Every machine is different.You can pull two brand new machines out of a crate and one might run faster than the other. First thing to check is the belt for wear, excess slack.If it has over 1 1/2" slack in the middle when you pull down on it(with a straight edge across both clutches) might be time to replace it. Around 45 mph best I remember on the 300 on average.Some may get more out of them. PLUS don't double post on the same problem in the same area! Causes confusion

 

about how fast is your 400?