car pops when going down road

[del.5.0 lx]

when going down the road if i ease on the gas my car pops but if i down shift and get on it it pulls really good with no problem . stsrted this about a week ago then stopped and when going around a car about 40 mph today it did it again untill i got on it a little more. car runs good most of the time though. it never did this untill i letf it sit and started driving our hoopty every where and found a new motor for the mustang

 

[jrichker]

Does the popping come from the intake or exhaust?

 

[del.5.0 lx]

timing was off so that took car of the popping
but time to rebuild car.... whent to start it friday @ lunch to go to bank and it cault on fire