my truck was running great when pulling trailer home the other day than ever since it has been getting weaker and weaker till it couldnt pull its own weight any more. Then we decided to advance the timing and started it and ran great a second backfired hard enough to shake the windows on surounding houses and continues to only back fire when trying to start it in every timing position. What could be wrong
The timing chain went in my 81 van with a 300. Mine didn't start getting weaker and weaker, it just got real weak all of a sudden! If you have a crapalitic converter, it may have clogged. I have to amend this post. The timing gear went bad! It's been a few years.
Here's a good site on big block sixes: http://fordsix.com/forum/viewforum.php?f=2&sid=2d4c2b575d374543021921797b1130b5 Rick
Could you post up what year the engine is? Knowing if it's a carb or EFI will help. I also remember on my 81 van, the electronic module going bad. I was driving about 50mph and it was just like you turned the key off. A split second later a jerk Boom and then no more run. It backfired so bad, it split the muffler wide open!
Check the coil, they are bad for internal arching. Look in between the windings and the laminated metal, if it is bad you will see arching marks. I have changed a lot of them.