i had the same problem, even 40:1 was fowling the plug and was smoking like mad. i was also getting an oily mess oozing down my pipe. eventually i decided to open my carb and yeh the jetting was out

i dont know if the '93 would have the same keihin carb as my '99 but this might help you, its a clip from a site i was looking at at the time:
" For standard bikes: The '99 and '00 had KEIHIN PWK36S carby with #158 as main jet, N7NW needle jet, #40 slow jet, '99 3rd groove/'00 2nd groove from top, and '01 and '02 have MIKUNI TMX36 and TMX38 carbs, '01 has #360 main jet, '02 has #350, '01 has #50 slow jet, '02 has #45 slow jet, '01 is 4th groove, '02 is 3rd groove from the top. All air screws are 1.5 turns out except for '01 which is 2 turns.
I think your jetting is the issue, I would start from factory standard jetting and work from there unless you have a modified engine. I have an '02 kx125 with 2mm over size bore on standard main jet at 40:1 on MOTUL 800 and it runs good.
Yeh I read it straight from the manual. "
Good luck
