Fuel pumps - series or parallel

[justin.schoeman]

OK - I know this has been discussed before, but I can't find the thread anywhere...

Now I need two electric fuel pumps. Is it best to connect them in series, or parallel? I need the least variation of pressure when one or both pumps are running...

Thanks,
Justin

[Gompou]

viewtopic.php?f=13&t=309&hilit=facet

hope this help.

[nicow]

viewtopic.php?f=13&t=309&hilit=facet

hope this help.

[justin.schoeman]

Thanks. That is the one... Series it is then!

[tigger_na]

After studying this article by Robert Metzler, who is a Hawk pilot in the States and wrote a comprehensive series about the Hawk in specific and all sub-systems in general, I went for a parallel installation:

Actual engine failures are rare. Almost all so called "engine failures" are returned to service without ever working on the engine itself. It's the systems which support the engine that usually fail and fuel systems fail more often than all other things combined. Most of those failures can be easily prevented and this article will show you how.

FUEL
RON and MON are two slightly different methods used to determine gas octane. European countries use the RON method only. Our American pumps have a label on them which shows that the advertised octane was calculated as RON plus MON divided by two. If your engine manual says to use 91 RON then you can use American 87 octane gas since its the same thing as 91 RON.

Aviation fuel is often called 100LL. It is 100 octane but the low lead part is misleading. It does have less lead than aviation fuel once had years ago but it still has much more lead than any automotive gas. That lead is hard on our type engines because it fouls plugs and causes rings to stick if we use it on a regular basis. A few gallons of aviation fuel once in a while doesn't really matter much but I still avoid it as much as I can.

Some "experts" claim that the volatiles evaporate out of gas so fuel goes "stale" fast. There is a tiny bit of truth in that but it certainly
doesn't happen very fast. How long does gas have to sit in your car or lawn mower before you notice any difference? In my plane 3 or 4 weeks certainly doesn't cause any problems and I have gone twice that long with only a very minor difference in performance ... and I even premix the oil with my fuel. Don't get too paranoid about "stale" gas.

Gas picks up small flakes of rust and dirt as it is pumped hundreds of miles through underground pipe lines. Slugs of water are inserted into the pipeline between different shipments and some of that gets mixed with the gas as it travels through the pipe. Large storage tanks at your local pipeline terminal always have some water and crap in them. Even the truck that delivers gas to your local gas station may contribute and of course water and crap in the gas station tank gets stirred up when the tank is filled. Gas companies try to filter the gas and most even have a filter right at the gas pump nozzle but some water and crap ALWAYS gets to our tanks. A lot of fuel problems happen shortly after refueling from a new source and even airports can have contaminated fuel.

Some pilots filter the gas going into the tanks on the plane with chamois cloth, felt or a Mr. Funnel. It doesn't matter what you use or how careful you are to avoid condensation in the tank sooner or later there will be water and crap in your tank so you should plan for your fuel system to be able to handle some of it.

ELECTRIC FUEL PUMPS
The Mikuni pulse pumps are designed to suck fuel up to the engine and can easily handle the 3 or 4 feet of vertical lift needed with our engine and tank locations. Faucet recommends mounting electric pumps below the fuel tank because they can NOT produce much suction.

Many people think that an electric fuel pump will provide an extra margin of safety. That's okay but make sure that you understand what you are doing or you can wind up LESS safe. Dual pumps can be used safely but you need to do it correctly.

If you use pumps in series each pump must provide enough fuel pressure by itself in case the other pump fails. The second pump can
boost the pressure of the first pump so high that the carb inlet needle valve can't close and then fuel will squirt out of the float chamber vent. To prevent that you need a pressure regulator ... but then you depend on that ONE regulator to stay in the air. Wasn't your orginial concern that you depended on ONE fuel pump? Pumps in series usually add more potential problems than they remove. I do NOT recommend series use.

For parallel use we usually use the Facet 40104 or 40105 4PSI pump when the pump is in the wings and the Facet 40106 6PSI pump when the pump is mounted lower in the plane so that the carb sees the correct pressure. NONE of those models have a check valve built in so any time there is more pressure at the outlet port than is being created by the electric pump fuel will be forced backwards through the pump. If the electric pump gets turned off then a large part of the fuel flow from the Mikuni pump will flow backwards through the electric pump. Click on http://www.facet-purolator.com/cat_part_finder.php to get technical info on those fuel pumps.

