Hi Bruce, you prob don`t need a rebore the oil is most likely worn valve guides, mine dont last long, i need to bite the bullet and fit bronze ones. I run an ngk bp4hs and its a bit better, ive borrowed carbs from bikes that rev clean and hardly work at all on either of mine, i think you have to just muck about with settings and jets, cubs vary so much.i use a modified villiers but hear there is a good amal available now, i think cubs were made to be frustrating so the reward is greater when you get it right, dont give up all you need is time and cash!