 months, I completed my annual, and I did my 5-year rubber replacement which included - replacing my fuel and oil hoses with Aircraft Specialty conductive Teflon hoses, and I had Lockwood check and replace the rubber in my Carburetors.

Although I installed electric carb heat a few years ago, I had never used it. After my first sputter this last time, I turned on carb heat but it didn't help, however I didn't check the carb heat during my annual to make sure it was working.

I have used 93 octane auto fuel 100 percent of the time during the last 1 1/2 years, and about 50 percent of the time prior to that..

I have always had my fuel return line going to my gascolator. I believe one of my new fittings on the return line has a restrictor which I didn't have before.

I was wondering if I should remove the fuel lines and carb float bowls and check for any debris since I just replaced all the hoses.

I would appreciate any advice or recommendation the forum could provide. I would hate to have this problem on takeoff or flying over long stretches of water.

Thanks,
Jim

  • Re: 912ULS Sputtering Intermittent

    by » 6 years ago

    Without the restriction in the return line, there is minimal fuel pressure to the carbs.
    As long as the flow volume is OK it works. With the restriction, it works better.

    I will assume this happened recently when the temperatures are now near summertime highs.
    Did it happen just after a throttle reduction?

    Many people return the fuel to the gascolator and I will catch flak for this...

    If the return line is attached to the Gascolator, and vapors in the fuel lines return and accumulate in the Gascolator as they have nowhere else to go.
    Eventually, the Vapor volume gets large enough that the carbs suck fumes for a second and purge the Gascolator vapor supply.
    The engine stumbles until the vapor is purged and the fuel supply returns to normal.
    It defeats the whole purpose of the return line.

    If the return line returned the fuel and vapors to a vented tank, the vapors would be vented and never starve the carbs.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: James Depfer
      
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  • Re: 912ULS Sputtering Intermittent

    by » 6 years ago

    Thanks for the info.

    I was changing my throttle often during the problem times. I was going low then higher, and circling looking for swimming dolphin near the beach.

    I did forget to mention that I run my electric fuel pump continuously while flying my high wing Zenith CH750.

    If this is a vapor problem it would seem to have something to do with my change in fuel lines, since I've never had this problem before.

    I think it would be very difficult to now add a return line back to my fuel tank fitting.
      
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  • Re: 912ULS Sputtering Intermittent

    by » 6 years ago

    Are all your fuel lines in Fire Sleeving (Thermally Insulated) and well away from Cylinder Heads and Exhaust pipes?
    Heating of the fuel produces more vapor in the lines.

    If you can return the fuel to the tank, it needs to go INTO the tank.
    Not just Tee'd into the fuel supply line AT the tank.
    It needs its own dedicated fitting.

    A secondary issue with returning to the Gascolator is that the fuel keeps running around a short path.
    It gets heated by the engine and returns to the Gascolator to quickly return to the engine to get heated again.
    If it returned to the Main tank the heat would be diluted by the mass of fuel and dissipated by the larger tank surface.

    The fuel pump can move ~30 Gal per hour.
    You are only using maybe 5 per hour.
    The other 25 goes around in circles getting heated.
    If the Gascolator holds a quart, it would get cycled every 60 seconds or so.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: 912ULS Sputtering Intermittent

    by » 6 years ago

    When I first installed the new fuel pump I rebelled against the relatively major job of returning the excess fuel back to the header tank. Instead I, not having a gascolator, placed a tee in the fuel supply line just after it emerged from the firewall. This worked well except... Sometimes I would get that same stumble. I figured the air would vent through the carb vents, and it did, mostly. When it came time to do my second 5 year rubber thing I bit the bullet and drilled and tapped and routed that return line back to the header tank behind the seats. No more stumbling.

    Alan
      
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  • Re: 912ULS Sputtering Intermittent

    by » 6 years ago

    Since you didn’t mention checking the carb bowls especially after a hose change then that is what I’d do. It’s one of the most common issues and it’s cheap and easy. To rule in or out. Start with the most common cause as exotic issues aren’t usually he problem. Since you have a new return hose back to the header tank I wouldn’t put a vapor issue at the top of my list especially since you were in flight flowing cooler fuel and it wasn’t that hot. I wouldn’t put carb ice at the top of that list either at this point.

    In Arizona it’s 105F.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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