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?