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Thread: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

  1. #1

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    Default Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Hello all,

    I have yet to find a thread on this from previous, so I thought I would mention...

    I have come to realize that the mechanical fuel pump on the 912 is very robust, but still offers a remote chance for failure as noted from previous threads. This mechanical diaphram pump is designed to push no more that 5psi to the carbs of the 912ULS. Any pressures much greater than 5psi can potentially push past the needle valves and jetting.

    Currently planning a back up pump (boost pump) to operate at the same max pressure, as offered by LEAF Aviation. (Facet Electric fuel pump, #H7278, 2.5 to 4.5psi). I am aware that a boost pump is not necessary, but is a nice to have option in the event of a vapor lock scenario in the fuel line above the engine. I have been informed I could run this boost pump in line with NO secondary bypass and check valve, and be just fine. John McBean informed me none of his factory airplanes have a secondary boost pump, they are not needed on this gravity feed system.

    Where the boost pump shines to my understanding, is when people will use auto gas with or without ethanol, that the likelyhood of vapor lock is greater. With a boost pump, the pilot has the ability to clear the vapor lock and keep the engine running. Same holds true for overcoming a the mechanical fuel pump from Rotax if it were to ever fail. The other net benefit as noted in previous threads, is the boost pump will fill the carbs' float bowls full of fuel for quicker cold starts.

    So with all this on the table, the question I want ask is: What have the majority of builders installed in their projects? I am inclined to run a secondary boost pump, without the the by pass check valve. Thinking underneath the flap console, just after the fuel valve, and before the firewall. Boost pump from LEAF is a free flowing pump with no check valve, max pressure at 4.5 psi. I have been informed the likelyhood of this restricting fuel is highly unlikely on an in-line install. Installing a one way fuel check valve would be for redundancy.

    Other considerations is under the seat pan with plenty of space, but if the seat pan were to ever collapse, it could rupture/compromise the hard fuel line.

    In addition, to alleviate vapor lock in fuel line above the engine in high heat scenarios, I plan to run a t-fitting from the fuel line after the Rotax mechanical pump, and before the carbs, t-fitting installed preferably closer to the carbs to get away from the heat generated from the engine. The t-fitting would reconnect to main fuel line, forward of the firewall. Question here is, should this fitting and hose connection be of a smaller inner diameter than the fuel line to create a higher back pressure to the main fuel line? Or can this be of the same inner diameter as the existing fuel line. Any thoughts or pics of installs would be greatly appreciated!

    Without a t-fitting installed in a high heat scenario, followed by parking the aircraft, it has been said this can create over-pressure within the fuel line, possibly flooding of the engine on restart, creating hard starts.

    Thank you everyone, I think everyone needs to think seriously about best practices here!

    Comments?
    Last edited by FlyinFish N928M; 03-01-2019 at 03:00 PM.
    N928M
    Mike
    West Chicago, IL
    Kitfox 5 Speedster, building
    912ULS, Airmaster 332

  2. #2
    Senior Member Dave S's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Do a search on "vapor lock"
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  3. #3
    Senior Member HighWing's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    I flew a Model IV-1200 for nine years and almost 1000 hours. I flew alongside a bunch of guys for many hours as well and we kept pretty close tabs on each other. My one fuel flow experience is the only one I remember amongst the bunch of us. It happened after the 500 hour rubber replacement was accomplished. My mistake was to mis-locate the clamp on one end of the fireshield and over tightening a bit. What it did was compress the fuel line as it entered the engine mounted fuel pump reducing fuel flow to a trickle. From start-up through run-up everything went fine. Then taxiing to the runway, I suppose, the carb bowls refilled and lift off was fine until about 400 ft. when a severely rough running engine ensued I had a Facet pump, as you describe, mounted under the seat very near the header tank outflow port with the on-off switch mounted just to the left - within thumb distance - of the throttle. It was immediately turned on and reducing throttle, the smoothed engine was able to maintain altitude for the dreaded 180 turn and back home. I have exactly the same configuration on my new IV - no check valve, no return line. I have never had a vapor lock issue, cold start or hot start. Maybe it is my climate - Central California temp ranges high 20s to low 100s. Glad I have it, seldom use it.
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Thanks Lowell for your input. I am coming up to speed with where I should be with this 912. A lot to learn. Making the most out of the installation manual and the Rotax Blog for added input. I am a firm believer in keeping things simple and well thought out.
    N928M
    Mike
    West Chicago, IL
    Kitfox 5 Speedster, building
    912ULS, Airmaster 332

