Regarding 80 cc big bore kit and drive belts

[comicjenius]

I have a Yongfu 50 cc scooter that I put an 80 cc big bore kit in. No increase in speed and it's shredding drive belts now. Need help, please.

[fugaziiv]

Nov 27, 2018 11:39:11 GMT -5 comicjenius said:

I have a Yongfu 50 cc scooter that I put an 80 cc big bore kit in. No increase in speed and it's shredding drive belts now. Need help, please.

We're going to need a whole bunch more information. Brand of kit? Transmission Info? Stuff like that.


Matt

[FrankenMech]

You have come to the right place.

[comicjenius]

Big bore kit is Fix Right Pro. Ordered from Amazon. Transmission is stock. Uses 669 30 18 belt.

[SMALL CC TEK]

So did you do anything to the transmission first off ? like change weights or contra spring or anything ?

Did you inspect the pulleys at all?  The front pulley is the drive pulley the rear is the driven  then the clutch and bell out back 
inspect all for wear  ..  Just  pull the belt off and grab the pulley the part that slides on one side , if you can pull it to one side or the other it should be changed .it should be tight no play at all because the  play makes your pulley lock up and not operate properly and it then overheats the belt and boom failure broken belt .   If your scooter shifted weird  or basically you felt it shift that  could be your issue , you don't feel a cvt shift tell you get some serious HP most times lol   Check the driven for that play , or did you get a Cheap Belt  ? 

[ratdog]

Do you have a tach? Stock, the engine will acclerate at around 7500 RPM. It is very common for the extra torque from the BBK to move that RPM up. From my experince, it is very common to have the bike jumping up to 8500 rpm after the BBK. While I haven’t seen it, I’m guessing there are some that do more then that. While that in its self may not effect belts, I would recommend you get that better controlled before you look elsewhere. It’s really hard to select the correct weights without a tach, but in general you will move that RPM down by going to heavier weights.. based on just the small number I have worked on, I find 6 g seem to work well. In my personal bike ( well really my Nephew’s) I use 6.5 sliders. My theory is to take advance of the added torque so we don’t spin the engine as much.

Just some general thinks about the BBK, are you using premium gas? BBK incresse your compression. The engine starts out at 10.5 :1 with a very mild cam. That by its self indicates mid grade gas, plus every one I have seen is jetted lean the builds heat as well.

I would love to see your sparkpluk that has been in the bike. A close up of the electrode would give us a hint as to your mixture.

Did you only do the BBK? Didn’t change the head or cam. Stock carb and air cleaner? No change in the CDI or coil?

Was the bike running great before BBK, or did you do the BBK to correct some problem?

While there is a chance you wouldn’t se a great deal of difference in top speed, I would think you would have better acceleration, unless there is something very wrong with your upgrade.

At any rate, not having any real info on your RPMs, I would think going to slightly heavier weights would be a good place to start.