Fourtrak starting problems


I have a diseal fourtrak and all of a sudden it started giving problems when i try to start it. The starter keeps turning but it starts just after about 40-50 seconds. I have checked heater plugs,connections etc but to no avail. I changed the diseal filter but still the same result. Has anybody got any ideas?

Thanks to all

Yes the problem was a fuel pipe letting in air but not leaking diseal. Thanks to all who posted comments.