If you're like most home owners who mow their own lawns, you probably find that the first time you go to start your lawn mower in the spring, it is a lot harder to start than it is for the rest of the lawn mowing season. Maybe you get frustrated with it, forget that you aren't a teenager any more, and throw your back out giving the pull-start a furious yank. Maybe you then think to yourself “I wish I had one of those electric start lawn mowers”. If you have an electric-start lawn mower, maybe you are thinking “man, this thing has electric start and it still sounds dead. Thank god I don't have one of those manual start models.”
By the time you have been trying to start your lawn mower for a few minutes, you have probably tried some with the choke on, and then maybe your “helpful” neighbor came by and told you that you probably flooded the engine. So maybe you try starting it a few times with the throttle opened up a bit and the choke off. If you have a manual-start machine, you may be contemplating your impending chiropractic bill, and wondering whether hiring the neighbor's kid to mow your lawn might not be such a bad idea after all.
So why can it be so hard to start an engine that has been sitting idle for six months? If you examined the spark plug under a microscope, you might figure out the problem. If you look at the tip of the spark plug with your naked eye, it will probably look fine, but if you replace that plug with a brand new clean dry sparkplug, you would probably find that your lawn mower will likely start in seconds.
The key to the mystery lies in something at the microscopic level the happens on the surface of the ceramic insulator of a spark plug over time in a machine that is stored outdoors. As the temperature and the relative humidity cycles, day after day, over time, micro-droplets condense on and re-evaporate from the surface of the ceramic insulator of your spark plug. Each time these micro-droplets form, they rearrange the tiny carbon particles that were deposited on your spark plug the last time you ran your engine. The growing of the droplets as they form pushes those particles together into conductive pathways that wind up providing an alternate path for electrical current (across the surface of the ceramic insulator instead of across the spark gap) when you try to start your lawn mower.
The carbon particles don't quite short things out, but they provide a path whose electrical resistance is low enough so that there is no spark, or the spark has so little energy that it won't ignite the charge in the cylinder. Once the engine starts, the heat of the burns within the calendar clears up this problem in a few minutes, so when you go to start the engine again the next week, it starts fine.
One easy solution for this initial-start problem is to replace the spark plug. Of course, if you want a cheaper solution that doesn't require a trip to the store, I have one for you, and it comes in the form of a hot flame. A propane torch works best, but if you don't happen to have one handy, a butane lighter or a gas stove burner will do. You see, there is a reason that the ceramic insulator surrounding the center electrode of your spark plug is made of the particular material it is made of. The surface properties of that material actually catalyze the burning off of carbon deposits when the ceramic gets hot enough. Of course, when your lawn mower has not yet started, nothing has yet gotten it “hot enough”.
To solve this problem, remove the spark plug from your lawn mower, and get a hot clean flame ready (either a propane torch or a butane lighter, or gas burner on a stove). Holding on to the end of the spark plug that usually connects to the spark plug wire, stick the other end (the spark end) of the spark plug into the flame for a few seconds while rotating the spark plug a few times, heating the center electrode and the surrounding ceramic. It only takes a few seconds. After you take the spark plug out of the flame, take a look at the ceramic material surrounding the center electrode.
You should notice that the ceramic insulator surrounding the center electrode of your spark plug is now bright white, where before it might have been off-white or gray. Now your spark plug is as good as new. Put it back in your lawn mower, and enjoy how it roars quickly to life!