By Vic Syracuse, EAA Lifetime 180848
This piece originally ran in Vic’s Checkpoints column in the March 2025 issue of EAA Sport Aviation magazine.
One of the most often asked questions I get from buyers is related to the fuel system, specifically whether it is carbureted or injected. Many don’t understand the differences, but “they’ve been told” that fuel injection is better.
In a future column I will share some thoughts on the advantages and disadvantages of each type of system. In this column, I am going to outline the steps to change from a carburetor to an injection system. So far, I haven’t had any requests to go in the opposite direction.
The biggest reason I hear for changing to injection is for aerobatic capabilities. In all fairness, you can perform a whole bunch of aerobatic maneuvers with a carburetor just by staying in a positive-g flight environment. As an aside, at my age, I’m perfectly fine with just positive-g maneuvers, and the lower the g’s the better.
Injection systems are a bit more complicated than a carbureted system. You should plan ahead to have all the components you will need. I would encourage you to speak with an engine shop or a local builder who has the same kind of airplane that you do. If you can find one locally, ask when they might have the cowling off so you can come and take pictures. It would help if the owner had installed the engine so they could answer your questions.
Unfortunately, with so many non-builder-owners out there today, many do not understand the firewall forward. The pictures might still be beneficial if everything was installed right. It might be best to review with an A&P mechanic. Many of the providers, such as Airflow Performance, have great instruction manuals available.
A word of caution is warranted, as a high percentage of accidents during Phase I are fuel system related, most of which were due to modifications from the plans. While this conversion may or may not be considered a major modification requiring some Phase I flight time, you should certainly consider some local test flying above the airport for a few hours after the installation.
Removing the carburetor and installing the fuel servo will begin the process. This alone will create some complications, as the height of the fuel servo is usually different. Usually, you will need a spacer to place the airbox in the same position as it was with the carb so that the airbox lines up with the air intake on the front of the aircraft. It’s easy to have a spacer made, and companies like Airflow Performance will make one for you. The carb heat box and associated parts can also be removed, as there is no need for heated intake air with injection.
Next, mixture and throttle arms are usually in a different position, possibly necessitating a different bracket or mount, and perhaps even different length cables. The fuel hose from the pump to the new fuel servo will also need to be replaced. Another one will have to be installed going from the fuel servo to the fuel distribution unit, sometimes called the spider, which is on top of the engine.
A fuel servo will have to be purchased and mounted on top of the engine. Stainless-steel injector lines will need to be fabricated to run from the servo to the new injectors. The injectors are usually mounted into the top of the cylinder through a 1/8-inch national pipe taper (NPT) hole, which is right by the intake valve. Don’t forget to secure and inspect the injector lines every 6 inches, as per Lycoming Service Bulletin No. 342 G.
A big difference between a carbureted fuel system and an injected fuel system is that the fuel pressure is much higher in the injected version. A carburetor operates just fine on 0.5 psi to 8 psi, and the injection system pressure can be upward of 40 psi and usually needs a minimum of 14 psi to operate correctly. So, we’ve got a few other changes to make.
The engine-driven pump, and the electric pump, if installed, need to be replaced with high-pressure pumps. Replacing either of them can be difficult at times. The electric pump needs to have a good filter ahead of it, and installing a filter is usually a new addition and will require some fabrication of new fuel lines and/or routing. The engine-driven pump has the same external form factor but is usually tucked away on the back of the engine with everything else in the way, from exhaust systems to cabin heat ducting.
Installing the new engine-driven pump requires a few tricks. You must hold the plunger rod up in the engine that drives the pump arm up and out of the way while you work both fuel pump mounting bolts into the accessory case. Most A&P mechanics have some tricks to help with this. My advice is that if you haven’t done this, get some help. I’ve seen people shorten the bolts so they could install one at a time. Not good!
Most carbureted systems also have a gascolator, which is used to trap water. It’s important for a carburetor, as a carburetor will not usually pass water. However, an injection system will pass water (up to a point), so you can do away with the gascolator. However, if you still want the gascolator, or your regulatory body requires one, you must change it out for a high-pressure one if it is installed downstream of the electric fuel pump.
When removing the gascolator, go ahead and remove the primer lines to the cylinders if installed. Plug the holes at the cylinders with 1/8 NPT plugs using some thread sealant.
Relocating the fuel flow transducer, if you have one, will also be in order. For the best indications, it should be installed in the AN4 hose between the fuel servo and the fuel distribution unit. In this location there are no pulses from the pumps, and therefore the readings will be more accurate. I have my RV-10 configured this way, and the aircraft fuel totalizer will usually only differ by about 3/10 gallon from the FBO fuel pumps when filling up the tanks.
Remember I mentioned that many owners want injection so they can do some negative-g aerobatics? Well, get ready to spend some more money and time. You will need two other items — an inverted fuel pickup and an inverted oil system.
The inverted fuel pickup is accomplished by installing a flop tube, usually only in one tank, which should be placarded for inverted fuel. The pilot will have to remember to place the fuel selector in the correct position. Most builders do not install a flop tube when building, so now you will have to remove the tank and install a flop tube and a flapper valve inside the tank to keep the fuel in one bay while inverted.
You will also need to cut another hole in the fuel tank to relocate the fuel quantity transmitter. Otherwise, it will bang into the float and arm, and neither will work properly. Don’t forget to put the O-ring on the end of the flop tube or you could end up with some unsightly dings on fuel tank skins.
The inverted oil system can be a little complicated. The most common one I see is the Christen inverted system. It’s certainly tried and proven, so I would recommend sticking with it. The inverted system will also require a few new hoses to be fabricated and installed.
Once completed, the fuel system should be pressurized and carefully checked for leaks. Remember, it’s much higher pressure now. Leave the mixture in the idle cut-off position so you don’t flood the engine while doing this.
A quick test I do is to watch the fuel pressure while turning off the electric fuel pump. The pressure should hold steady for quite a while. Moving the mixture forward will immediately show a decrease to zero in the fuel pressure.
Now it’s time to run the engine, adjust the idle rpm and idle fuel mixture, and recheck for any leaks. If all is good, it’s time to go fly and enjoy the increased fun factor.
Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA mechanic, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 11,000 hours in 75 different types. Vic founded Base Leg Aviation, has authored books on maintenance and prebuy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.