Even a careful reading of Chapter 1 might lead you to believe that the ignition and starting systems of typical small gas engine appliances require very little repair. After all, the only moving parts in the ignition system are the sturdy-looking flywheel with its permanent magnets and the breaker points; and the only replaceable parts are points, condenser and spark plug. The typical starter is a spring type with a pull cord.
Appearances can be deceiving, for the overwhelming amount of the service work on a small gas engine is devoted to the ignition system. See 6-l. Although the starter is not normally troublesome, if it does fail, there
are some intricacies about its service you should know.
Ignition system service begins with the spark plug and its wire. There are ohmmeter tests for spark plug wire, but here are the practical approaches for the homeowner:
1. If the wire is cracked or oil-soaked, replace it.
2. If, when the engine is running or being cranked, you see sparks jumping from the wire to the engine metal, replace the wire.
3. If the plug wire terminal is loose on the plug, look inside the rubber boot on those plug wires so equipped. Perhaps the terminal can be tightened by squeezing gently with a pliers. If not, replace the wire.
4. If the magneto coil looks good, the flywheel magnets pass a test for magnetism, and the breaker points are properly adjusted, replace the wire if there is no spark to the plug (Magneto coil inspection and flywheel magnets’ testing are described later in this chapter.)
5. If the plug wire is an integral part of the magneto coil, the number 3 check and part of check number 4 are academic.
THE SPARK PLUG
The spark plug itself is a much misunderstood item, and a lot of books have dwelt heavily on it. In fact, a reasonable discussion of the plug could fill a book all by itself. Rather than ply you with the kind of information of primary interest to an automotive scholar, we’ll limit spark plug discussion to just what you should know to be moderately informed, so that you can buy the right plug and to install it properly. We covered the basics of spark plug operation in Chapter 1. Now let’s take a closer look, referring to 6-2.
The high voltage electricity travels from the spark plug wire into the center electrode of the plug. When it reaches the bottom, it is supposed to jump to the side electrode, which is an electrical ground because it’s part
of the metal structure of the plug that is threaded into the engine’s cylinder head. If the electricity jumps across the air gap of about .025 to .030 inch, it ignites the air fuel mixture and all is well.
Sometimes the spark won’t jump the gap, in which case the engine won’t run. The most common reason is that the ceramic insulator that separates the center electrode from the side electrode isn’t functioning as an
insulator, and instead is allowing the current to leak along the ceramic to an electrical ground. Without a current-jumping gap, you get no spark and no engine operation. Why would an insulator suddenly stop working?
Here are the most common reasons: Coatings of oil, carbon (from a rich fuel mixture) and lead may form an easier path for the electricity than the gap between the electrodes. Or the gap between the electrodes may get sufficiently large so that even an incomplete coating on the ceramic insulator may be an easier path for the current. The spark plug fires thousands of times per minute and in time the electrode tips wear away, increasing the gap between them.
Electrode wear is a normal condition, and although the spark plug can be readjusted, it generally isn’t worth the trouble. By the time the electrodes are badly worn, just a normal accumulation of lead deposits makes it worthwhile to replace the plug. Why not clean it? The only way to really clean a plug is with a sandblaster, which costs about $40, at least.
But sandblasting is not recommended by some engine manufacturers, including Briggs and Stratton. New plugs are perhaps a dollar, so even if the manufacturer of your engine approves sandblasting, the equipment would be a poor investment that would take a couple of lifetimes to pay off in savings on spark plugs.
Thick carbon accumulations on the plug are abnormal. They indicate one of the following: 1) The clogged muffler (or exhaust port clogged on two-stroke engine) leaves exhaust residue in the cylinder. 2) The carburetor jet needle is adjusted for too rich a mixture. 3) The jet and/or needle is worn, allowing excess fuel into the mixture. 4) The choke is sticking closed, or the air filter is clogged, either of which can overenrich the
mixture.
A certain amount of oil acclunlllation on the plug is normal on ;I twostroke engine, abnormal on all but a very worn four-stroke. The reason is that in the two-stroker the oil is mixed with the gasoline to provide engine lubrication, so naturally some of the oil will get 011 the spark plug. If you mix in too much oil, you can expect oil soaking of the ph~g. But don’t skimp on oil either, or the engine will burn out. The two-cycle spark plug is designed to operate with a fair amount of oil on the insulator. When it comes to a significant oil coating on the insulator, you’ve got to be practical, particularly with a four-stroker. The normal causes of oil-fouled plugs on a four-cycle are these: 1) too much oil in the crankcase, 2) wear of the piston rings, or 3) wear of the guides in which the intake and exhaust valves move. If the oil level is right and the engine runs for a reasonable amount of time before the plug becomes oil-fouled and requires replacement, you may be better off leaving the annoyance rather than spending money for an engine overhaul. It takes just a few minutes to replace a plug, the price is low, and there are worse things to worry about.
Post time: Mar-30-2024