I have an 18hp with a 40 series and I feel like the engine is to much for the TC. If I am not careful when I hit the gas the belt starts to burn up. Just a thought.
The 40 series was factory rated to 18hp- you can bet your bottom that they would have left a pretty generous error margin in the rating. 25hp sounds reasonable.
I remember Blaz noting that they've had big twin cylinder karts in excess of 20HP working perfectly with a 40 series. Comet under shot the 40's potential but got the 20/30's just about spot on.
I have an 18hp with a 40 series and I feel like the engine is to much for the TC. If I am not careful when I hit the gas the belt starts to burn up. Just a thought.