Popping at 4500 ??

[Coupe50h]

Well my friends 93 gt developed a faint pop thru the exhaust starting around 4k to 4500, he gave the car a tuneup, and replaced the coil....... i checked codes, everyting seems fine?

It idles great, and has power, almost sounds like it's losing fuel at that rpm, his mods are: typhoon intake, 70mm tb, 75 pro-m, 24lb injectors, 373 gears, no cat's, and shorty headers.

I told him to get a fuel pressure regulator, he has no idea what size the fuel pump is since he just got it a year ago.......also, he said it started happening right after he got on the gas one night and hit the rev limiter on accident, you think he could have hurt a valve spring? i checked timing, all good there......
I dont know what to tell my friend, but im thinking i need to pull the valve covers.......thanx for any suggestions....jeff c.??


edit: he said he just put on a starter solenoid, i know that doesnt have anything to do with it, but he said he might have had some wires put back wrong? just a thought.