While trying to show off at the beach, I learned that if I sit way back on the rear rack, the SP90 will pull a wheelie very easily. Unfortunately for me, it does it a little too easily and I ended up on my butt. My son thought this was hilarious.

 

Pechmial you now have a 93cc machine my friend. On the filter issue, Change it. Go with a Uni filter. You will have to go up to a 85 jet for the carb but it is easy to rejet.

 

Just out of curiosity, how do you figure it's a 93cc?

I know it's probably not the 110cc's the head kit claims but there is a visable difference between the head diameter currently on it from the stock one. If it was 89cc's to begin with, I am certain it's over 93 now.

On the filter issue I've still not decided what to do. The kids DO take this through puddles and mud so any filter change will require some sort of new airbox. Rejetting the carb will probably happen at some point in the future, as will some modifications to the tranny. I'm taking it slow, though, as some of the riders on this machine are still running with the throttle restricted.

 

The cylinder you recieved is a 53mm cylinder. The stroke on the SP 90 is 42mm. R2*3.14*42= 92.6cc Stock your cylinder was a 52mm. Hope that answered your question. It should have given your machine a little more bottom end. The difference might have been to slight to notice tho. Did your kid feel any difference?

 

I obviously can't ride the little thing, but all the kids who have noted quite a difference in power. There was no increase in top speed (which I believe is a restriction of the tranny), but the thing scoots up hills much better than when stock. My eldest son, who is 12 and certainly not an expert, says there is "20% more power!"

Now you got me wishing I'd taken some measurements. The CC numbers really aren't that important to me (who wants bragging rights on a mini?) but the increase in performance is fairly visable.

I bought the kit from here where they advertise it as a 90cc to 110cc kit. This is what I've based my figures on. That and the increase in performance (which, as you stated, I wouldn't see with only an additional millimeter).

 

Great! I'm glad your kids like the addition. Most people that modify these engines start with their cyclinder. Us included. I usually recommend to go with this cylinder kit for a new top-end. The speed is limited mainly by your sprockets. If the front is 19 tooth then that is as big as anyone has for the front. The rear can go as low as 25. I think the SP has a 32 stock. The take-off speed is mainly controlled by transmission mods and horsepower. Tommy Richardson