With engines mounted high and fuel pumps mounted low even with no pumps running there is one PSI pressure on the pump outlet port for every 37 inches in height just from the head pressure of the fuel in the line up to the carb. For this discussion call that 1 to 2 PSI head pressure at the electric pump outlet when all pumps are off.

Fire up the engine and the Mikuni will add 4 to 7 PSI on top of that head pressure to give about 5 to 9 PSI total at the electric pump outlet. The electric pump can only pump 6 PSI so fuel from the Mikuni can flow backwards through the electric pump at high engine speeds EVEN WHILE THE ELECTRIC PUMP IS TURNED ON.

Electric pumps can force fuel backwards through the Mikuni pump too if it has grit in the internal flapper valves. Reverse flow can cause fuel starvation and other problems like unexpected fuel transfer to a different tank depending on how the pumps are connected.

McMaster-Carr sells a good check valve to prevent reverse flow. Part # 7775K51 only takes 1/3 of a pound of pressure to open and it has a Viton seat which is rated for gasoline but you need two # 44555K131 barbed adapters to install one in our fuel line. I strongly recommend a check valve on each pump outlet anytime there is more than one pump in the fuel system. I also recommend a fuel filter on each pump inlet so if one filter stops up the other pump can still provide fuel.

If you provide a seperate fuel tank dip tube and fuel filter for each pump you can have two completely independent fuel systems as long as the two systems don't connect together except on the carb side of the check valves. There you can use a 4 port manifold, a cross fitting or two tees to provide for one line from the electric pump check valve, one line from the Mikuni pump check valve and two lines to the carbs.

MIKUNI PULSE OPERATED FUEL PUMPS
Two stroke engines use the piston to suck the air/fuel mixture from the carb through the intake manifold into the crankcase each time the piston moves toward the combustion chamber. As the piston moves away from the combustion chamber the mixture in the crankcase is pushed from the crankcase into the combustion chamber through the transfer port. This creates a small vacuum and then a small pressure inside the crankcase which we call a PULSE. A connection through the crankcase wall is connected to a chamber inside the fuel pump by a piece of tubing so the fuel pump diaphram can be moved by these pulses.

Condensate forms when warm fuel/air vapor hits the cooler pulse line. If that condensate can not drain back into the engine it will collect in the pulse line or the fuel pump pulse chamber. Fluid trapped in a pulse line will weaken the pulse at the pump. Fluid trapped in the pump restricts the diaphram movement. Either one means less fuel gets pumped.

Rotax drills a tiny drain hole in the Mikuni pumps which they supply with their engines. That hole is located at the pulse port connection to drain both the pump pulse chamber and the pulse line if the pump is mounted right side up and lower than the engine pulse connection. Click on the link below to see how to drill that hole. Some models have the hole in a 90 degree brass fitting at that location.

http://www3.sympatico.ca/murray.j/mikuni_pump.htm

I don't do that because the hole weakens the pulse and the hole may get plugged with crap. I do mount the pump right side up but above the engine pulse connection so that any condensate at the pump diaphram will drain out into the pulse line and the pulse line will drain back into the engine. Do NOT bolt the pump solid to the engine. Use mounting which isolates the pump from vibration and engine heat.

Long pulse lines, leaks in the pulse line and thin wall tubing which can expand and contract with the pulses also weaken the pulses. Weak pulses move the diaphram less so less fuel is pumped. Pulse lines must be less than 18 inches long so the pump must be located near the engine pulse connection. On Hirths you can use the engine pulse connection on the other cylinder to help keep the pulse line short but Rotax only has one engine pulse connection.

It's not easy to do all that so some plane manufacturers take the easy way out and then problems develop over time after condensate builds up in the fuel pump or pulse line.

I recommend the large round Mikuni pump since it can pump up to nine gallons (35 liters) an hour ... but that is only when they are installed correctly. Remember that they will pump less if they have installation problems. Both outlets come from the same internal chamber so cap off one outlet or tee the two outlets together if you only need one outlet.

Its VERY rare to hear of any problem with a correctly installed Mikuni pump. They have been used on many brands of ATVs, cycles, jet skis and snomobiles for many years. They don't use electrical power and they have few moving parts. Those parts are replaced during a low cost rebuild which is so easy that anyone can do it.

