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Re: UL: Re: Electric Fuel Pump & Mikuni

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  • Robert Metzler
    ... In that case, he might want to consider using the 4-6 PSI electric pump to get enough pressure at the carb(s). A few other things to keep in mind when
    Message 1 of 1 , Oct 1, 2002
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      Hendrik Maroske wrote:
      > Robert Metzler wrote:
      > > See explanation above about head pressure.
      > Robert,
      > this is a very good explanation. Thank you
      > for the insight. Two weeks ago I have seen
      > a Scheibe Uli II with a Hirth motor, Mikuni
      > fuel pump, primer and Bing carburetor. The
      > Bing was mounted with some solid plastic,
      > "Bakelite" or similar looking, piece to the
      > motor.
      > The owner is planning to mount an electric
      > pump and I'll forward your article to him.
      > Of course the tank is behind the seat and
      > the motor up front on the main tube, exactly
      > the case you are considering.
      > Thanks again,
      > Hendrik

      In that case, he might want to consider using the 4-6 PSI
      electric pump to get enough pressure at the carb(s).

      A few other things to keep in mind when designing a fuel system
      are below.

      NOTE...I expect others to post about easier/cheaper ways of doing
      it. I could care less. When it's my azz in the cockpit I want
      it done right.

      SUPPLIERS I think we should buy from the folks who support us
      like Mark Smith at http://www.trikite.com and Lockwood at
      http://www.lockwood-aviation.com when possible. For items that
      they don't carry I use McMaster-Carr at http://www.mcmaster.com

      FUEL LINE You can buy clear plastic tube almost anywhere. Some
      of it might even stand up to fuel....for a while. The only clear
      line that is designed for fuel is Tygothane. It doesn't get
      "gummy" or brittle but it does turn yellow on the inside from gas
      deposits like all the others. To be safe, I replace all mine
      somewhere between 1 and 2 years even though it stays flexible
      much longer. The size is 1/4 ID by 3/8 OD. Use metal fittings
      only. Plastic ones get brittle and break easily. If you use the
      plastic snap clamps to attach tubing to fittings, replace them at
      least once a year since they get brittle with age.

      PULSE LINE Tygothane size 1/4 ID by 7/16 OD. Yeah, I know
      the fittings look larger but this is the right size. Any
      substitutes here can cause big problems later if they collapse on
      the vaccum part of the pulses or expand on the pressure part of
      the pulses. To be sure the pump gets a strong pulse this line
      should be kept as short as possible and NEVER be longer than 18

      PRIMER AND LINE I firmly believe that a primer works much
      better than a choke. I went to a cycle shop and bought all the
      pieces to add the choke (actually a fuel enrichment system) to
      mine. Complete waste of time and money since it has never been
      used after the inital tests.

      If you use a squeeze bulb type primer, install a fuel filter
      ahead of it to keep trash out of the internal check valves. Also
      install a fuel filter after it since there are reports of them
      "shedding" black rubber particles which could get into the fuel
      pump or carb. I have also seen reports of failures of their
      internal check valves and/or cracks in the rubber. Change it at
      least once a year.

      If you use a plunger type primer, provide a seperate fuel tank
      connection for it. If you just tee into the main fuel line, the
      primer will leak air past a worn or broken internal O ring into
      the main fuel line eventually. Use 1/8 ID by 1/4 OD Tygothane
      for primer lines.

      MIKUNI PULSE PUMP I recommend the double pump even for a
      single carb. A short piece of tube with a 1/4 bolt in the end
      can block off the unused output port or you can tee both output
      ports togeather if needed. It doesn't matter since both come out
      of the same chamber in the pump. The pump MUST be mounted higher
      than the engine pulse connection so that any fluid in the pulse
      line drains back to the engine. It should be mounted so that the
      rubber diaphram is horizonal and the pulse port connection is on
      the bottom. Viberation is not good for pumps so don't mount it
      on the engine. I even use viberation isolators on mine. I know
      certain planes come from the factory with the pulse pump mounted
      differently but if it were my azz involved I would fix it. The
      aviation versions of pulse pumps come with a very tiny hole
      drilled in the bottom of the pulse port fitting. This tiny hole
      is to help drain whatever fluid enters the pump. A very short
      duration vacumm pulse sucks the diaphram down and then a very
      short duration pressure pulse pushes the diaphram up during each
      revelotion of the engine. Any fluid agianst the bottom of the
      diaphram restricts the travel of the diaphram in that very short
      time span and reduces the amount of fuel pumped. I replace the
      pump (not rebuild) every 1 to 2 years.

