motor problem...???

[flipthatjig]

i have an old 18hp rude fastwin my problem is the motor wont start on nutral but it will only start if i set the motor on forward and turn the handle to fast. sometime when i do get it started it wont beable to go as fast but then it will take a minute or two then it will then beable to go faster. sometimes it will just go fast right at the start. does anyone know whats wrong with my motor. any help would be great. thanks in advance