90 Trooper Starting Problems

I have a 1990 Isuzu Trooper w/ 2.8 Ltr. Vortec V6, Manual Trans.

Every once in a while I try and start it, and it always cranks just fine but it just does not ketch. If I turn the key off and turn all accesories off (Ie: A/C ect.) then try again sometimes it starts sometimes it doesn't.

Any help on this.

Thanks

when is the last time you had it tuned up?