      ELECTRIC PUMP Should be mounted below the bottom of the tank.
      If the pump is mounted low (like below a seat back tank) and the
      carb inlet is real high (like when the engine is mounted over
      head) use the 4-6 PSI version since you will lose 1 psi of pump
      pressure for each 37.4 inches of vertical distance the pump has
      to pump the fuel. The idea is to have 3-5 PSI available at the
      carb inlet. Check it with a temporary gauge AT THE CARB INLET.
      Check it under all possible pump combinations BEFORE you leave
      the ground. A mistake can be fatal.

      FILTERS A clear sintered bronze filter clogged up on me but I
      couldn't see anything wrong with it. I replaced it and laid it
      aside. After the fuel evaporated out of it I could see very fine
      particles. Paper filters let you see crud better but the paper
      can deterioate. The best filters are the fine plastic mesh or
      stainless steel mesh types. Reguardless of type, get a big one
      that is clear so you can see the crud getting caught. Change it
      often even if it still looks good. Oil premixed with the fuel
      will pass through all of them with no problem.

      FUEL I know Rotax says their engines only require 87 octane
      and I know that higher octane does not produce any more power. I
      still use 93 octane because a higher octane can not cause any
      harm but too low an octane can destroy engines. I also know
      that it doesn't take long in storage for the octane to start
      dropping. When my azz is on the line, I pay the extra dollar per
      tankfull. I prefer Amoco because it seems to leave less
      deposits. 100 LL is strictly for emergency use only since it
      fouls my plugs quickly.

      TANK I just read yet another post about some poor guy finding
      residue from resin breakdown in his fiberglass tank. Seems to
      happen way too often for me to trust any fiberglass tank.
      Gasoline is very hard on most fiberglass resins and most
      plastics. Pick a tank from a reputable source and be damn sure
      it is rated for gasoline. Make a probe from 1/4 or 3/8 metal
      tubing hooked to the end of a siphon hose. Use it to suck water
      and crud off the bottom of the tank occasionally after it has had
      a chance to settle out of the fuel overnight.

      TANK CONNECTIONS All my tank connections are on the top of the
      tank and the fuel connections use dip tubes that go down to the
      bottom of the tank. Any tank connections that are lower on the
      tank will eventually cause problems that range from gas smells to
      large unstopable leaks.

      Bulkhead fittings are designed to pass a tube through a surface
      like the top of a fuel tank. They have a tube connection on each
      end like a coupling but the middle part is threaded outside with
      a nut and is designed to mount solid to the top of the tank.
      Some let the tube slide all the way through them but others have
      an internal shoulder that is easily drilled out. Install brass
      bulkhead fittings for 1/4 inch OD tubing in the top of the tank
      then use 1/4 inch OD copper tube to slide through the bulkhead
      fittings down to the bottom of the tank. Aluminum tube is too
      fragile and brittle. Notch the bottom end to make sure it can
      not get plugged agianst the bottom of the tank and keep the end
      1/8 above the bottom of the tank.

      Ever think about what would happen to your fuel if your plane
      flipped over? Seal off the existing vents in your gas cap and/or
      tank. Mount a bulkead fitting in the top of the tank (without a
      dip tube) as the new vent. Put a short piece of 1/4 inch OD
      copper tube in the top of the bulkhead fitting and slip 1/4 inch
      ID Tygothane tube over it. Route the Tygothane down below the
      bottom of the tank and install a fuel filter on the end of it to
      keep the bugs out. This filter can also be used as an emergency
      replacement filter if your main fuel filter gets clogged up 100
      miles from nowhere.

      SERIES VS PARALLEL PUMPS A lot of planes use a 2-4 PSI Facet
      electric pump in series with a Mikuni pulse pump in an attempt to
      increase the safety factor. This system will supply 2-4 PSI
      additional pressure at the carb inlet with both pumps working
      which may be enough to cause fuel to flow out of the carb

      Lets look at some failures and see how a series system handles

      1 The internal check valves fail to seat properly on the Mikuni
      due to trash. The Facet would get you home.

      2 A pulse line failure stops the Mikuni. The Facet would get
      you home.

      3 The diaphram ruptures on your Mikuni. You are going down
      since the Facet will pump fuel through the rupture then thru the
      pulse line into the engine below the piston.

      4 A tank connection or fuel filter clogs. You are going down
      since both pumps go through that connection or filter.

      5 A fuel line or fitting breaks. You are going down since both
      pumps use the same lines and fittings.

      True parallel systems would use seperate tank connections,
      filters and lines. Each pump outlet would go through a seperate
      check valve to prevent backflow into the other system. You would
      get home in every case above if plumbed correctly and your carb
      would never recieve excessive fuel pressure.

      Unfortunately most existing so called parallel systems depend on
      the internal check valves in the pumps instead of seperate new
      ones. Previous posts discussed some of those problems.
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