I think they are more reliable than electric fuel pumps so I use a Mikuni which I replace or rebuild every two or three years and I don't have any other pump on my plane. If I was concerned I would install a second Mikuni pulse pump for a backup. Hirth engines have a pulse connection for each cylinder and there is no reason that you can't use a seperate pulse pump on each connection. Rotax engines are limited to one pulse pump since they have only one pulse connection. Do NOT try to use two pulse pumps on the same engine pulse connection. That will cause both pumps to recieve weaker pulses and any pulse line problem will affect both pumps.

FUEL TANKS AND THEIR CONNECTIONS
Be a little leary of fiberglass and other composite fuel tanks. Every once in a while we hear about a tank which partially disolved because it could not handle ethanol or some other fuel additive. I suspect the wrong resin was used to make those tanks or they were intended to hold water instead of fuel.

I don't care what you use to filter your fuel or how careful you are there WILL be crap and water in your fuel tank. Your first line of defense is to leave a little room at the bottom of the tank where the crap can settle out without getting into the fuel lines.

I don't like holes anywhere in a fuel tank except at the top. Sooner or later holes in the sides or bottom will leak no matter what you do. The dip tubes described below use a hole in the top of the tank so there are no leaks. If you already have a hole in the side or bottom of your tank you should use part #FTG-12240 from Wicks Aircraft. Those grommets are made of Viton instead of black rubber so they will stand gasoline service much better.

With no drain on the bottom, you need some other way to get the water and crap out of the tank. A siphon hose will do that but I use a piece of 1/4"OD stainless or aluminium tubing long enough to reach the bottom of the tank through the filler opening with enough extra for me to hold as I guide the probe to the low spots in the tank. 1/4"ID Tygothane tubing goes over the probe and connects to a squeeze bulb so I can suck out trash and water. I don't do it everytime I add fuel but I do make it part of my regular maintaince routine. Do it after the crap and water has had time to settle to the low spots.

Only planes capable of inverted flight need flexible lines inside the tank. Flexible lines can be a real pain in the butt and may collapse from the vacuum needed to suck fuel out of the tank. Do NOT try to use them. Do NOT use screens or filters inside the tank. They will get clogged sooner or later and you will have a hard time cleaning or replaceing them.

A clear Fram G1 filter at the pump inlet will protect everything plus it's cheap, eazy to inspect and easy to replace. If you install it with the
inlet on the bottom and the outlet on the top it even tells you when its getting clogged up. When its new fuel fills the bottom part but the top part stays full of air. As the bottom part of the filter element gets clogged the fuel automatically rises to get through the unrestricted partt. Replace it when the fuel covers the entire element.

The fuel line connection to the tank should be a metal dip tube which goes through the top of the tank and ends about 1/2" above the bottom of the tank to keep it above any water and trash in the tank. Do NOT let dip tubes lay against the bottom or sides of the tank. That can seal off the open end or let vibration rub a hole in the tank. Fuel will corrode copper or even brass but aluminium or stainless steel will work fine. Very large engines need as large as 3/8 OD X .035 tubing for dip tubes but 1/4 OD X .032 will easily handle 4 or 5 gallons per hour.

If the tank doesn't have any connections on top install a stainless steel panel mount coupling through the top of the tank. McMaster-Carr
#5182K375 fits 1/4 OD tubing. It is like a normal tubing coupling except it has a threaded section in the middle so it can be inserted through a hole and mounted solidly in place by a nut. If you install a 7/16 viton, nylon, or PTFE washer as a gasket outside the tank it will seal fumes. McMaster-Carr #90295A186 is nylon. Use a metal flatwasher between the gasket and mounting nut to protect the gasket. The tubing nut and ferrule inside the tank are not used so that the dip tube can be removed later. The tubing nut and ferrule outside the tank will hold the dip tube in place, seal around it and allow removal of the dip tube. There is a small shoulder inside the coupling which needs to be drilled out with a 1/4 inch bit before the tubing will slide all the way through.

Most tanks have a small filler opening so here are some tricks to help install connections. Large drill bits make a sloppy hole in soft plastic so drill a small hole then use a tapered hand reamer to enlarge the hole to the correct size. Don't forget to deburr the hole. Drop a string down through the diptube hole then fish the string out through the filler opening. Remove everything from the coupling body then push the string through the coupling. Tie a weight on the end so the string can't pull out of the coupling and drop the coupling in the filler opening. The string guides the coupling to the diptube hole as you pull it out. Install a viton, nylon or PTFE gasket with a stainless flat washer on top to protect the gasket. Use Locktite when you tighten the mounting nut. Put both pieces of the ferrule and the tubing nut loosely on the top of the coupling and slide the dip tube through the coupling to the bottom of the tank. Pull it back up at least 1/2 inch before you tighten the tubing nut.

