Coping with Ethanol-Containing Gas
1. The Problem: I remember when unleaded gas replaced our leaded fuel in the 1970's. Everybody said that it would burn up the valves in our vintage bikes. Many solutions were proposed but the one that finally worked was to simply ignore it. I have yet to see a burnt valve that was the result of unleaded gas. Now we are faced with a new threat, ethanol-containing gas. This is a much more real threat, however. Although gas containing up to 15% ethanol seems to burn quite nicely in our period machined it does present problems. Ethanol, unlike gasoline, is hygroscopic. That means that it loves water. Sometimes that is good. I used to add a pint of ethanol to my car's gas tank once a year to purge any water that may have condensed in it. The water would be dissolved by the ethanol and is simply burnt. But today ethanol's love affair with water soon becomes too much of a good thing. Vintage bikes are not usually used on a regular basis and often must sit patiently for weeks or even months between rides. Any moisture in the air above the fuel will be absorbed by the ethanol and eventually when the ethanol becomes saturated it will form a green sticky varnish that plugs up carburetors, promotes rust in the tank, and kills the combustibility of the fuel.
You must have noticed that this never seems to bother modern cars and trucks. That is because they have sealed gas tanks. There is a one-way vent in the tank that allows air to come in to displace the burnt fuel but it will not let air go out. It is the sealed tank that causes the gasp of air that escapes from your gas tank when you take its lid off on a hot day. The result is that very little moisture is presented to the fuel. Older motorcycles generally have a vented gas cap that allows air to come in to replace the lost fuel. The problem is that a simple vent also allows air to leave that tank. Thus the tank can breathe. A bike stored in a shed or unheated garage will cool down at night drawing moist air in and then it will warm up during the day expelling air. Day after day more and more moisture is taken up by the ethanol. The obvious way to solve this problem is to seal the tank when the bike will be parked for any extended period of time. More on how to do that below.
2. Get 100% Gasoline when You Can: Some filling stations have ethanol-free gas, usually sold as "marine gas". Always use that if you can get it. However, even 100 gasoline deteriorates fairly quickly in a breathing tank. Not as fast as gasahol but fast enough to still pose a problem. When gas goes "sour" the bike will not start. Sour gas has a turpentine smell to it and is easily identified. When that happens the only solution is to drain the tank, find someplace that will dispose of your spent fuel and refill it with fresh. The varnish problem is not as severe as with gasahol but if you left you petcock open the carb may get varnished even with 100% gasoline.
3. Store the Bike with Dry Carbs: The carburetor is just a mini fuel tank. The float bow contains an ounce or two of gasoline and it also has a simple vent. That fuel can also take up moisture and any varnish formed there will gum up the carb requiring a time-consuming (or expensive) carburetor cleaning. This problem is fairly easy to solve. Almost all carbs have drain plugs allowing you to drain the float bowls to remove any water that may have contaminated the fuel. If the bike will not be ridden for several weeks simply shut off the fuel petcock and drain the carbs. An even simpler approach is turn off the petcock and run the bike until it quits (usually 1 to 3 minutes). One note here is that some petcocks do not turn off 100% but rather weep fuel. In that case leave the drains open with a cup under them or pull the fuel line off the petcock so any leaking fuel will not go into the carb. Leaking fuel can be a fire hazard so if more than a drop or two per day is coming out fix the petcock or put a plug in the fuel line.
4. Store the bike with a Sealed Gas tank: I found the simplest way to seal the tank is to make a sealed gas cap. Obviously there is an infinite ways to attack this problem. I tried putting a small sheet metal screw in the vent hole but most gas caps are not that air tight. I therefore designed a simple storage cap that can be put on and taken off quickly. The left figure below shows the cap on my Triumph Bonneville. The middle figure shows the parts required to make the cap.
My cap is made of a disk of aluminum that was cut out of 3/16" sheet with a hole saw. The center hole is 1/4 inch. Any gasoline resistant material could be used. I tried Plexiglas but the gasoline attacked it. Nylon or polyethylene would work. You will need a gasket. I cut mine out of rubberized cork sheet I bought at the auto store. Be sure it is rubberized as plain cork sheet will not be air tight. Neoprene sheet would be even better. I first glued the gasket to the disk with Gasgacinch but gas fumes attacked it and it eventually came loose. The cap will still seal if the gasket is loose but it is easier to fasten if it is adherent to the cap. JB weld smeared between the gasket and the cap works best but must harden overnight with a weight on it to keep the gasket flat while it cures. Gas fumes will also attack silicone RTV even if allowed to fully dry.
The cross bar was 1/2" square aluminum stock who's length was equal to the diameter of the disk (2.5"). A 3/16" hole was drilled in the center of the bar and it was threaded with a 1/4-20 tap. The bolt is a 3" x 1/4" coarse thread stove bolt with two nuts. Finally you will need a 1/4" neoprene rubber washer. I obtained mine from the hardware store. Assemble the parts as shown in the right panel. The two nuts were tightened together with a drop of locktight to ensure they do not come off. The jam nuts keeps you from accidentally dropping the cross bar into the tank when you loosen the screw to remove it. To put the cap on move the crossbar to the bottom of the screw and insert the cross bar onto the tank one end at a time. Then gently pull the bar up against the top of the tank and tighten the screw until it draws the disk down firmly onto the fill spout. Do not over tighten. Just finger tight should be enough to make a good seal. Even gasahol will stay fresh for months in a sealed tank. Reverse the process when you want to remove it and put the stock cap back on for a ride. Not all tanks have the same size cap. BMWs and other European bikes require a 3.3" disk. Be sure to measure the fill hole on your bike before cutting the parts.