It is a play off between shock loading of the deck & belt and shock loading of the engine.Thanks guys. No, the deck is not hitting anything. Guess I'm just used to my Cub garden tractor with a very smooth clutch. Run WOT and engage the blades - smooth take-off. Totally different with this thing. And no deck rattle. Just that nasty sudden 'impact' when engaging. Have thrown the belt once already. Guess I'l keep starting 'er at low idle. Thanks guys. BTW, great forum.
Check that the tension pulley arm is free to move.
IT is there to take up those sorts of shocks but they do get stuck .
JUst did a Timecutter where the pivot hole had elongated to double its length so could now swing properly.
Result was 2 broken belts two days running.