86 has bad stumble and bucks if more than 1/4 throttle.

I have a 86 stang that if I use more than 1/4 throttle it bucks and stumbles, and sometimes even back fires... Here's the kicker if I slowly walk to the throttle it seems to run much better. So far I have changed plugs, fuel filter and bumped the timing 2 degrees. I noticed that when I checked the plugs that my vaccum line on the passeger side was burnt through by the equal lenghts, I replaced the vac line and re-routed.. I am still getting same stumble and buck. Any suggestions would be apprecited....
Thanks...

Thanks for the advice....