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First and foremost, your engine needs to fire to start. If your glow plug isn't glowing, your engine isn't going to start. The glow plug should be the first thing you check when you are having starting problems. Clean the area around your glow plug with motor spray and remove your glow plug, being careful not to lose the washer. Jam your glow plug into your igniter, you should see the coil glow bright orange. If your coil doesn't glow, your glow plug might need to be replaced. It could also mean that your glow igniter battery is dead and needs a recharge. Figure out which problem you have by trying another glow plug or another glow igniter. Fix or replace whichever part isn't working. Keep in mind that a glow plug that glows is not necessarily good, but a plug that doesn't glow is definitely bad. If you determine that your glow plug is the problem, do yourself a favour and throw it in the garbage. It won't fix itself laying at the bottom of your toolbox, save yourself the headache of trying it again later.
A flooded engine is another common problem that could cause a no-start. Nitro engines don't actually burn raw fuel; they burn a fine mixture of fuel and oxygen. Fumbling too much with your engine can result in too much fuel into the crankcase through over priming. You may have an excessively rich mixture setting as well. This raw fuel in your crankcase gets drawn up through the ports and hits the glow plug, extinguishing the hot coil and making it very difficult to start. Raw fuel can also get trapped on top of the piston once the ports have been sealed off, causing what is called a "hydraulic lock." Since fluid doesn't compress, this hydraulic lock prevents the engine from turning over. The problem of excess fuel in the crankcase is an easy one to fix, but you must be able to detect the signs of a hydraulic lock. If your engine has more trouble than usual turning over it is probably flooded and the issue should be addressed immediately. If you continue to try to rotate the crankshaft of a flooded engine you can do some severe internal damage. Instead, clean the area around the glow plug with motor spray and remove the glow plug and washer. Place a rag over the cylinder head and flip your engine upside down. Rotate the crankshaft a few times with whatever starting system you are using. All of the raw fuel trapped within your engine will come out of the glow plug hole and onto your rag. When it is all out, simply thread your glow plug back into place and try to start your engine again.
Sometimes your starting problems don't have anything to do with your engine at all. The problem could lie within your clutch. If your clutch shoes are melted or one of your clutch springs is broken, your clutch may engage at an extremely low RPM. This means that as soon as your engine tries to fire up the clutch will engage, try to rotate the wheels, and stop the engine again. Basically, if your wheels rotate when you are trying to start your engine, check your clutch for damage.
An engine will start when your air/fuel mixture settings aren't just right, but not as easily as when they are. If you have been working with your engine for a long time and you've been going crazy with the tuning screwdriver, you may be best to stop, set your carburettor to the original manufacturer recommended starting settings, get the engine started, and tune it from there. The air/fuel mixture settings can be very fine and a tiny turn here with a little twist there can easily tune you way out of whack!
It is always better to be slightly rich as compared to slightly lean when you can't seem to find the correct tune. A rich engine can be slightly frustrating, but a lean engine can cause internal engine damage. If in doubt, turn your mixture settings out (that means richen them!). Remember that being a little richer than normal will cause your engine to flood more often, so you may have to drain through the glow plug hole a few times during the initial tuning process, at least until you get the mixture settings closer to where they should be. Also remember that you should never fine-tune a cold engine. Run your engine under normal driving conditions for a good two or three minutes before you even think about making final mixture adjustments.

Dirt and debris in the carburettor can easily prevent your engine from starting. A quick flush of your carburettor with some motor spray or denatured alcohol in a spray bottle will usually dislodge any chunks of crap that may be blocking fuel flow.
Fuel goes bad, simple as that. If you've tried everything else you should never dismiss the idea that your fuel could be contaminated and causing your starting problems. It never hurts to try a fresh gallon of fuel. If the problems go away and your engine starts fine with fresh fuel, you know the old stuff was bad and you can dispose of it and move on. At worst the fresh fuel won't solve anything and you'll have extra fuel to use once you get the engine running.
Your fuel tank is a common area for air leaks that if left unaddressed will lead to difficult starting and erratic tuning. To check for air leaks, you must ensure that your fuel tank can hold pressure. To do this, empty the fuel tank of all fuel and unhook the pressure and fuel lines from the tuned pipe and carburettor respectively. Plug your fuel tank's fuel line with two fingers and blow hard into the pressure line while plugging it with the fingers on your other hand (the fuel line should still be plugged). This should pressurize your fuel tank. Wait about 10 seconds and let go of the pressure line. You should hear a "Psssssst" noise. This passing of air out the pressure line shows you that your fuel tank is able to hold pressure. If your fuel tank isn't sealed it will need to be replaced. You might as well throw some fresh fuel and pressure lines on while you're at it to ensure that no pinholes or cracks letting unwanted air into your fuel system.

Now you know all the steps necessary to get your engine going. Some of these tips are quick and easy and others require a bit of effort but the simple fact is this: if your engine still won't start after trying all of these tricks, maybe it's time to go electric!
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