I am using a new DA150 on a 3m extra 260. Its in its second gallon of lawnboy ashless fuel. After 5-6 litres, the engine is suddenly not able to run consistently. It starts upon choking, but once choke is opened, it starts and almost immediately quits. The H needle is at 2 turns and the L needle is 5/8th turns open. This is as per the manual. What could be the matter? Any suggestions?

Its sounds as if its leaning out after the choke is open. Is it running long enough that the choke would normally be open? (engine warmed up?) Make sure there is no fuel restriction. (good flow to carb). Check to see if the bolts to the carb may have loosened causing a vacuum leak causing it to run lean. Open the low speed to a turn and a half and start from there. If the carb is tight and it starts to run good start leaning it out about an eighth turn at a time until it runs how you like on low end. I know on mine the low end will lean out to a point and then not want to start. If you have a fuel filter make sure it is clear. I know the engine is new but if all this fails take the cover off and look at the screen make sure no trash is in it.

On our DA150 we use a servo to operate the choke. On a cold first run of the day, we start with the choke closed and the ignition on, carb at idle. While one guy props the engine the other guy holding the transmitter is ready to flip the servo switch. When the engine starts the choke is opened and momentarily re-closed if the engine begins to quit. This process is repeated until the engine is running smoothly and will accelerate. After the first flight of the day the engine is started by closing the choke, ignition off, and the prop flipped twice. The choke is then opened, ignition on, carb at idle, and the prop flipped once or twice. The engine starts easily.

If you are using a non servo operated choke try starting the engine with the choke closed. When it starts and quits try opening the choke about 25%. It should start and run rich. When it warms up open the choke.

In India the temps are not that cold.  In fact the temp that I operate in is rather a nice warm 25-28C.  Now, the engine always fires with the choke closed and throttle at full stick.  Then I open the choke and throttle at idle, the engine will fire and run for about 5 seconds and quit.  I also tried the throttle at 50% and flick the prop.  Now the engine will start and a higher rpm and the rpm ocillates pretty rapidly.. as if I am playing with the throttle from idle to mid.  The engine does go to full power, but then after a bit it will quit.  Seems like there is dirt in the carb.  I will check it tommorrow.<div>
</div><div>thanks.</div>

You certainly seem to have a fuel flow problem like a kinked fuel line, air leaks, or most probably dirt in the internal carb fuel strainer.
Do you use:
A three line tank setup (fill, vent, carb)
a felt clunk
NO t-line in fuel line

The above three conditions is a must in gas engines.