'91 5.0 Stumbles, backfires and sluggish.

[BlueBronk]

Stock '91 5.0 w/ MAF running in an automatic. New injectors, plugs, and plug wires were installed when this donor motor was put in. Motor was put in a year ago and never have got it dialed in. Starts very quick. Idles pretty good with a very slight miss. Occasionally idles up and down some???? Every once in a while it will die when idling. When driving it for a few minutes it seems to run good. 10 minutes into the drive, and the rest of the day...it runs, but backfires and stumbles. If you step on the gas...it usually goes away until you slow back down to cruising speed and then it will start doing it again. Its a muffled backfire sound maybe through the exhaust..hard to tell. Will definitely do it under load on a hill when not giving it much gas, but if you put it to the floor...it goes away again. I have used gas treatment, ethanol free gas, etc.. Any ideas on these problems? Could this bve timing? Also..what is my timing supposed to be set to?

Thank you,

Andy
BlueBronk

 

[Mustang5L5]

Timing should be 10 degrees btdc. That is factory setting

Have you run the engine codes and performed a cylinder balance test?