I pull the rope and nothing happens, now what? That's a fairly simple question, but it may require more than a simple answer. Let's take a look at the situation from a repairman's point of view. Air-cooled engines need three basic things to run. Take away any of one of the three and it's no start time. So, let's review these things and see if we can simplify that question.

1. The engine must be able to develop compression.
2. The engine must have ignition, better known as spark.
3. The engine must receive fuel and air through the carburetor.

If the engine cannot develop enough compression to draw fuel and air from the carburetor then it cannot run. Normally you can feel the resistance of compression when pulling the rope. Electric start models can be a little harder to tell without the right equipment. Keep in mind that you have to obtain a significant amount of engine speed when you pull the rope or you will not develop enough compression. A low battery can cause the same problem. When I check compression using a gauge, I usually like to see 60 pounds or more after a couple of pulls. Low compression can be caused by a worn engine or burnt or sticky valves. Stale fuel from setting over the winter can cause even a new engine to have a valve stick in the open position. Attachments to the motor that cause it to bind up or put and excessive load on it, can keep it from turning fast enough to develop compression. An example of this is a mower blade that has become wrapped with debris.

You must have a good spark to fire the fuel mixture once it is in the cylinder. A good way to test for spark if you don't have a tester is to take an old spark plug and cut the electrode off, so the spark has to jump from the center to the side of the plug. Lay the plug on the engine so that it can ground and spin the engine. If you don't have a good spark check for a faulty switch or interlock device before condemning the Magneto. The new solid state ignition systems are pretty reliable. The spark must be steady and a bright blue color.

The engine must receive a proper mixture of fuel and air, so that the spark can ignite it and start the engine. Besides the fuel and air, the engine governor must open the carburetor throttle to allow the mixture to reach the cylinder. A stuck throttle shaft can cause a no start, so check the simple things first. If you suspect that the carburetor is not supplying fuel you can give a small squirt of gasoline into the air intake (usually with the air filter removed) and see if the engine fires momentarily. If it does then you probably have a plugged carburetor. Once again we are back to the stale gasoline problem. Even a good engine may not start on stale gasoline, so stick your nose in the tank and take a whiff, if you come out gasping for air, then you'd better get rid of that fuel. One other thing to keep in mind is that many of the newer electric start engines also utilize an electric fuel shutoff valve. Check this both electrically and mechanically if you suspect the carburetor is not supplying fuel.

If you resolve the three problems above, the engine will run (usually). I know someone will make a liar out of me, but in most cases, one of these three items will be at fault. So, next time your unit will not start, make a quick check and see if I'm right. Good luck to you.

Also see Complete Engine Rebuild,  Step by Step Two Stoke Carburetor Rebuild, Carburetor Rebuild, Starter Rope Repair, Carburetor Troubleshooting Chart or Loose Valve Seats for more troubleshooting.

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