Check that the blade is secure. A loose blade - apart from being dangerous - will remove some of the flywheel effect.
If that's not it, and assuming it runs properly when it does start, then the compression release mechanism has probably worn out/fallen apart. Spares are not available, so you will either have to replace the entire camshaft, or live with it.
phil_saunders(a)bigfoot.com
Phil Saunders, August 2009
