Few things.
1. Four stroking is normal when cruising or accelerating.
2. On speed runs, I as well have WOT and the idle lowers drastically thus killing the engine.
3. If there is a bad exhaust gasket, it isn't necessary an air leak like you would have with a bad crank seal. You will have a higher idle though which is normal. Most important, technically this would lean it out. Just as cutting a 1/2" hole or removing the exhaust cap.
I would take into consideration, if everything was fine before but introducing a carb caused problems, exploring that vicinity.