  5. #5

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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Mike, you need an orifice on the return line. I think it's either .035 or 0.040 but check the manual. I'm installing one on my Model 4 with a 912UL right now after a rubber replacement. We used a 4 way T and machined a fitting with an orifice for the return line. After much consideration, i'm going back to the header tank instead of the gascolator (which is basically what you're talking about doing). I know lots of guys go back to the gascolator, but our reason for going back to the header tank is to avoid pumping hot vapors right back into the fuel pump.
    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

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    Senior Member fastfred's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Does anyone have a process to prevent a vapor lock? I think I had my first one yesterday. It was warm and muggy with extra DA of about 1000 feet we had a small issue on a soft field so I had to stop the engine several times. Then on the last time before taxi it powered down like it ran out of fuel then started right up on restart. I tested it for a while before TO and it ran fine flew for any hour with no issues. does this sound like a vapor lock?

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    Senior Member jiott's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Yes, fastfred, it sounds to me like you had a vapor lock incident.
    Here's my "process" to prevent vapor lock in Rotax 912 engines:
    • Make sure engine compartment fuel lines are in firesleeve, and routed away from hot exhaust areas.
    • Definitely install the Rotax recommended fuel bypass line with proper orifice restrictor and check valve and preferably routed back to the header tank, rather than to the gascolator.
    • 100LL is most resistant to vapor lock; next best is non-alcohol Mogas; worst is Mogas with alcohol.
    • If you shutdown for about an hour or less on a hot day, open the oil door to let trapped heat out of the cowl. Then when you start-up be very aware of possible vapor lock and don't takeoff right away-ground run the engine for a while to see if it stumbles. Let cool fuel from the bypass line circulate for awhile before committing to a takeoff. The vapor lock stumble will occur quite soon after start-up if its going to occur at all.
    • I'm not saying these rules will eliminate all possibility of vapor lock, but I believe it will come very close to solving the problem.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  8. #8

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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Quote Originally Posted by fastfred View Post
    Does anyone have a process to prevent a vapor lock?
    Fred,

    Good chance you encountered vapor lock, seems like all the right conditions were present. Next question to ask is whether you were running 100LL or auto gas. Most auto gas bought in and around big cities is formulated with ethanol for lowered carbon emissions. Even premium fuel is blended with ethanol. You have to go out of you way too find straight premium. To my understanding, the ethanol is a big culprit, not to mention the ethanol pulls moisture from the air, and thus settles down to the lowest point in your fuel system which is why gascolaters and header tank purge valves are so important to check prior to flight. This hold very true especially if you run your wing tanks are at 1/2 a tank or less, the added are air space in the wing tanks work to promote water in the tanks. Or if the airplane sits for an extend amount of time with low fuel in the tanks.

    You either had a vapor lock or water in the fuel line that may have caused your power outage. For me, to rule out the vapor lock scenario, I installed the rotax file block which is supposed to siphon out the vapor lock prior to your carbs. Simply run an added fuel return line from the fuel block, back top the top of the header tank.

    Can anyone else speak on behalf of todays auto fuels? Would be nice to have some kind of measurement tool to check the quality of fuel prior to use.
    N928M
    Mike
    West Chicago, IL
    Kitfox 5 Speedster, building
    912ULS, Airmaster 332

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    Senior Member fastfred's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Thanks .I do use non ethanol premium auto fuel and I mix in 10% 100LL. I also have the return vent line to the tank. But like I said we turned it on and off several time plus powering to full to get it to move on the wet field s I could have got warm under he hood . It ran fine after even at full rpm.
    Is there anyway to prevent it?

  10. #10
    Senior Member fastfred's Avatar
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    Default Re: Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock

    Thanks. ALl that is done except venting out the heat. I was stressing it out pretty good hopefully I won't get myself in that position on a soft field again. My cfi was messing around as I knew it was wet. Luckily it didn't happen on climb out.

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