I own a scooter which I blew the engine in Dec 2016. I since replaced the defective parts in it and it now runs great aside of one thing:

Bottom end of the throttle is slugish (I'll elaborate)

It appears that I'm running fine on Idling and top end but between 25%-50% throttle the response is not good, it bogs down.

This is really obvious if I reduce my throttle to that range from having it full throttle.

Another one is when I accelerate, it goes up then slightly down until it picks up more speed then goes high again.

After I changed the parts, I noticed that I was running way too rich on Iddle and mid, so I changed my pilot jet, which fixed the iddle but it was still doing what it currently does on the bottom end of the throttle. I lowered the needle to the lowest position. I do notice that if I have more oil in my gas it seems to run better (I'm assuming here that I must be too rich still?)

I do have some dark sludge coming out of my exhaust pipe and it does smell rather bad (smells more than when it was a 50cc)

I was hoping for some advice on that.

Kind Regards!!


  • Welcome to the site. If I'm understanding you correctly, when you rebuilt the engine, you put a larger cylinder on it and took it from 50cc to 70cc? Did you change out the carb or just rejet it? If you used the same carb, I'm not understanding why it would have been running rich. Larger displacement requires more air/fuel to make it run. Could you fill in some of the other details? May 11, 2017 at 22:24
  • your post is a bit unclear as you seem to have left out a details regarding everything done in the rebuild. that said, humor me and put you bike up on the center stand then make your observations again. though you didn't state it, I suspect you might be reporting an issue with the CVT and acceleration rather than your trouble with combustion at any particular throttle point. if you find the same issues​ than disregard, but if you notice it less or not at all, then start looking at the CVT. though to do this test right you should actually disconnect the crack shaft form the CVT completely
    – axa
    May 13, 2017 at 9:46
  • by the way how much improvement in power would you estimate you got from the big bore kit. you didn't state which engine/bike you had, but at least some 70cc upgrades are actually just 63.5... which is what is available for the 50 that I have and wonder if it's worth it.
    – axa
    May 13, 2017 at 9:50


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