The rules for changing the plug are simple:
I. The new spark plug should look almost exactly like the old one, with a hexagonal section to the electrode. The length of the threads should be the same, and The gap between the insulator and the metal structure should
Similar.
2. Stick to name-brand spark plugs, these spark plugs are numbered and designed closer to the original product. Miscellaneous brands may be basically similar in design features, and you won’t be able to see the difference. But they may or may not work.
3. The plug wrench you use is almost immaterial, because the small gas engine plug is wide open, or else you must remove a sheet metal cover to get at it. And once the cover is off. the plug is wide open. If you have a ratchet and deep socket, fine. Otherwise an inexpensive plug wrench will do.
4. Apply steady force to loosen the old plug. If it doesn’t break free, squirt penetrating oil down onto the edge of the hole; allow it a few minutes to work its way in; then try again with the wrench.
5. If you’re installing a new plug, round-wire spark plug feeler gauges are best, but breaker point feelers will do. To increase the gap, pry up the side electrode, using a small thin screwdriver braced on the base of the plug. To decrease the gap, gently tap the side electrode with a small hammer or a rock. The specified gap for most small gas engines is .025 inch, with Briggs and Stratton an exception at .030 inch. The store selling the plug should have a chart you can check. If the set of feeler gauges you
have does not include the appropriate thickness, use two or more. Example: A gauge of .016 inch and another of .014 inch together equal the .030-inch thickness for a Briggs engine.
6. If the spark plug has a gasket, thread it in until finger tight, then with the wrench one-half turn more. If it hasn’t got a gasket, thread in till finger tight, then just enough more until it feels tight.
It’s possible to do other ignition work on the mower and blower with the engine on the chassis; but this is not always a good idea. You would have to brace the engine and chassis on its side in the case of the typical reel mower and blower, and a crack in the chassis might result if the chassis slipped off and hit the concrete floor of your garage. With a rotary mower, the ignition system is at the top and in-chassis service is perfectly acceptable. On chain saws, in-chassis service is the only way on most, and it’s practical on all.
Removing the engine where possible and servicing it on the work bench, even if not absolutely necessary, does have its advantages. You generally have your best lighting at the work bench, plus a vise, and you can work in the convenient standing position.
Begin by disconnecting the spark plug wire from the plug. Cover the terminal of the wire with electrical tape to prevent accidental firing. Take the metal cover off the flywheel. It’s easy enough to identify because the starter spins the flywheel, the cover containing the starter is the one that
covers the flywheel. You don’t have to remove the starter from the cover.
If you have a mower or a blower with an electric starter, the starter must engage the flywheel. So locate the starter, and there you’ll find the flywheel and cover. The metal cover normally is held by screws of the slot or Phillips-head variety. If the screws are also hex-head and are unusually tight, use a socket wrench to remove them.
The exact side-cover removal procedure varies according to appliance and engine. Refer to Chapter 5 for guidelines and some examples. In a few cases, it may be necessary to remove some other parts and covers first.
Once the cover is off, you’ll see the screen guard for the flywheel. (Or it may have been at least partly visible before YOU removed the cover, another tip-off to the location of the flywheel.) This screen prevents pebbles from damaging the flywheel’s fins, which serve as a fan, aircooling the engine. The chain saw does not have the screen guard.
Turn the flywheel slowly by hand to make sure that it at no point comes in contact with the magneto coil assembly. If the parts touch at one or more points, there is a strong chance that the magneto coil has been damaged. And if you are tracing a no-spark problem, you probably have
found it.
If the magneto coil is tight on its mounting screws and the gap seems to be adequate, leave it alone. Many coil mounting screws go into elongated holes, so that if the gap is inadequate, the screws can be loosened and the coil moved away from the flywheel to avoid contact. Do not move the coil any farther away than specified, or the magnets may not permit sufficient magnetic induction of current through the coil. In any case, do not disturb the adjustment until you’re familiar with the possible effect on ignition timing, covered later in this chapter. Happily, most magneto coils are simply screwed down into position, and you can’t disturb them.
If you wish to check the gap between the magnets embedded in the outer circumference of the flywheel and the coil surface just opposite, use a brass or plastic feeler gauge. You really shouldn’t use an ordinary automotive feeler gauge because it is made of steel, which will be drawn to the flywheel magnets and will give a false sensation of drag. Instead use a brass gauge (available from automotive supply stores) or a plastic gauge (sold by some small gas engines parts outlets), either of which is nonmagnetic.
Note: On those few flywheels with ring gears, the coil magnets may be inside the flywheel, because the gear teeth on the flywheel exterior may prevent location of the magnets there. The only way to check this internal gap is to slip the feeler in place as you temporarily reinstall the flywheel.
Then turn the flywheel to feel if it turns with moderate drag. Also check the flywheel or motor magnets for strength. They cannot be remagnetized. If they have lost their magnetism, the flywheel must be replaced. A simple check is to hold a lightweight screwdriver by the handle, with the tip % to an inch away from each magnet. The magnet should attract the screwdriver from this distance.
Now proceed to remove the flywheel, which is admittedly not the easiest job on most small gas engines. We covered most of the basics of this job in Chapter 5, as part of the disassembly sequences. We’ll now review
the subject in some additional detail and with illustrations of flywheel removal procedures that differ from those for the engines disassembled in Chapter 5. Where appropriate, references to illustrations in Chapter 5 are given.
Except on Briggs and Stration, and most chain saws, flywheels are held to the crankshaft by a combination of retainers: When a nut is tightened, it pushes the flywheel onto a tapered section of the crankshaft. This tapered section retains the flywheel even after the nut is removed. Then a halfmoon shaped key fits into the recesses in both the flywheel hub and the crankshaft to keep the flywheel from turning. If there is any wear in the key or in the recesses into which the flywheel fits, the flywheel vibrates; this can cause several problems, namely these: 1) The position of each magnet relative to the magneto coil varies slightly, changing the time that current is induced in the coil and the time the spark arrives at the plug, which can noticeably affect engine performance. 2) The flywheel vibration accelerates wear on the key and its recesses, and soon makes it necessary to replace the flywheel and crankshaft. Checking the condition of the key and recesses, therefore, is a very important aspect, and we’ll discuss how to do it a bit later in this chapter.
To remove the flywheel, remove the nut that holds it. This means you must hold the flywheel to keep the crankshaft from turning while you put wrench pressure on the nut. You could try to wedge a screwdriver be
tween the fins of the flywheel to restrain it, a procedure that will work nicely on most McCulloch chain saws because there is a raised shoulder for the screwdriver at a couple of opposite points next to the fins; see 5-178 in Chapter 5. If you’re lucky, the flywheel nut is lightly tightened, such as on the Sears Explorer II, and you can hold the flywheel with your hand, or shock the nut loose as shown in 6-7 through 6-9. However, on other engines, you can’t hold it by hand; and if you use a screwdriver wedged in, the result could be broken fins, and you still might not be able get the nut off. A replacement flywheel could cost you up to $20 or more. (Note: If you insist on using a screwdriver and break a fin, then intentionally break off the fin directly opposite-180 degrees away-to minimize the amount of flywheel imbalance.) These broken fins will result in a small loss of cooling air circulation, and the remaining imbalance still might be sufficient to measurably shorten the life of the engine. But if you tried the screwdriver procedure, you’re obviously a gambler, so why stop suddenly