1986 merc 4.3 occasional misfire/backfire above 3000 rpm

Hi all. I am kinda at lost here. I have an express cruiser with twin 4.3l mercs with thunderbolt 4 ignition. I had a couple tuliped valves on SB motor recently, so I removed the heads and had them reworked. Installed heads and all was well for about 10 hrs (sametime I installed new cap and rotor). Recently the SB motor has developed a misfire and occasionally a backfire through the carb above 3000 rpm. Below 3000 never misses a beat, and idles flawlessly. Yesterday the motor ran perfect going out, we stopped at our local spot for a few hours, then went for a cruise and it started missing and occasional backfire. Came off plane for a bit, then back on plane and ran fine.

Everytime the motor misses (it's seems and sounds like one cylinder is dropping off), my sync gauge and the SB tach go wacko. That leads me to believe ignition. It's a thunderbolt 4 system. Cap, rotor, wires, plugs and ignition sensor in dist. All new. (Plugs were replaced this spring). Water separator new also.

Were should I go from here? The new cap and rotor seem like the contacts are kinda burned but not sure if that is normal.

Thanks for all your time.

Re: 1986 merc 4.3 occasional misfire/backfire above 3000 rpm

What do you think caused the valves to warp?
If your backfiring then a valve is open. When you had the heads redone did they check for correct spring height? It could be you have a weak valve spring.