Why Does Your Two Stroke Engine Not Run? By James Goss    Our two-stroke engines are about as simple as an engine can get. They are easy to understand and easy to disassemble. For an engine to be so simple in design, they can really give a modeler a hard time at the flying field when they don't work like they should. There is not much that can keep your engine from running if we just stop and think before we jump to conclusions. How hard can this engine be to analyze? It only has three moving parts while the engine is running, if you count the piston and rod assembly as one unit. The piston assembly, the crankshaft, and the ball bearings (if used) are all that moves. The carburetor does have moving parts, but they are for control purposes and their movement will not stop your engine from running. It may not run proper, but it should at least run if you set the throttle at a fixed point and leave it there.      If the glow plug is hot and fuel is being pumped to the top of the piston you will have ignition. Lets take a new engine for an example and say that it ran fine for a couple of weeks and then would not run at all. The first thing that we do is to change the glow plug because that is the easiest thing to do. Most new engines, especially the high rpm jobs, will have some metal wear during break-in and will deposit a metal film on the glow plug that makes it run cooler. Changing the glow plug will either fix the problem or it will not. We will say that it didn't fix the problem and the engine still will not run at all.      With a good glow plug and a known good 1.5 volts across the plug, we now know that the problem must be in the fuel delivery to the combustion chamber. There are quite a few possibilities here because of the fuel tank and tubing associated with it. I would recommend that you do this first. Remove the supply tubing that comes from your fuel tank and also your pressure line from the muffler. Since you have been trying to crank the engine there will be some fuel in the crankcase that you need to remove to insure that the engine is not flooded. Remove the glow plug and place a paper towel on top of the engine to catch the fuel, turn the engine over with your electric starter to blow the excess fuel from the crankcase. You can check the towel for dampness and if dry you can replace the glow plug. So we now have a dry crankcase and the engine is disconnected from the fuel tank. At this time we need to see if the engine is capable of ignition. To do this place a few drops of fuel in the carburetor. With your glow battery in place try to start the engine, it should fire for a few cycles. If not, add a few more drops to the carb and try again. What you are checking is the fuel delivery system from carb to combustion chamber atop the piston.      If the above test went ok then everything from the carb output to combustion chamber is good. If the engine never fired, then you will need to check the bypasses and porting in the cylinder. On a new engine it is highly unlikely that these are stopped up, so the above test probably went ok. Most engines will have either 1,3, or 5 bypasses cast into the cylinder base. The fuel travels from the carburetor through the center of the crankshaft shaft (this is what front induction refers to) and then through the ports in the cylinder and arrives in the combustion chamber. All this occurs on the down stroke of the piston. When the piston moves up it sucks fuel into the crankcase, when it goes down it pumps fuel to the ignition chamber.      With the fuel tank connected the engine still does not run. We now know the problem is in the fuel tank system or the carburetor. Fuel is not getting to the output of the carb and we can see plenty of fuel at the input. To go from the fuel tank to the carb output the fuel must pass through the needle valve. The needle valve meters the fuel so the engine will not receive too little of too much fuel. The needle valve housing is the smallest area that the fuel must pass on its journey to the engine. If a small object (trash) enters this area it will become trapped and hinder the fuel flow. Nine times out of ten this will be the problem. This means removing the carburetor and cleaning it out. It is a standard practice to use a small piece of fuel tubing connected to the needle valve input nipple and blow through the carb. This may send the obstruction on through the carb or it may just wedge it tighter. A better way instead of using pressure is to use a vacuum to pull the obstruction out. Remove the needle from the needle block assembly and connect that same piece of tubing connected to a large syringe, like the Sig fueler, to the valve body. Give the syringe a hard pull and out comes the obstruction. The syringe is a good tool to have in your flight box for field repairs such as this. It may keep you from having to remove the carburetor from the engine and let you get back in the air sooner.      If fuel is not getting to the carb input or you can see bubbles in the line to the carb, we have fuel tank problems. The most common is getting holes in the tubing, either inside the tank or outside the tank. What I like to do here is temporarily connect a small fuel tank onto the side of the plane, at the same height as the main tank, with rubber bands. Connect the fuel lines as always. If the engine now runs we know the main tank has a problem. If the engine still behaves in the same manner, it is not the tank. If you find out that it is not the tank you have saved yourself a lot of time by not having to remove the tank and then reinstalling it.      I had much rather work on an engine that will not run at all instead of one that is intermittent in its operation. Take the case of an engine that runs fine and all of a sudden it stops abruptly. The question as to glow plug or fuel supply comes up again. First change the most likely thing, the glow plug. If that doesn't cure it the fuel supply is being interrupted. Knowing the path that the fuel must take it is just a mater of time until you find it. Again there may be some foreign material trapped in the needle valve assembly. This material may move around and cause different symptoms from one run to the next.  Using an inline fuel filter will help prevent a lot of problems such as this, but the filter can also be a problem if it leaks and allows air to be sucked into the line.      Air leaks are also a common thing with a two-stroke engine. A common place for an air leak is around the carb base, the needle valve, the head, the glow plug, and the back plate cover. I have had engines that after a few minutes of running time they would quit. Each time after the same amount of run time they would again quit. It turned out to be a loose back plate. After the engine heated up, the aluminum would expand and pressure would escape. When the piston moves down it produces pressure in the crankcase that pushes the fuel through the intake ports to the top of the piston for ignition. By having a leak in the back plate, the pressure is not great enough to force the fuel through the ports so the engine quits running.      A leak around the needle valve will prevent the engine from sucking the fuel; it draws air instead of fuel. A leak anywhere along the fuel supply line will do the same thing. This includes the tubing inside the fuel tank, the line with the clunk on it. If the pressure line from the muffler is leaking, the fuel tank will not develop its 1-psi of pressure and fuel will not flow to the carb the way it should.      As I said earlier, the engine is a simple device that can still give you a lot of trouble when they are not running the way they should. Just remember, if the glow plug doesn't fix the problem it must be a fuel flow problem, too much or too little fuel reaching the combustion chamber. All you have to do is to determine which it is, usually too little. Don't overlook the remote possibility of having bad fuel. Try some different fuel just to be sure.