Harley-Davidson: EFI to Carburetor Conversion Step by Step

In this article, we're going to make the dream of the most loyal Old School fans come true: we'll remove the fuel injection (EFI) system from a Twin Cam Harley-Davidson and replace it with a carburetor, so everything will be "as it was before."
First of all, I want to make it clear that the modification was carried out purely for experimental and aesthetic purposes, guided by nostalgia, since there was no real need to justify the change in this case: the injection worked perfectly and the engine ran without problems, although it can serve as an example to those who wish to carry out the replacement to eliminate malfunctions or defects in the system, such as in Magneti Marelli injections with high mileage and erratic operation.

Special engine-Special carburetor
To start, let's review the list of 3 elements we need to completely eliminate our EFI injection system:
- Carburetor with its intake.
- Fuel tap with adapter.
- Compatible ignition module .
We start with the carburetor: as in our case the engine is a 100ci Screamin' Eagle model with high compression and quite a bit of "bad temper" we are going to use an S&S Super "G" carburetor modified by the American specialists Triple X and chrome plated down to the last corner.

The main modifications consist of reworking the interior of the venturi, installing a circular diffuser or "booster," and adding two Thunderjet circuits. These circuits come into play under high demand and have an effect similar to an extra acceleration pump, catapulting us to infinity.

We selected the fuel tap and adapter from the Pingel catalog, products known for their high performance and reliability.

The ignition must be compatible with the original crankshaft rotation sensor ; this sensor will be the only one reused from the injection system.

This magnetic sensor will accurately indicate the position of the pistons, ensuring that the spark plugs deliver the spark at the precise moment. We used the P3 system from Altmann Motorcycle Ignitions , which is highly reliable, easy to install, and compatible with the original coil and the HD safety system.

Dismantle!
Now that we have all the components, let's move on to the fun part: we're going to remove sensors, intake body cables, the ECU, and any other component of the EFI system (except the crankshaft position sensor, as mentioned above).
After removing the injection pump fuse (it is important to consult the workshop manual throughout the process), we will attempt to start the engine to relieve pressure on the injection pump.

Next, we'll disconnect the negative battery terminal and remove the fuel tank. After that, we can uninstall the EFI induction module.

Next we will open the hatch at the top of the tank,

to be able to remove the injection pump, releasing the high-pressure hose, which will allow us to remove the quick connection from the fuel pipe.

We take a break
Congratulations, we're almost halfway there! Before installing the new components, we'll focus on removing all the sensors and cables from the system we'll no longer be using. To do this, we can take a look at the wiring diagrams at the end of the workshop manual. This will give us a much simplified and practical electrical system, while also improving the aesthetics.
It's time to take a break to review everything we've done and prepare for the next step: assembly.

Installing the carburetor

Let's continue to complete the downgrade or conversion from an EFI fuel injection system to a carburetor system. In the first part, we listed the new components and disassembled the components to be removed. All that remains is to remove the exhaust O2 sensors and replace them with a bung. Now we begin the reassembly.

The fuel tank
To resume the project, we'll start with the easiest part: after closing the top hatch of the gas tank again, we'll install the fuel tap adapter and the fuel tap, using a little Teflon or liquid sealant to prevent fuel leaks.

The admission
We start with the intake manifold, following the instructions in the workshop manual and the component manufacturer's instructions as always. We'll install it loose. We'll place the carburetor in its final position using its support and tighten the intake manifold bolts, marking their position to verify that they haven't moved. Now we can remove the carburetor and retighten it to the specified values, always making sure that the intake doesn't move from its position. This way, it will remain completely aligned with the carburetor and prevent air leaks.

Next, we installed the carburetor, bracket, and air filter. In this case, we chose a Performance Machine component to enhance the aggressive aesthetic while increasing performance.

In the following photo we can see the location of the 2 Thunderjet diffusers and their intakes in the carburetor bowl.

The ignition
Installing the ignition is very simple if you follow the instructions. You just need to connect the positive and negative wires, the coil, and the crankshaft position sensor.

We will choose the location of the module in a place sheltered from the elements and rain (for example under the seat).
It's important not to rush and to make connections and wiring arrangements as best as possible. Remember that 90% of electrical failures in motors are due to faulty connections.

After this we can select the positions of the 3 dials of the ignition module, where we will choose the advance curve (green) and its type (yellow) and the RPM limit (red).

Final tests
Once the carburetor, ignition, and electrical system are in place, we will proceed to remount the fuel tank on the motorcycle, connecting our new fuel tap to the carburetor with a suitable fuel line.
We reconnect the negative battery terminal and we can now start the engine. We'll check that everything is working correctly and proceed to adjust it.
Tech Tip: For the S&S and CV carburetors (original HD), the adjustment is very simple: warm up the engine slightly and tighten the idle screw to about 1000 RPM. Then, slowly close the mixture screw until the engine begins to sputter, and memorize this position. Open the mixture screw again until the engine begins to sputter again, and memorize the position. The ideal setting is roughly between the two positions. Do this and reset the idle to about 850 RPM. Voilà!
We can now put on our helmets and hit the road to experience the new way our beloved Twin Cam breathes (Figure 10). We will find that although the power is similar, the way it is delivered is more “raw” and the bike no longer asks permission to accelerate, it just does it!

Frank Burguera

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