Cold-Starting And The Nature Of Gasoline

“I’ve seen some fellows kick a machine 50 or 100 times…”

Ducati’s motor-driven dual-length intake system
Turn key, push button, easy peasy: Ducati’s motor-driven dual-length intake system, as seen on the Euro 4-compliant Panigale V4. When the four bellmouth extensions are raised, intake length becomes 25mm shorter.Ducati

Motorcyclists of today may never have encountered a problem in cold-starting, and that’s progress. Digital fuel injection thinks of everything, so all we have to do is turn the key and the engine is running. Let the great escape begin.

It wasn’t always so. An ignition spark can ignite mixtures of gasoline vapor and air between the ratios of 10:1 and about 18:1, but that won’t help us start a cold engine. Notice I said gasoline vapor. That means the gasoline has evaporated completely, assuming the gaseous state. If your engine’s cylinder(s) receive a mixture outside that range, you can crank it until the battery’s dead or kick until your leg cramps and it won’t start.

Now a basic fact about gasoline: It’s not a pure substance, all of whose molecules are identical. It is a mixture of 100 or more hydrocarbon molecular species. Some of them are light, with only five carbon atoms, and evaporate easily at room temperature. Others are heavier, with more carbon atoms, and so need more energy to leave the liquid state—to evaporate. So when we try to start a cold engine, the percentage of the fuel that actually evaporates is determined by the temperature.

Ignition sparks can’t ignite droplets of liquid fuel; they can only ignite mixtures of air and fuel vapor in the range stated previously. So what do you do if ambient temperature is only evaporating a quarter of the fuel? You enrich the mixture until the percentage of the fuel that is evaporating creates a fuel vapor and air mixture that is spark-ignitable.

That is the job of:

  1. Automatic cold-start enrichment, in the case of digital fuel injection.
  2. Use of a very rich miniature starting carburetor, which was part of the main casting on many Japanese motorcycle carbs.
  3. Use of a “choke,” which often takes the form of something that restricts (chokes) the flow of air through the carb, creating an extra-strong vacuum that pulls a lot more fuel into the airflow.
  4. In the most primitive case, by priming (raising the fuel level in the carburetor’s float bowl until liquid fuel puddles in the carburetor’s air passage). On bikes of this type, there was a button by which you could press the fuel float down, opening the float valve and allowing fuel level to rise. The presence of this extra fuel in the carburetor might do the trick. This was called “tickling the carburetor.”

The principle is that we must send the engine enough extra fuel—sometimes a mixture as rich as 1:1!—so that the portion of it that does evaporate at the prevailing temperature is enough to form an ignitable mixture. The engine can then fire and start.

When the engine does start, you will smell from its exhaust that part of the fuel not presently evaporating, which is passing through the engine unburned. This is why the EPA wants exhaust catalysts to “light off” quickly so they can burn up this extra fuel during engine warm-up.

Combustion quickly warms the engine, and this heat causes more of the fuel to evaporate, making the mixture richer, possibly enough to make it stall. Engines with digital fuel injection manage this phase wonderfully well, gradually leaning out the mixture as more and more of the fuel supplied is evaporated by engine heat on its way to the cylinders.

If your bike has a manual choke operated by a lever, you gradually close the choke or starting carb to lean the mixture as the engine warms. Cars used to have a choke pull-knob on the dash; the operator pulled it out all the way on coldest days, less far out on not-so-cold days. It was a matter of experience, which every motorist had. As the engine warmed, you’d push the knob in a bit every few seconds. It was second nature in those days.

Many a motorcyclist has been unable to start his or her machine for the first time as warmer weather arrives. Why is this? Because on many older bikes, the most volatile fraction of the fuel—some 10–15 percent of it, in fact—has over time evaporated out through the gas tank breather and has floated away into the atmosphere as unburned hydrocarbons (UHC). What remains in the tank and fuel system lacks the volatility to form an ignitable mixture even with use of the choke. All you can do at this point is drain out the old fuel and replace it with fresh.

This is the basis of a minor industry in upscale neighborhoods all over this nation. Damned lawn mower won’t start, even with a fresh spark plug? Hoist it into the back of the SUV and get even by taking it to the dump. Then buy a new one with what for our subject is small change; he or she made partner four years ago and is now on generous billable hours. The attendant at the dump carefully sets those perfectly good mowers aside, knowing that most of them will start normally when the old gasoline in their tanks is replaced by fresh. He has a lineup of such low-hours mowers, freshly fueled and ready to go at attractive prices.

Fuel stabilizer does not magically prevent the evaporation of your fuel’s “front end,” the most volatile fraction of the gasoline.

Yes, it’s also true that if you leave fuel in the carburetors of your treasured older bike and that fuel evaporates completely over winter, it may leave behind the dreaded “existent gum” that typically blocks the idle system, having the smallest jet, so your engine can’t start. And it won’t start until you, or your authorized representative, removes the carb(s), disassembles, cleans, reassembles, and reinstalls them.

Or maybe, having poured in a can of fuel “stabilizer,” you are expecting a better result? Sorry, fuel stabilizer does not magically prevent the evaporation of your fuel’s “front end,” the most volatile fraction of the gasoline. So there you may very well be, kicking or cranking without result.

Therefore, it’s good to bear in mind that there is such a thing as “old gasoline,” which has lost its most volatile part to evaporation. It’s terrible at cold-starting, but burns normally in a warmed-up engine.