MDT Cub Cadet Briggs engine wont start?

This thing has always been a nuisance to start (spinning for at least 2 mins before starting) but now wont go at all. It is a 2 cylinder ohv engine. It does fire occasionally but never runs. I have checked the spark and tried different plugs but no good. I have tried a different carb but no different so then I removed both plugs & put drops of petrol in each cylinder but still nothing. I also did a compression test but found no problem. There are 2 black plastic adjusters on top of the carb - anybody know what they're for? & what to try next?

Many Thanks for any advice.

Nick.
November 2008