A hand pump primer connected to the fuel pump suction line can leak air into the fuel system if the primer goes bad. I use a seperate 1/4"OD dip tube just for my primer to avoid that.

A cheap fitting to use for vents in the top of tanks is a metal valve stem for tubeless truck tires from an automotive parts store. They have a nut for solid mounting in the top of the tank but throw the rubber seal away and use a viton, nylon or PTFE washer as a gasket. Remove and discard the valve core before you slide Tygothane tubing over the fitting. If you route that vent line below the bottom of the tank fuel will not leak out if you flip the plane over. If you extend the vent line out the bottom of the plane it will keep fumes and any slosh overflow out of the cockpit. I stick a fuel filter on the open end. It keeps bugs out and its my spare fuel filter on cross country flights. Any vent built into the filler cap can be sealed off to prevent leakage and fumes when the tank is vented this way.

New tanks and lines usually have construction crap left in them which often causes serious problems on the first flight. Clean and flush them out before you trust them.

HEADER TANKS
Some installations with more than one wing tank use a smaller header tank installed lower than the wing tanks. Fuel drains out of the wing tanks into the header tank which is then used as the main fuel tank. Header tanks often have serious problems with leaks so I recommend only an all metal tank with threaded connections welded in place for every opening needed. It should also include a welded connection for a drain valve to drain the water and trash which will collect there. Since the fuel uses only gravity to drain down into the header tank each wing tank must have a connection on the bottom and that connection often leaks. You should be aware that the vent used on the header tank must extend up as high as the vents on the wing tanks to prevent overflow. Without that vent, fuel may not drain into the header tank because air trapped in the header tank will try to work its way up to the wing tanks through the wing tank drain lines.

FUEL, VENT AND PULSE LINES
Saint-Gobain is a very large company that includes several subsidary companies, factories and/or divisions. Combined they manufacture
many different kinds of plastic tubing known by several brand names includeing Chemfluor, Fluran, Norprene, Pharmed, Tygon, Tygothane, Versilic and others.

Tygon is probably the best known brand name but remember that there are many different types of Tygon tubing. Most types are NOT suitable for our application so be sure to check the formulation number. There are also other companies and they each have their own brand names of tubing made with different chemical compounds.

Years ago some manufacturers tried to identify their products with different colors so that buyers could tell a supplier to "Give me 30 feet
of the green stuff". When the "orange stuff" proved to be a better product or more popular competitors quickly colored their brand orange
too. It quickly got to the point where color tells you absolutely nothing about what the tubing can safely handle.

Today there are hundreds of different types of tubing available so we need to understand and correctly use some industry terms. Do NOT tell a manufacturer that that his product will be used with "gasoline", "fuel" or "ethanol" because those mean three completely different things to him and none of them accurately describe what we really use to him. We use a 10% ethanol 90% gasoline mixture. Be certain that he understands that so he can recommend the correct product.

We need a product that lets us easily see air bubbles inside the tubing. You will see several words used to describe how well you can see through the tubing. "Translucent" means that you can see light on the other side but you probably won't see details like air bubbles. "Opaque" means you can't even see light through it. We need "transparent" or "clear". Sometimes a color is opaque and sometimes its like the tinting on your windshield. Its there but you can still see through it fairly well.

We need a product thats flexible. If the minimum bending radius is more than four or five times the tubing OD then it may get kinked shut
in our application.

We need a product thats soft enough to seal easily on some of our fittings so it needs to be less than about 90 on the Shore "A" durometer
scale. It also needs to withstand hose clamp pressure so it needs to be more than about 60 on the same scale.

We need a product thats able to stand a strong vaccum. Mikuni fuel pumps can suck hard to get fuel up to them through clogged filters.

We need a product that remains stable, resists UV well and lasts at least 4 or 5 years without getting brittle enough to crack or break with
age and vibration. Most get brittle and crack after a year or two.

My choice is clear Tygothane which is Tygon formulation C-210-A. I left some on my plane for over 5 years with no problems and it was still as flexible as when new. I finally replaced it only because the additives in gasoline had stained it brown on the inside so it was getting hard to see air bubbles. Tygothane is sold by Lockwood Aviation, Mark Smiths Tri-State Kite Sales and McMaster-Carr sells it as Part #5549K47 for 1/4ID X 7/16OD. Part #5549K31 for 1/8ID X 1/4OD. They also have other sizes.

