Nitrous Cam Timing

In relation to intake duration, what would be considered too much exhaust duration? I'm fighting a problem with popping out the exhaust and its starting to stump me. I'm quite a bit leaner on the tune-up than I was with the old motor, but its still showing rich. I'm also pulling out less timing than before as well.

I'm thinking of retarding the cam to see if I can delay the opening of the exhaust valve enough to get over this problem without having to pull the cam and have it reground. No time for that before the upcoming races. Any comments on this idea?

This is a 15:1 434 SBC and I'm only running a very small shot right now. 63 nitrous jet and a 61 fuel jet in a NOS sniper plate. 5.4 psi flow through a #70 Holley jet and 1000 psi bottle pressure though a -4 line. 38 degrees total timing (locked out) and 3 degrees retard from the MSD box. -9 NGK plugs, VP C-16 fuel.

I kept pulling fuel out of it yesterday and got the popping mostly gone except in the upper rpms. Then I noticed that my carb's fuel pressure regulator was only showing 3 psi, so I upped that back to 7 psi where its always stayed before. Then the popping only showed up rate at the finish line (1/8th mile). So was it rich or was it lean? LOL. The plugs show a tad rich, but not pig fat. The timing mark is more toward the tip than the curve of the ground strap. Some black specs on the porcelain, but they don't look shiny.

I also replaced the coil yesterday and it seemed to help some. Am I just blowing out the spark? .035" gap on the plugs with a MSD Digital-6 box.

Any ideas or input would be greatly appreciated.

Thanks.