its cannot seem to hold a smooth rpm count.

spark plug looks normal,
air intake is clean,
fuels getting in to the carb fine.
plug looks to be getting enough power.

worked well for a couple of months (if not a bit slow) now revs drop, seems to work better if doing very high revs. 8-9k.

just thought,,, are these bikes 2 stroke? i havent been putting oil in the petrol tank??? any ideas?

EDIT> bike is old, i mean really old, light cables are not attached any more, kind of rusty, speedo stopped at 60k km, rev counter and geers counter (n , 1-3 working, 4 not , 5 temperamental)
plates possible from another bike, also back fires from time to time