2000 5.7 mercruiser not firing

[Hayes2406]

Hi guys 

My 2000 mercruiser 5.7 engine with thunderbolt 5 ignition is not firing it is turning over on the starter fine. this problem has come from nowhere so it hasn't been getting worse over time we can see the carb is getting fuel and we have changed the filters. The butterfly at the top of the carb is in the fully open position we tried closing it manually but it still would not fire any suggestions on what might be wrong would be well appreciated. 

[Woodski]

@Hayes2406: Here is a troubleshooting tree to follow, first item:  

https://www.google.com/search?source=hp&ei=d_xNXKFBpr2PBMLmo6AP&q=mercruiser+thunderbolt+5+troubleshooting&oq=mercruiser+thundervolt+5+trou&gs_l=psy-ab.1.0.33i10i160.163.7147..9901...0.0..4.1304.9001.3j8j1j1j2j1j4j2......0....1..gws-wiz.....0..35i39j0i131j0j0i10j0i13j0i13i30j0i8i13i30j0i8i13i10i30j33i299j33i160.h-uK0pXjha0