black backfire through carb

[84fiveohore]

we got my 79 351w put in my 86 stang, when we fired it up she backfired through the carb (fire and black smoke/carbon stuff) now.. the heads that are on it are d00e that my buddy put together.. he said he nicked the valve seat a little on the intake valve so i was wondering if that would do it... or what else should i be looking at.. we rulled out timing problems and carb.. any other stuff im missing?
also we had to grind the bottom of the rocker a little on the exhaust springs cause the rockers were touching the spring retainers ... why would this do this.. the pushrods are the corect length.. the long ones.. i dont know this is very discouraging..
thanks guys

 

[marine_2003]

I had the same problem on my 85, but my problem was the valve springs, you could push down on them with your hands and open the valves.

 

[Modular2v]

you sure you have the timing and firing order right?

 

[84fiveohore]

ya its a 302 crane fireball 2 cam.. i tryed both firing orders.. and tripple checked the timing.. thats all good.. i think these heads are messed up.. they have e6 valve springs, stock rockers for 70 d00e . do i need a longer pushrod? 70 d00e heads and a 79 block.?