multiple misfires tried everything ,someone help!
 

Recently pulled my carbed 502 ,had the valve train and lower end looked at and had it dynoed with my crane ignition box... motor pulls 591hp no reversion with shops wet exhaust all is well.Did this because the motor was reverting water last year with the shorty risers and silent choice at the time.Just had a set of Lightning headers made up with the the dumps right before the tips. Just put it back together sounded ok in the drive way then put it in the water.At all rpms the motor stumbles all over itself but no backfiring and no valvetrain noise.Some plugs are now turning dark,pull the headers and see 2,3 and 8 have puddles at the bottom of the tubes.Regarding the misfires I am using my original 454mag engine harness with the ignition mod and knock mod disconnected,and the same trigger type dist that has not been removed or turned from the dyno test.I even dumped out the carb and started it with an external fuel source,at 2000 rpm you can still here it missing from both banks.Is my motor reverting because it is missing OR is it missing because it is reverting (I thought once you have throttled up you blow any water out)and why is it running so bad in the boat but perfect in a test cell. Ihave gone over the wiring many times cannot find anything,This is elementary stuff but I am baffled,all grds and power sources good The twisted pair of pickup wires rerouted away from EMF.The timing is 34 degrees ,516cubes, holley 950, 9.5/1 comp,.592/231,.611/236cam 112 lobe sep 7 psi fuel press.Again this motor ran great in the cell with this configuration other than the engine harness and headers why are all the cylinders missing while in the boat?

A buddy of mine had a similar issue, found the inner tubes on his CMI headers were cracked. Maybe something similar as yours.