Mason Delacroix
The process of converting a gas carb to alcohol involves several modifications to the carburetor and fuel system. The main changes include increasing the size of the main jet, adjusting the idle jet, and modifying the ignition timing. Additionally, the accelerator pump orifice may need enlargement to ensure the engine runs properly. This is because alcohol requires a richer air-fuel mixture, and engines running on alcohol typically consume a higher volume of fuel. To achieve this, the fuel lines and jets must be enlarged to accommodate the increased flow. It is also important to consider the compatibility of materials used in the carburetor, as some rubbers do not handle alcohol well and can harden over time. Corrosion may also occur if the carburetor material is not treated regularly. Overall, while it is possible to convert a gas carb to alcohol, it may be more economical to opt for fuel injection or modern ignition engines for improved performance and ease of use.
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What You'll Learn
Increasing fuel flow
If your engine has a carburetor, you will need to increase the size of the metering jets to allow for more fuel flow. You can do this by boring out the main jet orifice by 20-40%. This will make the air/fuel mixture richer. You should also increase the size of the angle passages that feed from the main well to the booster supply passage. You can do this by hand, using a drill bit mounted on a pair of locking pliers. Be careful not to drill past the outer wall of the main well passage.
You will also need to modify or replace the carburetor's float, to allow the float bowl to retain a larger quantity of fuel. You should also install a high-energy ignition system, as alcohol does not vaporize as well as gasoline, especially in cold weather.
If your engine has a fuel pump, you will need to upgrade it to provide all the extra fuel required. For example, a belt-driven pump will offer much higher flow capabilities than a pushrod-driven pump. You will also need to ensure that all fuel lines are large enough to allow the increased flow, otherwise, you will starve the engine.
Finally, you may need to adjust the accelerator pump. This is a mechanically activated plunger or diaphragm that injects a stream of gas directly into the throat of the carburetor when the accelerator is suddenly depressed. You can either enlarge the orifice by about 20% or adjust the stroke length of the pump arm to pump in more fuel.
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Accelerator pump changes
When converting to alcohol, it is essential to consider the increased fuel demand of alcohol engines. Alcohol requires a richer air-to-fuel ratio, which means boring out the main jet orifice to increase its diameter by 20 to 40 percent. This allows for a higher volume of fuel to be delivered to the engine.
To accommodate the higher fuel volume, the accelerator pump's capacity and delivery volume may need adjustments. The pump cam and nozzle (or squirter) size can be modified to ensure the engine receives the necessary fuel during rapid throttle openings. The accelerator pump's timing and synchronization with the throttle opening are also critical factors to optimize performance.
Additionally, the accelerator pump circuit should be tuned to ensure efficient fuel delivery. The circuit uses air speed through the boosters to create low pressure, drawing fuel from the float bowl into the engine. Adjustments to the pump linkage and clearance, as recommended by Holley, can help prevent hesitation and ensure proper fuel injection timing.
It is worth noting that the accelerator pump changes may vary depending on the specific carburetor model and engine requirements. Holley carburetors, for example, have specific recommendations and components, such as the accelerator pump squirter and pump cam, that can be tuned for optimal performance.
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Main jet changes
The main metering jet in your carburettor is the first thing to tackle. In most carburettors, this will be a threaded brass plug with a hole drilled through the centre. This hole is called the main jet orifice and its diameter directly affects how rich or lean the air/fuel mixture will be when the engine is running at normal speeds. The smaller the hole, the leaner the mixture; the bigger the hole, the richer the mixture.
Since alcohol requires a richer air-to-fuel ratio, it's necessary to bore out the main jet orifice when using ethanol fuel. The increase will be on the order of 20 to 40 percent. To do the conversion, you will need a screwdriver, wrenches, vise-grip pliers, a putty knife, a pair of needle-nose pliers, and an electric hand drill with bits.
First, remove the carburettor's air filter housing and all its hoses, tubes, and paraphernalia from the engine. When the carburettor is free from the engine, turn the unit upside down to drain out any gasoline that may be in the float bowl. Remove the carburettor air horn and locate the main jet. The jet will usually be in the main well support but it could also be right in front of the float bowl body. Once you've removed the main jet, size up its jet diameter using a drill bit (the largest bit that fits).
Do not attempt to drill the jets without removing them from the carburettor. The shavings will fall into tiny passages and create problems. A drill press is best for this job, but a hand drill can be used if caution is exercised. You may be able to avoid drilling. Auto parts dealers stock jets in various sizes for many types of carburettors. They are inexpensive and if you can find one in the proper size, it will save time and labour.
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Idle jet and timing changes
Idle jet changes are necessary because an alcohol system needs more fuel than gasoline. This means that the idle circuit must be enlarged to allow enough fuel for the engine to idle. The idle jet size may need to be changed, and the idle mixture screw at the base of the carburetor may need to be loosened to allow more of the air/fuel mixture to pass through. The seat into which the tapered screw extends may also need to be enlarged by up to 50%.
Timing changes are also important when converting to ethyl alcohol. To take advantage of the antiknock qualities of alcohol fuel, the ignition timing must be advanced by turning the distributor housing in the opposite direction. Gasoline engines tend to perform best with the timing set at 22 degrees, whereas alcohol engines perform best at 21 degrees of advance timing.
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Dual-fuel system
A dual-fuel system allows a vehicle to run on either gasoline or alcohol. Flexible-fuel vehicles (FFVs) are based on dual-fuel systems that supply both fuels into the combustion chamber at the same time in various calibrated proportions. The use of dual-fuel systems in vehicles was a response to the oil crisis and the goal to improve air quality.
To convert a gas carb to alcohol, you will need to make three changes: the main jet, the idle jet, and the timing. The main metering jet in your carburetor is a threaded brass plug with a hole drilled through the centre, known as the main jet orifice. As alcohol requires a richer air-to-fuel ratio, it is necessary to bore out the main jet orifice when using ethanol fuel. The increase will be between 20 and 40%.
To convert your gas carb to alcohol, you will need to remove the carburettor from the engine. Turn the unit upside down to drain out any gasoline that may be in the float bowl. Remove the carburettor air horn and locate the main jet. The jet will usually be in the main well support but could also be right in front of the float bowl body. Once you've removed the main jet, size up its jet diameter using a drill bit (the largest bit that fits). You may be able to avoid drilling by purchasing jets in various sizes from auto parts dealers.
Additionally, you will need to ensure that all fuel lines are large enough to allow the increased flow of alcohol fuel. Alcohol engines use a significantly higher volume of fuel than gas engines. For example, a 280cc Herbranson engine using alcohol gets through a gallon of fuel in about 10 minutes.
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Frequently asked questions
The basic steps are to disassemble the carburetor, drain out any gasoline, locate the main jet, and increase the size of the jet orifice by 20 to 40 percent. You will also need to adjust the accelerator pump to increase fuel flow.
You will need a screwdriver, wrenches, vise-grip pliers, a putty knife, a pair of needle-nose pliers, and an electric hand drill with bits.
It is important to remember that an alcohol system requires more fuel than gasoline, so you may need to upgrade your fuel system. You can also modify the choke to increase airflow and polish the venturies for better performance.
Be careful when drilling the jets as the shavings can fall into tiny passages and create problems. Also, remember that once the metering blocks have been converted to alcohol, they cannot be used for gasoline anymore.
