After I changed the fuel filter, (was over-due & something might have got past the filter). I have had trouble starting engine, air in lines?? It's a hard start if machine has not run for days, crank it 5-8 times rough start, blows black smoke, then runs well; always starts quickly if only off for 1-4 hours. What is the best way to remove / check injectors or should I look in another direction? Thank you for any help.