Originally posted by Big Wig
If your engine will idle all day, your low end mixture is definitely too lean!
When I tune my engines, I get the high end mixture where I want it then go to work on the low end. I only want my engine to be able to idle for around 10 seconds before it starts to load up.
I think your problem is a tuning issue... check out the recent article in RC Nitro on engine tuning (2 or 3 issues back I think), this should be a big help.
Maybe his engine is slightly too lean on the bottom end, but it's not the root of the problem.
There are many reasons and engine will not run longer than half a tank. The most obvious is a problem with the fuel tank or the fuel system.
Check the tank for leaks. If there's a crack in the seam on the tank or a leak around the fitting where the fuel line attaches to the tank, the problem is not like to be exposed until the fuel level drops that far.
Check the lid of the fuel tank for a proper seal. If the pressure insise the tank is leaking out because the filler cap isn't sealing properly, then engine will start to lean out as it nears half a tank, because it's having to work harder to draw fuel into the carb.
Check the pressure line from the muffler. Make sure it's not plugged or crimped with a zip-tie. This can cause the same symptoms as a faulty filler cap.
Engine related issues could simply be a bad mixture setting. Often people tune engines so they run well right after start-up, but as the engine warms up, the fuel mixture leans out and the engine will start eating glowplugs and flaming out.
The piston and sleeve could simply be worn. An excessive worn piston and sleeve will allow the engine to start when it's cold, because all of the tolerances are tighter when the engine is cold. But, as the engine heats up, the piston and the sleeve start expanding at different rates, the sleeve gets farther away from the piston, and there's not enough compression to keep the engine running.
These are what I think could be the primary causes of your problem. If you can eliminate some of the posibilities, it would be easier for us to help you narrow it down the the actual cause.