Watched both videos. The engine is jumping out of gear in the first video. You better get that gearcase back off and check the shift rod connector. Recheck the fine adjustment on the shift handle as well. Continued running like this will do permanent damage to the clutch dog and gear lugs, if it is not too later already..
The second video shows that you are pushing the carb butterfly open without advancing the throttle/timing…. This will normally created a lean condition and a lean spit, so perhaps there is nothing wrong. The puff coming under the mag plate is from the excess pressure caused by the back/fire lean spit. The pressure may be more than the upper seal can handle….
