Survival Manual

How To Convert A Gasoline Engine Into Ethyl Alcohol Engine

Carburetor Conversion

There are three changes that need to be made to convert a carburetor engine over to an Ethyl Alcohol engine. These three changes are main jet, idle jet, and timing.

MAIN JET CHANGES The main metering jet in your carburetor is the first we will tackle. In most all carburetors this will be a threaded brass plug with a hole drilled through the center of it. This hole is called the main jet orifice! This holes diameter directly affects how rich or lean the air/fuel mixture will be when the engine is running at normal speeds. Smaller the hole less fuel/leaner... bigger hole more fuel/richer.

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 obviously.

Keep in mind that removing a carburetor involves dealing with gaskets. Unless you planned ahead and have spare gaskets or have some brand of gasket in a tube for high temperature uses then you are possibly going to end up with an engine that doesn't run.. on alcohol OR gasoline.

In order to take the carburetor apart, you'll first have to remove its air filter housing and all its hoses, tubes, and paraphernalia from the engine. If you need instructions on this then you probably shouldn't be attempting this anyway. This article assumes you have the knowledge and engine experience to remove a carburetor.

When the carburetor is free from the engine, turn the unit upside down to drain out any gasoline that may be in the float bowl. Remove the carburetor 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 (largest bit that fits). Now if the original hole/jet is .056" you will want to increase it by 40%. Which comes out to about .078". Remember we are wanting a 40% increase to the holes diameter, not area. So the formula is simple and doesn't involve using pi (3.14). Just multiply .056 * 1.4 to get .078!

Hold the jet with your vise-grips or in a vice if you have one and bore out its central hole. Clean any filings away before using.

If the carburetor uses a metering rod instead of a jet. Then you will have to take a lot of time and try to sand the rod down to a smaller size. Smaller rod... more fuel.

NOTE: The hole does not have to be exactly 40% larger. That is a great starting point to shoot for. Too small a hole could run the engine too lean and ruin the engine where as if the jet is to large you will waste fuel. But you can't easily go smaller so be careful and take your time and do things right the first time.


Fuel in most carburetors will only pass fuel through the main jet when the throttle is used to rev the engine past idle. When idling the idle orifice is the only route of fuel to the engine and therefore the fuel flow will need to be increased at idle as well.

Luckily on many carbureted engines it is a simple matter to adjust a screw with a flathead screw drive to adjust the amount of fuel allowed through at idle. Yippee!

But all is not quite as rosy as it might seem. Since you will be backing the screw out further than it was designed you might need to take measures to be sure the screw does not vibrate out when the engine is running. Boring out the seat might alleviate this issue. You can go the alternate route of shimming the idle mixture screw spring with small lock washers or any other method that prevents the screw from turning on its own.

With an motorcycle or ATV you might be finished. But with an old car or truck there is something known as a power valve that allows fuel to blend with the air when the engine is revved. This vacuum controlled valve is spring loaded, and shuts off when it isn't needed in order to conserve fuel.

For most conversions changing fuel flow is important only from an efficiency stand point.

Virtually all automotive carburetors utilize an 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. The fuel is injected through a small orifice located in the throat wall before carburetors throat narrows.

You might get a car to run without messing with this but you will want to to get the car to run properly. So you need to enlarge the orifice about 20%.

If you are lucky you can forgo this and instead adjust the stroke length of the pump arm in order to pump in more fuel. Consult the manual to see about this option. In lieu of that enlarge the orifice.

That is it. You are finished! You should now have an engine that runs passably on alcohol. There are other adjustments that can help but aren't essential. They are listed as follows:

But before any of these extra measures are taken be sure you have already proven that the engine will run on alcohol. These are fine tuning adjustments.

Installing a manual chock is better than an automatic choke designed for gas. Conversion kits are available for cheap.

IGNITION TIMING In order to take advantage of the great antiknock qualities that alcohol fuel provides, you'll have to advance the engine's ignition timing by turning the distributor housing in the opposite direction that the rotor spins. Moving about 20 degrees is a ball park figure to work with.

Increasing the compression ratio will not be covered here. If you already know how to do this fine. If not then you shouldn't be messing with this when your life may depend on the engine running.

The ratio can be increased to 14 or 15 to 1. Gas is usually at about roughly 8 to one. It isn't imperative to go all the way to 14 or 15 any increase will give you some of the benefit.

Be Aware
Alcohol engines are harder to start in cold weather.

Alcohol will act as a cleansing agent and a clogged fuel filter is something that might get clogged up soon after the conversion. Keep this in mind for troubleshooting your engine after it stops working but was working previously.

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