Re: UL: Re: Electric Fuel Pump & Mikuni
- Hendrik Maroske wrote:
>In that case, he might want to consider using the 4-6 PSI
> Robert Metzler wrote:
> > See explanation above about head pressure.
> 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
> 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,
electric pump to get enough pressure at the carb(s).
A few other things to keep in mind when designing a fuel system
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
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.