There are four things that affect starting lawnmowers:

1. The fuel should be "fresh"
2. The carburetor should be clean (part of the "fresh fuel" issue);
3. The ignition system should be functional.
4. The fuel should be volatile;

Overview

Ignoring for the moment mechanical problems and the carburetor gumming up from stale fuel, the condition of the fuel is a major issue with starting lawnmowers. This relates to "freshness" and Reid vapor pressure.

Fresh Fuel

Fuel is formulated to have sufficient stability to not form gum deposits while stored at a gas station, and in the car's tank. Since most cars are filled at least once a month, the fuel can be presumed good for 3 months. (Aviation fuel will last in excess of 5 years, but contains lead.)

Fuel stabilizing compounds are available for overwinter storage, but an alternative is to pour the old fuel into your car's fuel tank. The additives in the reminder of the fuel will be adequate to prevent problems, and you can then get fresh fuel for the lawnmower.

Volatility or Reid Vapor Pressure

Volatility is the ability of the fuel to evaporate. It is measured as Reid Vapor Pressure (RVP). Air quality considerations dictated that gasoline have limited volatility, typically between RVP 7 and RVP 9. This works well in a car, which has the ability to run with much lower RVP numbers. In the case of a lawnmower, the fuel must be volatile enough to start.

RVP decreases as fuel is allowed to sit in the tank. So while fuel with RVP of 7 would be sufficient to start the lawnmower, after a month or more, the RVP decreases.

Increasing RVP

I was looking for a way to increase RVP in lawnmower gas. One problem is I don't have a gasoline car, so I had no readily handy place to dispose of gasoline. I was reluctant to give away the fuel because I didn't want to know about every drivability problem that could happen to show up in someone else's car. So I needed to do something to the fuel.

It turns out that one of the components that petroleum refiners extract to reduce RVP is butane. That's the stuff that comes in lighters.

So I went to a drugstore to get a bottle of butane. They were out, but had lighters for $1.00. "Okay, I'll buy one."

Now cigarette lighters aren't the type of thing that one wants near a fuel tank. They make loud noises, and some more unpleasant effects. Fortunately, cheap lighters use a wheel and a flint; otherwise they don't ignite. It's easy enough to press the valve without striking the flint, but I figured it's almost as easy to just remove the flint striker wheel. Even if there's a 1/1000 chance of my striking the wheel, that's too risky. I pried off the wheel and the flint went flying across the room.

So now I had a lighter that doesn't work. When dealing with a tank of gasoline, that's a good thing.

Then there was the matter of getting the butane into the fuel, which was the whole idea.

I found an electrical cord which I had already destroyed when I wanted to obtain a section of wire for my car. I cut out another section of wire, and carefully removed the plastic sheath protecting the copper strands. This was of course a flexible tube, which fit over the flame tube of the lighter. Now the tube could be inserted into the fuel, the lighter's valve depressed, and butane injected into the gasoline.

Since this particular lawnmower had no primer, I splashed a little bit of MEK (a solvent) onto the air cleaner. I don't recommend this because I can't guarantee this won't start a fire, but it worked for me. Meanwhile the fuel in the tank has enough butane in it to be sold as pump gas in the 1950's.

Cautions

Lawnmowers are a major source of income for hospital emergency rooms, both from the mower and flying objects. Be careful, since you don't want blood all over the mechanism. Do not start a mower with spilled fuel on the deck, and don't fuel a running mower.

Everything Else (called "Maintenance")

There are of course the other things, such as clean spark plugs, clean carburetor, and working ignition. I'll leave that stuff for others to write about.

Next Lawnmower

Next time, buy an electric lawnmower. They pollute a lot less, even if one considers the power and batteries used on the rechargables. And I heard they're easier to start.

Cool Lawnmowers

Flymo

- These don't use wheels.

Old Lawnboys

- Some of these have two cycle engines, which run at about twice the speed of ordinary 4 stroke engines. Many of these have magnesium cases. If you want to wake up the entire neighbourhood, install a tuned exhaust and wear heavy-duty hearing protectors!

Reel-type push mowers

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