Engine Trouble Bike wont go faster than 24mph with 36t sprocket

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BikePete

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Hey everyone my name is Peter and i need help. My bike is a 2009 zoombike and it has a ported outtake just wanted to throw that out there maybe that will help with my problem. Anyways my problem is that for some odd reason my bike wont go faster than 24mph with a 36t sprocket. I don't get it. Before i had a 44t and did 24-25mph! I think im running fuel ratio fine because i did a spark plug check and its black but dry. Thanks
 

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Meh...it looks rich to me...a good color for a plug is chocolate brown in color with no oily residue.

Is it the stock carburetor?
 
Its the performance carb or HP carb which ever u guys call it. Lol. Im using a heat lever 7. I have a 5 and 6 too. Should i go lower? The temps here are low 70s. And also right now im running a size .70mm jet and the next smallest i got is a .64mm u think thats too small? Thanks for answering by the way.
 
Do you have any other mods done like porting or a different exhaust?

The HP carb is a clone of a 15mm Delortto carburetor.
 
The only thing that i think is special about this bike is that the outtake is a oval instead like a circle. I believe that would be called ported cylinder outtake right? And i have a stock exhaust
 
Outtake meaning the manifold the carburetor is attached to?
 
Sorry mean the hole where the exhaust goes to. Its a oval
 
Ok..thats what we call the exhaust port.

Is it smooth running from the start all the way to top end?
 
If its smooth then the needle is probably ok but i would re jet with a smaller jet as that is what regulates the fuel at wide open throttle...going from a 70 to a 64 is a big jump but most stock motors with the stock 14mm NT carb seem to like a mid 60s jet size.

I would try the 64 jet but change your plug with a new one and check it after a top end run..if its got any ashy white to it then jet back up untill it turns a chocolate brown in color.

Investing in some jets is a good investment.
 
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