850 Ultra XP lean bogs when secondaries open

90% of the time, when you have a lean bog like that, on a fully built engine with big ports, big single plane intake, huge cam etc. the issue is insufficient pump shot. I The first thing I would do is to get a 50CC accelerator pump kit, and an accelerator pump tuning kit. The pump tuning kit is expensive, but if you work with Holley carbs, you will be using that kit for the rest of your life. Start with the 50cc kit in the back of the carb and see how that works, the 50cc may cure the lean bog by itself, but you will probably need to work with the cams and bigger squirters to get it to run exactly where you want it.