Hello there!
I have replaced the belt on my Murray 16.5 hp, 42"deck, model # 42544x99 and it is still loose even though I engage the blades and they do not turn, and if they do, they are very slow and will not cut. I think the tension pulley (don't know the correct name) needs adjusting but I do not know how to do that. Any help would be most appreciated...

Thank you!
Chris

Probably not routed correctly if the belt is the proper size. Recheck the routing.

Did you purchase an oem murray belt? If so many have the routing diagram on the belt sleeve package. And have you checked your idler pulley tension spring, and is the pulley moving side to side on the bracket freely when you engage deck?