mower starts. sometimes but won't. keep running changed carb and filt.

Mine has this issue, and all I do is push forward on the metal slider under the carb / safety stop at the end of the metal cable that goes from the safety stop handle to the engine, once I pull back the handle and before I pull the starter cord. I am pretty sure if I replaced the cable (I wrapped it around the handle one too many times and I think it stretched it out) that it would correct the problem, but that does the trick every time. Hopefully it also works for you! If it doesn't, good luck to you and this is at least free to try.