To check the timing without taking the flywheel off, mark TDC on the flywheel with a pencil, and make a corresponding stationary mark on the crankcase or whatever is convenient. Back the timing mark up anticlockwise a couple of inches, grab hold of the live end of the spark plug lead with one hand, and move the flywheel clockwise with the other hand, until you feel an electric shock from the plug lead. (Turn it slowly, and speed up a bit if you can't feel it). Make a second mark on the flywheel at the firing point. The firing point had better be to the right of the TDC mark (engine runs clockwise, and it has to fire before it gets to TDC). Measure the distance from the firing mark to the TDC mark, and measure the circumference of the flywheel. Divide the firing mark to TDC mark distance by the circumference, and multiply the result by 360 degrees. That tells you how far before top dead center it fires. It should be 6 degrees. It increases to 26 degrees when the engine is running. Be very sure it is firing before TDC, not after TDC.
If it consistently starts and stops after 15-20 seconds, you have a fuel feed problem. You haven't left the fuel tap in the off position have you? If the tap leaks a bit, that could be the problem. Or the tap could be restricted. Or the float needle could be stuck close to the closed position. 15 to 20 seconds is probably about the time it takes to empty the float bowl.
