kx440 ltd missing at high rpm

okay so ive got an 81 kz 440 ltd that fires right up, valve clearances are great, idles like a champ, but theres the strangest bog that occurs around half to three quarters throttle. ive replaced plugs, carbs are clean, no induction leak, a slight exhaust leak except the studs are so rusted that i dont dare touch them....theyre staying as is. original air filter, no current fuel filter. when you reach that half to three quarters throttle range in any gear it suddenly bogs so har and consistently that it prohibits you from reaching the higher rpms. heres the wierd part. if you back the throttle off to the point just under which it started bogging, the motor will climb through the rpm range without problem, it just does it extremely slowly. when in a high gear and downshifting it does not hesitate at all going to high revs, and as you slow, it backfires like its mad rich. this is why ive decided its not a spark advance problem, but a mixture problem. but as ive been typing this, it occurs to me that i have not checked the spark plug caps themselves yet. come to think of it im not sure the coil itself is up to spec. so heres my question then: what resistance can i expect from the coil, and does anyone have any idea what my issue might be? thanks for the help everyone...

brian

well brian, i would start off by getting a new air filter. And i know i heard from some one on here, i think the gringo. The air boxes are really finiky on the 440's, they have to be put back on perfectly or the bike will bog.

Yeah Foose the first thing you need to check is that the air cleaner is installed correctly. In order to check it open the seat, grab the back of the filter and try lifting up on it. If the box moves you have it installed wrong and the bike won't run right. There is a slot in the airbox where the bottom of the filter fits into prior to attaching it with the two top screws. This will only take a minute to check.

i just went out and sure enough i had installed the air filter wrong....after fixing it i have it another ride and the problem persists....perhaps is even worse than before. okay so heres something dumb that i did that i neglected to remember until now....the foam element in the filter was shot so i grabbed another piece of foam from somewhere in the shop, shot it with air filter oil, and replaced the unit. you think im choking it with this foam? will the bike run without the element, but the air box itself installed? i notice the airbox has venturis, probably to stabilize the air, but just to get it running well, do you think i could take the foam out?

i just got back from the best ride this poor old 440 has seen in the last 6 years! i took that foam strip i installed off and like a cheerleader on prom night, the hesitation was gone! gotta hand it to you guys, youre very crafty in your ways! thanks again for all the help....ill post some pictures and the whole story of this bike pretty soon.....its come a long way from the compost pile under a pine tree down the street.

no problem, im glad we could help you out. And yes pictures are a very good thing here at kzr.