A friend has had simular good results for 3 years with Tygon formulation F-4040-A but it has a couple of drawbacks. Its tinted yellow so its not quite as clear. Its too soft for metal hose clamps or twisted safety wire. It doesn't handle vacuum quite as well and its not made in a heavy wall size so its not as good for pulse line or pump suction line. McMaster-Carr sells it as Part #5552K25 for 1/4ID X 3/8OD and
#5552K23 fot 1/8ID X 1/4OD.

Heavy wall tubing should be used for the impulse line and on the suction side of the fuel pump or vacuum may cause those lines to collapse. The lines on the outlet side of the pump and the vent lines can be a thinner wall thickness if you want but I use the heavy wall for everything.

Suction lines should maintain a steep slope all the way from the fuel tank to the fuel pump so route the tubing to avoid haveing high spots
and low spots. Tiny amounts of water in the fuel can accumulate to fill low spots and small air bubbles can accumulate into one very large
air bubble to fill high spots. A large slug of water or a large air bubble can cause problems.

Automotive type worm gear hose clamps often let air leak into suction line connections because the small size we need doesn't form a circle very well. McMaster-Carr sells nylon double snap grip clamps which work well. Part #9579K63 is typical but get the right size. They also sell spring steel wire formed into a circle as "constant tension spring" clamps. Part #5324K61 is typical but get the right size. They also sell a band tightened by a nut and bolt. Part #5412K48 is typical but get the right size.

If all else fails wrap two turns of safety wire around the tubing and twist it tight but don't cut into the tubing.

FUEL FILTERS
There is a lot of discussion about fuel filters. Some Cessna pilot "experts" claim that any small stuff which goes through the gasolator
screens they use is so small that it goes through their fuel pumps, carbs and engines with no problem. Obviously that has worked for them for a long time but you need to remember the differences in planes and engines.

Their engines are much larger so they use larger jets in their carbs while our smaller jets are easier to clog up. The main problem is that our fuel pumps depend on internal flapper valves being able to seal properly up to 100 times a second. Grit that gets through screens can keep those flapper valves from sealing shut properly. Most of us install a fuel filter just before the fuel goes into the fuel pump to protect the pump and carb.

Purolator (and others) make a filter with a glass sediment bowl that you can take apart to replace the filter and clean out the bowl. It works fine on the pump output and there is no problem with them there. Unfortunately we need a filter on the pump intake and there is suction there. You may have already noticed that suction is harder to seal than pressure if you had a hard time keeping air bubbles from leaking into your suction line connections. The seal at that filter bowl was NOT designed to handle suction and if you try to use it on the suction side of fuel pumps it WILL suck air into your fuel system. Many UL pilots have known this for years and avoid them but sometimes a newbie will try to use one then post that he has a problem with bubbles in the suction line.

Some internet "experts" claim that filters should not be used on the suction side of pumps and use those posts as "proof". You should
remember that many of the "experts" use wing tanks to gravity feed fuel to a lower engine. Their fuel doesn't need to be sucked up to the
engine so they are not familar with our fuel pumps or the suction needed. They claim that the bubbles are "vaporizeing" out of the fuel because a high vacuum is needed to suck fuel through a filter.

It is true that a high vacuum in fuel lines or high temperatures can cause "vapor lock" when fuel vaporizes but that is NOT what is happening here. The vacuum used normally isn't much more than we generate when we suck on a hose to siphon gas and the bubbles are present even when the temperature is below freezeing. Its real easy to prove that those bubbles are simply air leaking into the suction line. Replace the glass bowl filter with any different type of filter and there are no more air bubbles in the suction line.

These same "experts" have claimed that our other filters leak air into suction lines. Thats not true. The ONLY reports of air leaks are those glass bowl filters and tubing connections which use the wrong clamps. I did see one report where vibration unscrewed one brand of so called "high performance" fuel filter. They are a metal cylinder which screws apart so you can replace the very small filter element. I do NOT recommend them because the elements are too small.

Some fuel filters use a sintered bronze element. That element has a very small surface so it doesn't take long for them to get cloged up.
I do NOT recommend them. It's hard to beat the plain old pleated paper type fuel filters which have been used in cars, boats, cycles and many other applications for years. I highly recommend them. The Fram G1 is large and clear with connections that fit our 1/4"ID tubing. Get them from Mark Smiths Tri-state Kite Sales or from Lockwood Aviation as part #414-3636 if you can't find them locally.

