Backfire at 4k
 

Series 4 with mikuni 6 port side draft carburetor na and 85 coil/distributor in a engine swapped dunebuggy. check out the video below if you may have ideas.

https://www.youtube.com/watch?v=PnHf...ature=youtu.be

rotorholic helped me troubleshoot the issue and found that my Main Air jets are too big, this led to the issue of trying to die at 4k rpm when the main jets kicked in and let in too much air. the main air is a part of the main jet circuit.