Rough idle turned to crank then ka-thunk

So i went from what looked like a semi common issue to one i can't info on. My dart was stuttering then dieing then would die at idle even while moving but only after warming up. I checked all the basics fuel, spark, distributor, battery voltage. But i couldn't find the issue but now when i crank it over it'll crank but stop with a thunk but if i take the cap off the distributor i can crank till the battery dies. So to me it seems like bad timing but I'm lost and don't want to cause more damage than might be there.