If you don't know how to tune the engines yourself, then there are HOST of things that could be causing the problems. Lean runs are probably the most common problem with pilots that don't know how to check the high speed needle. Many things can affect the high speed needle setting, temperature, humidity, and barometric pressure being the most common. You could have a blockage in your high speed needle valve. That blockage could be from debris from a fault fuel pump to bad fuel line handling practices to worn fuel lines.

Lean runs cause several problems. The most common, and easiest to fix, is burning out the glow plug prematurely. For an engine that is running fine, and then starts deadsticking and transitioning poorly, these are the first signs of a bad glow plug. In my troubleshooting regimen, this is nearly always the first thing I look for.

In ABC engines (or ABN), the way they are designed has some self protecting features. A lean run will cause the cylinder to overheat, causing a loss of compression, which causes the engine to stop running. Again, this fits the description of the behaviors your describing.

What color is the exhaust from your engines. It should be white, not clear. If its clear, you're running too lean. What color is the exhaust residue? It should be clear or very slightly colored. If its black or dark brown, your running too lean. Can you hold your fingers in the exhaust stream an inch or two from the exhaust outlet (two strokes only). If not, its too lean. This is also a good way to check the quality of the oil in the exhaust.

Bearing failures on used engines are another major culprit to look for. An engine will run with bad bearings, but the metal particles can and will be pulled into the cylinder. These particles will ruin the glow element (at best) or score the cylinder and piston. The best way to inspect the bearings in a complete tear down. For a used engine which you have no history on, this is probably a must before you fly the engine. You can check the bearings by removing the glow element and then turning the engine by hand. This will give you a feel for the bearing condition. Any grinding or roughness should be reason for suspicion.

Finally, peeling of the nickle or chrome from the piston liner is easy to look for. Simply remove the head and then remove the piston liner (after noting the orientation in the cylinder. Look for wear, scratches, or loss of chrome. If you see these, you may need to consider a new piston and liner set.

Good Luck,
Brad