Some "experts" claim that ethanol in the fuel absobs water and that water will clog up paper filters. It is true that ethanol absorbs water but neither gasoline, ethanol, water or any combination of them will clog up paper filters. Anyone with a paper filter and a squeeze bulb can easily suck any or all of it through the filter. Think about the millions of paper fuel filters used in cars for years.

You should trap excess water and trash in the fuel tank before it ever gets to the filter anyway. Cessna "experts" should think of our fuel tank as our big gascolator where water and trash settle out. We just use a filter at our fuel pump intake instead of a screen inside our "gascolator".

Some "experts" claim that its better to use a gascolator because filters will clog up. The reason filters clog up is because they are doing their job by keeping stuff out of our pumps and carbs. Rather than clean a screen we replace filters. As long as we replace filters when they need replacement there is never a problem.

SQUEEZE BULBS, CHOKES AND PRIMERS
A squeeze bulb will only pump fuel to refill the carb bowls and lines which have drained down while the plane was sitting. It will NOT squirt fuel into the intake manifold to help the engine start like a hand pump primer. It will NOT enrichen the mixture to help cold engines run smoother like a choke. Some people have had so much trouble with squeeze bulbs that they refuse to use them. Squeeze bulbs should be replaced once a year to minimize problems like splitting, leaking, shedding rubber particles into the fuel or failure of their built in check valves. I do NOT recommend them.

Our "choke" is actually just a seperate fuel path inside the carb which allows fuel to flow through a seperate jet. That enrichens the mixture to help cold engines run smoother but it doesn't "prime" the engine or fill the carb bowls and fuel lines. I don't use mine at all.

A hand primer pump squirts a little fuel into the intake manifold every time you pump it. Proper use eliminates any need for a squeeze bulb
or the choke. It does take a little experience to learn how many times to pump it before starting the engine and when to pump it to keep the engine running while the carb bowls and fuel lines refill and the engine warms up a little. I recommend them highly.

I mounted my primer near my right shoulder when I'm seated in the plane to keep all fuel behind the pilot. That keeps the primer lines
short and lets me reach across and pump it with my left hand to use it as an emergency fuel pump. I can also reach it from outside the
plane to prime the engine as needed when starting the engine.

To start my plane I lean in to make sure the throttle is all the way back and the kill switches are in the "run" position. I position myself so that my left leg is against the front of the right main tire. If the engine were to rev up the plane may run over a chock but it can't run up my leg before I can kill the engine. I look over the wing to watch fuel move through the primer line as I pump the primer. Once it reaches the carb I give it a couple more pumps to squirt fuel into the engine. The exact number of pumps depends on my past experience with the plane and the current temperature. I look to be certain that the area near the pusher prop and the area in the prop blast are clear then yell "Clear prop". Last I lean in and hit the starter. It may take another pump or two to keep the engine running until it warms up. It doesn't take long to learn when to pump it.

FUEL PRESSURE GAUGE OR LIGHT
I'm a firm believer in KISS (Keep It Simple Stupid) so I won't put one on my plane but some people like to have a fuel pressure gauge or a
light that comes on when fuel pressure drops below 2 PSI. Thats okay as long as you understand what you are really seeing.

You need to see the pressure right at the carb inlet. If you just tee into the fuel line there and run a line down to a gauge mounted on the panel you will see the head pressure of the fuel in that line added to the actual pressure at the carb. You need a 10 PSI gauge but normal gauges are not very accurate near the ends of their scales and can not survive vibration very long. You have also added the potential problem of fuel leaks and fire in the cockpit. Electronic sensors for an EIS or a pressure switch for an indicator light installed near the carb inlet solve those problems but are they really useful?

If trash blocks the carb inlet needle valve or the main jet the engine will starve for fuel but the light will never even flicker because there will still be normal pressure at the carb inlet.

If air is leaking into the pump suction line there will be air bubbles in the fuel lines. The light will never even flicker as small bubbles pass through the carb inlet valve and get vented because the pressure in the the fuel line stays normal. By the time there are enough air bubbles in the fuel line to drop the pressure at the carb inlet below 2 PSI you won't get much advance warning from the light before the engine quits.

