850 Ultra XP lean bogs when secondaries open

Pump shot can be one of the hardest things to get right. Takes time, patience, and a slew of cams, nozzles, and maybe a bigger pump. That’s your issue though. Put the jetting back to normal.. I recently worked on a 340 dart with a 750. Guy has driven the car for 20 years with a stumble on the secondaries. A few hours and several hole shots later the stumble was gone and the car picked up significantly at the track. I ended up with a big shot on the back of that carb to get it to transition. Pump arm adjustment is also critical, 90% of the carbs I work on have the adjustment screwed up.