I have a blueprint engines 408 stroker in my 68 dart. Its has 10.1 comp, edelbrock aluminum heads, comp roller rockers, a comp hyd cam and a air gap intake, The trans is a coan and has a coan 3500 converter and 3.55 gears with a summit brand electronic box with rev limiter and a pro comp distributor. EVERYTHING is new on the car Except the carb (was all rebuilt and tuned but bought from a friend used)and it has about 1000 break in miles on it. Just recently I took it to the track and pulled a 12.66 @ 106, which is respectable BUT it breaks up and pops through the exhaust at 5800, I fixed a couple of vacuum leaks and replaced the base of my holley cause I found the shaft was worn and whistling Its much better but still has the issue. I was thinking the ign box or maybe the valves arent set right?? Anyone have any ideas before I go tearing this thing apart. Thanks Pete