Ok I'm at a loss I've checked/replaced vacuum lines, increased jets, decreased jets to no avail. It is a 350 .040 over, mild cam, edelbrock performer intake headman shorty headers, MSD Street fire distributor, MSD Street fire capacitive discharge box, and a Holley 750 Vacuum secondary square bore carb. Extremely fast from 1500-4,000. But at 4k it literally does a nose dive. Hacks and coughs and has no power. Also at idle it sounds strange like it's not firing on all cylinders and low speed cruising it surges. Any idea's?

 

What do you have for heads? Valve springs in particular.

 

Nothing special as far as I know they're cast iron double humps, rebuilt about 15K ago when the motor was done. This all started when I dropped from an 800CFM DP to the 750 SP VS

 

Are you sure that they are double humps? You could be floating valves, or you may just be running out of air. Usually they dont hack and cough though, when they run out of air. What cam are you running?

ETA: What happens when you unplug the box and just run off the distributor? Does it still do it?

 

I had the same thing at 5k rpm and discovered that my ignition coil was going bad. I had changed valve springs, fuel pump, had the carb rebuilt, changed plugs and wires and still same issue, tested my ignition coil and found out it was going bad, Now I spin to over 6500 rpms and no problems.

 

I'll check the coil too but I have an inline fuel pressure gauge and when I hit the throttle by hand it dips down to about 2 1/2-3 PSI, so I'm thinkin my pump is screwed

 

I agree with you. When you hit a wall at a certain RPM only under WOT it usually means you're outrunning your fuel supply. If you can see pressure dip just revving the engine you definitely have fuel supply issues. It should hold onto roughly 6 PSI like a pit bull under all conditions- idling, part throttle and WOT throughout the entire rev range.

 

Last night I discovered a gigantic leak in my dual feed fuel rail. Pressure is much better but it still drops to about 4-4 1/2lbs, so I'm still replacing the pump and I ordered a light weight fuel pump push rod