The most common problem is crud building up in the fuel filter. How fast that happens depends on how much crud is in the fuel and how
much filter surface area the fuel filter has. Refueling with heavily contaminated fuel can clog a small brand new filter very shortly after
takeoff. Normal fuel usually takes well over 100 hours of flying before problems ever start. Once the fuel filter can not pass as much fuel as the engine is useing the fuel level in the carb bowl starts to drop which opens the float needle valve more. The PRESSURE in the fuel line starts to drop as the needle valve opens more because the fuel FLOW is restricted by the fuel filter instead of the float needle valve.
Eventually the fuel PRESSURE drops below 2 PSI and the light comes on. Notice that the engine is still getting some fuel and that fuel FLOW may be enough for the engine to run for hours. The light tells you that the fuel PRESSURE has dropped below 2 PSI but thats all it can tell you. You have no way of knowing why and you have no way of knowing how long the engine can continue to run. You need more information before you can make a decision about what to do.

NEVER base a decision to make an emergency landing on an idiot light alone. That light should only be used to tell you to check the EGT gauge. The EGT will tell you if your engine is getting enough fuel or not so base your decisions on the EGT instead of an idiot light.

Heres why. As previously explained when the fuel system can't deliver as much fuel FLOW as the engine is useing the level in the float bowl starts dropping. As that level drops the jets deliver less fuel so the mixture starts getting lean and the EGT starts to climb. As long as the EGT is normal you have no reason to make an emergency landing. If the EGT is high you should look for a landing site but also reduce throttle to see if the EGT will drop to normal when the engine uses less fuel. If the EGT drops to normal you can limp a long way to a better landing site at reduced throttle so you don't really need to make an emergency landing right away.

If the float level drops too far the engine will start surgeing. That is a short increase in RPM and power when the mixture gets very lean as
the engine runs out of fuel. Once the mixture gets too lean to ignite the engine coasts to a stop unless it gets another shot of fuel to run or
surge again. The prop windmills for some time after the fuel is gone. That keeps the fuel pump working so its common for the engine to
surge several times before it quits completely. Now you have a real emergency but you may still be able to limp to a better landing spot if
you can use the hand primer to help the fuel system deliver enough fuel and/or reduce the throttle so the engine doesn't use as much fuel.

Most fuel problems show up first at full throttle because the fuel system works the hardest then. Get in the habit of checking for high EGT during all full throttle operations such as climbout. When you start to see that happen, its time to replace your fuel filter and check the rest of your fuel system.

The pressure gauge, idiot light or EGT will not give any warning of a sudden serious fuel system failure so you should fly like the engine
could quit at any time and practice emergency landings so you will be ready for a real one.

FUEL INJECTION CONSIDERATIONS
Fuel injection requires a high pressure fuel pump. Most of those pumps need a return line to be run from the pump back to the fuel tank because they use a built in pressure relief valve as a form of pressure regulator for the outlet port. When the pressure at the outlet port rises higher than the set point of the pressure relief valve the pressure relief valve opens to divert fuel from the outlet port to the return port until the pressure at the outlet port drops back down to the set point.

That means that the fuel flow into the fuel pump is much greater than the fuel flow required by the engine alone. The return line from the pump back to the tank and the suction line from the tank to the fuel pump inlet port must be big enough to handle the maximum fuel flow that the pump can produce. Note that the fuel filter in the suction line must be able to handle the larger flow also. The line from the fuel pump outlet port to the fuel injection can be much smaller since that flow is only the amount actually used by the engine but that line must be able to handle the high pressure that is at the fuel pump outlet port.

THE BOTTOM LINE
You should NEVER attempt the first takeoff until you are absolutely certain that your new fuel system will provide enough fuel under all
conditions. Tie the plane down and run it at full throttle to be certain that it won't starve for gas during takeoff and check closely for leaks. Always be prepared for the engine to quit on takeoff.

[Turbo]

Careful connecting pumps in series (connection pipework dependant) - if the first pump fails IN FRONT OF the second, the second pump will not be able to pump fuel -

Parallel (next to each other) is the most reliable way of pumping fuel - we use this on all the road and modified drag/drift cars every day! you want both pumps pumping together (not pumping INTO each other, raising the pressure) but side by side so that if one fails the other still continues regardless and there will be no drop in pressure or flow.

Connecting these units together is key and a one way valve will be required to make sure the pumps to do fight one another. Another Good idea is to build a surge tank as a buffer.