Rust never sleeps, and gasoline doesn’t keep. Do any of your motorcycles–or car, lawnmower, boat, etc.–sit for more than a couple weeks between uses? The best advice I can offer is…don’t let them. Running your vehicles every week–or two maximum–until fully warm is the best way to prevent fuel delivery problems with contemporary gasoline. When you can’t run them, here’s what I do to minimize problems with my small collection of bikes, and my generator, string trimmer and lawnmower, even spare fuel in cans.
Half Full, Half Empty
On carbureted bikes with steel gas tanks, half the fuel system should be drained, and the other half kept full. Carburetors and their tiny air passages and jets can become plugged with aged fuel that deteriorates into sticky varnish over time. Since carb internals are made of non-ferrous aluminum, brass, plastic and rubber that won’t rust, if it’s practical to drain them (shut off the gas manually first or look for a vacuum-operated-type petcock that is off whenever the bike is), this is your best bet for trouble-free operation when refilled. Most carburetors have drains in the bottom of their float bowls operated by turning a screw. Don’t overtighten that screw, and only drain carburetors (into something please, not just onto the bike and floor) when the bike is off and cold, then run the bike until it dies to suck the rest out. I once bought a Honda multi that had been stored in a basement for 15 years with the carbs drained and stabilized fuel kept in the tank, and it was rust-free and fired right up without carb service. If you’re clean and careful, there’s no reason you can’t return the drained fuel to the tank.
On the other hand, steel tanks on carbureted or fuel-injected bikes can rust inside, so it’s best to leave them at least 3/4 full of fuel to which you have added fuel stabilizer (more on this later). Some newer models have plastic-shrouded aluminum or plastic tanks, in which case it’s up to you, but make sure you stabilize it if you leave fuel in the tank. In really humid environments I would still keep an aluminum tank full.
Fuel injection systems seem much less susceptible to the ravages of stale fuel, and once full of stabilized fuel are almost care-free. In fact, some manufacturers warn against running their EFI bikes entirely out of fuel.
If you can’t drain carbs, after adding stabilizer to the fuel in the tank run the bike long enough to insure stabilized fuel has filled them, then shut off the bike and petcock. I carry a small bottle of stabilizer with me when I take out one of my less frequently ridden bikes, and add it at the gas station before riding home. Err on the side of adding more stabilizer; you can’t overdose (within reason) with the products I mention below. Stabilized fuel in the carbs does not guarantee that they won’t suffer from plugged passages or jets, however, and you should still run bikes kept this way at least every three weeks. More often is simple insurance that you won’t need an expensive carburetor overhaul–just make sure you run the engine until it’s fully warm (to burn off water and contaminants in the oil and exhaust). While you’re at it, pump the fork and shocks and work the brakes, clutch and shifter to keep seals flexible and lubricated.
A Stable Relationship
Your best friend and ally in the fight against bad gas and fuel delivery issues is fuel stabilizer. Three we’ve found to provide consistent and reliable results with motorcycles are Seafoam Fuel Additive, Spectro FC Premium Fuel Conditioner & Stabilizer, and Star Tron Star brite Enzyme Fuel Treatment. There are others, but we lean toward these simply because they include motorcycles in their literature and FAQs and that gives us a warm, fuzzy feeling. All make lots of claims about their effectiveness that we have no way of proving or disproving, so just buy some and use it, or spend hours online researching them before you just buy some and use it. Spectro and Star Tron both offer smaller bottles and/or with measuring devices built-in to make carrying and using it while out on the bike easier.
The instructions for each will tell you how much to use, how long the fuel is usable when treated, etc. There are some consistent rules of thumb. You generally only need to stabilize fuel if you won’t use it up within two months (but carbureted bikes should still be run every couple of weeks as described above). Adding a little new gas or stabilizer to old gas won’t renew it, nor will adding more stabilizer to old stabilized gas extend its usable life. Overdosing is not an issue (unless you drink it, duh), and in my experience none of them will cure a plugged-up carb no matter how much you add to the fuel. Your best bet is to avoid plugging it in the first place.