ditch it.use 86 manifold, as 87 on have an angled manifold due to different shock location than 83 thru 86.hth.K.
the 86on manifold wont fit the 85 { 4bolt vs 6 bolt }
a new 85 manifold should be very reliable as the hole for the boost bottle is oem {not drilled}
the boost bottle is suppose too improve lowend power and MPG
Dont know how well they work but i dont think all the big maufacturers would spend the extra cash fitting them if they did not work
yamaha first Yamaha Energy Induction System (YEIS) used on many models from the 70/90s and still used it on some of the later ypvs models dt125 rd125etc where the gains should be less
Suzuki Intake Pulse Control system (SIPC), have used on many model ts rgv
not sure what honda called there system but i think the boost bottle was fitted too some NSRmodels
if you do remove the boost bottle on the kx you may need to rejet,