| This is defiantly one of the biggest bummers, especially because it happens when you need it to work the most or if your new to the hobby there are little things that cause this that can be overlooked.
There are 3 things that cause this problem 99% of the time. Fuel, air, glow, sounds simple but apparently it gets harder.
Step 1 (Engine won't start)
If the engine won't start first step (granted you had the wits to fill up the tank) is to check the glow. If you are using a hotshot glow igniter make sure the batters are charged, regardless of what method you are using a simple check is to remove the glow plug and see if its glowing. The glow plug should be a nice orange, most plugs run on about 3 volts or so. If its an old plug even if it still glows replace it because certain deposits can inhibit the catalytic action even though the plug glows it may not work like it should.
check two: fuel systems.
is make sure you primed the engine. Do this by opening up the carb to full throttle with the glow off place your finger over the carb barrel and flip the engine over for a short burst with an electric starter or by hand until you see fuel run through the line to the carb and most times you will feel the fuel hit your finger. If your not getting fuel check the fuel lines first. Check for kinks, all the bends should be smooth. Next and MOST IMPORTANT check for pin holes, cuts and blockages in the fuel line. Remove the line and inspect it carefully if any doubts replace it. Next step check the carb settings the main jet should be about 2 turns out. Finally check the pressure fitting to the muffler make sure its line is not pinched or cut and pressure is free to flow into the tank, also make sure the pressure fitting and fuel line are not hooked up in reverse. If all these check okay and it still won't start there may be dirt in the needle jet ports that will require you to take apart the carb to clean. To prevent this use a fuel filter.
Step 2 Engine starts but won't stay running or doesn't run the tank down past half.
First check carefully for pin hole leaks in your fuel line, kinks, cuts and blockages. If none check your pressure fitting to muffler make sure there are no kinks or cuts. After that has been taken care of check and try adjusting your carb if its only running down ½ a tank make the mix a little richer maybe try a different fuel (less nitro perhaps). You should shoot for a running temp of 350 F.
You should also take note of the position of the main jet as well as the idle adjustment. Vibration from the engine maybe causing them to move on their own.
Next check and perhaps remove and disassemble the carb clean it out make sure the jets are clear of dirt. If you still are having problems it could be because the engine is worn out and is low on compression. If you remove the piston and sleeve and slide the piston up the sleeve (make sure its in the same position as it was in the engine Never rotate anything off its normal wear marks) the piston should not slide more than ¾ of the way up the sleeve. If it does (pop its head over the sleeve) its worn out.
Good luck and hope this helps