I don't have much experience with the 87-era KDX, but I was having similar problem in my 89 kdx. I was running 32:1 Spectro; now I run 42-48:1 and that cleared it all up! Changing jetting wouldn't help smoking with the 32:1, with an 8 series plug. Now it burns perfect (had to richen jetting a tiny bit on MJ) with no visible smoke once warmed up.
But...what kind of useage does the bike get? If it has stock sprockets and chain I'll assume not very much, but it may be time for new rings. Check ring end gap first, they may just be worn out; gap should be .010-.012" roughly. Good luck. :thumb: