I just had installed a fairly new 454 engine in my 1969 Chevelle. With a Holley 750 and TH-400. The engine ran OK for a couple days, but now it barely runs and pops and bsckfires like crazy. The mechanic can not figure it out. He's tried troubleshooting the carb, distributor, plugs, plug wires, timing. But can't get it to stop popping and backfiring. It starts right up and idles fine, bue when you give it throttle, it skips, pops and backfires constantly. Has anyone had a similiar problem or any ideas what could be the problem?  

Sounds like something's screwed up with the timing.

I'd suggest trying a different distributor and see what happens. Get one you know is good and install it on your motor.

Also, when it's doing this, are you getting smoke out of the tail pipe? Consider worst case scenario, the timing chain may have slipped a tooth or two.  

i wonder what kind of distributer it has...    hei or aftermarket???

got a timing light????  hook it over one of the spark plug wires... and fire up the motor... watch the flashes... see if it is steady flashing... or random...  test all 8... plus the coil wire if it has a separate coil.....   poor mans scope......

does it have enough power to the coil?????

if it has HEI...  pull the cover off the top of the cap... examine the coil primary wires.. one red.. one white or yellow...    one black.. the black one goes to a corner coil hold down screw...    make sure the ground strap that goes under the coil and allows the center wire in the 3 wire plug up from the module to ground the coil frame...  when this is left out... it can cause all kinds of problems,....

a long time ago... my brother in law put a reman motor in his chevy..   on  its first trip out of his town... it started blowing coils and modules.... violently...  BANG!!!

he called me...  i ask if when they put the motor in... if they put a new cap on the hei... yes they did... did they change the ground strap over from the old cap?????

what ground strap..??    i gave him the napa part numbers and told him to find one of their stores... as i knew they are popular where they were.... but i did not know which one was closest...   so i called... described what i wanted... and what they looked like. and to have the parts waiting for them....

i heard from him a few weeks later...  on a different subject... when i brought up how did it work...   he stopped for a second...    he said it was weird...  when he got to the store..    one he had never been to before...  they greeted him by name as they walked in the door and had the exact parts on the counter ...     he said it was really strange..   then i told him i had called the 3 local napa stores and filled them in...  

Fire it up at night, pop the hood, look for plug wires arcing to each other or to the engine.  You said the mechanic checked the ignition, but I'd pop the cap anyways and grap ahold of the rotor and make sure it isn't loose, it should turn a bit and move up a bit as the distributor rides up the cam gear, but not excessively. Plus check the physical condition of the rotor and inside the distributor cap.