ENGINE WON’T START
The most common problem is that the engine fails to start, so let’s begin with this one. There are literally a thousand possible causes of starting failure, but a few tests to check out the most frequently-occurring possibilities normally isolate the problem very quickly. The ingredients of a successful engine start are these:
1. The engine turns over (the job of the starter).
2. The piston develops adequate compression. (The engine must be in reasonably good mechanical condition.)
3. High-voltage current is delivered to the plug, and it jumps the air gap between the electrodes.
4.The spark arrives at the right time.
5. A mixture of air and fuel in reasonably proper proportions is delivered to the cylinder.
QUICK CHECKS
A series of quick checks will isolate the problem to one of the five items. It may not give you the solution, but it’s the only way to begin. When you pull the starter cord you can feel and hear if the engine is turning. If it isn’t, the problem is the starter. (Refer to Chapter 6.) Disconnect the spark plug wire and hold it ‘/s to G-inch from the spark plug terminal. (If the plug wire has a rubber boot, wedge a paper clip or coil spring into the metal connector it covers; make sure you use a clip or spring that is long enough to project from the boot.) See 4-1. Crank the engine and you should see a spark jump to the plug at least a couple of times during each complete pull of the starter cord. If you have an electric starter, the wire should discharge a spark at regular intervals for as long as the starter operates. If you get the sparks, the ignition system up to the plug is in good condition. If you don’t, check the troubleshooting chart; then review ignition system service as explained in Chapter 6.
Caution: If a spark plug wire’s insulation is defective, holding the wire with your bare hands could lead to an electrical shock. To avoid this danger, hold the wire with a pair of small sticks, chopstick fashion. With a spark to the plug, the next check begins with leaving the plug wire off and cranking the engine several times. Now remove the plug quickly and inspect it. If the fuel system is delivering, the plug will be wet with gasoline. If it isn’t, you have a fuel system problem. You can doublecheck a no-fuel problem by removing the spark plug, pouring a couple of teaspoons of gas through the hole into the cylinder, reinstalling the plug and trying to start the engine. If the engine now starts and runs for an instant, lack of fuel is the problem, caused by anything from an empty tank to a plugged fuel line. Note: On chain saws, use a half teaspoon of fuel-oil mix. Also inspect the spark plug’s general condition. If you have any doubt about it, reconnect the plug wire and using rubber or an insulated pliers as a holder, rest the plug against the cylinder head while a helper cranks the engine.
You should see the spark jump the gap across the electrodes. See 4-2. If you had a satisfactory spark up to the plug and none jumping the gap, the plug is fouled and should be replaced. If fuel is wetting the plug, you know that some amount is being delivered, but you don’t know if it’s too much. To check out this possibility (flooding), open the choke and the throttle all the way; then try to start the engine. If the engine now gives at least some indication of firing (or if it starts and runs), flooding is indicated. A strong odor of gasoline at the carburetor is a confirming clue. These problems are covered in Chapter 7.
The next check is for adequate compression. This can be done with an automotive compression gauge on those engines for which compression specifications are published. Unlike an automobile engine, however, the small gas engine compression reading is a very rough indication. In fact, Briggs and Stration refuses to publish compression specifications on the theory that too many people regard specifications as gospel, that mistakes are easily made taking the readings, and that unnecessary repairs result.
Inasmuch as Briggs is Number 1, this philosophy cannot be ignored. It suggests that you pull the flywheel cover and spin the flywheel counterclockwise against the compression stroke (spark plug reinserted and tight).
You can identify this stroke by turning the flywheel until you encounter resistance, which will be the piston pressing against the air-fuel mixture. Give the flywheel a flip motion counterclockwise, and it should rebound very sharply. A slight rebound or none at all indicates poor compression. If the manufacturer specifies a compression reading, have a helper hold the compression gauge firmly in the plug hole while you crank the engine with the rope starter half a dozen times. See 4-3. Most small gas engines will read at least 50 to 60 pounds, although those with a compression release feature (for easier starting) might read 40 to 45 pounds. These are not universal readings. For the McCulloch MAC-10 series, a minimum reading would be 100 pounds. It’s a good idea to make this test on your engine when it’s new, so that you know what it should be, then allow a 25 percent drop from normal engine wear.
Low compression normally is caused by 1) a loose spark plug, 2) a defective cylinder head gasket, 3)valves that are burned or failing to close, or 4) worn or stuck piston rings. Unless the starting problem occurs at the start of a season, when aging and poor storage could have caused the valves or piston rings to stick, the loose plug or cylinder head gasket is the most likely possibility if the engine ran normally the last time it was used.
A head gasket failure can occur suddenly, and when it does, the loss of power is instantaneous. If the engine very suddenly ran poorly during the last use, bet on the head gasket or the spark plug. Poor performance is not a certain indication of a head gasket failure though, for the gasket could rupture as
the engine is being shut down. If the fuel, ignition, compression and starter all check out, ignition timing is suspect by process of elimination. On an engine where coil position controls timing, take a look at the magneto coil to see if the screws are holding it tight. On an engine whose plate position adjusts timing, remove the flywheel and check the breaker plate bolts with a wrench, even slightly loose bolts can allow an unwanted timing change.
