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
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.