Hawk 250 problem after changing rear sproket
 

I changed out my rear sprocket and it seem like I don't go any faster, I was at 62 with 50t went to a 45 and struggled to get to 62 not sure what happened?

Any other mods or still stock? Both of my Hawks wouldn't go beyond 62-63mph until the exhaust was installed and the carb rejetted. They are just too choked up from the factory to push any more. Beyond that, if the bike is modified more than a rear sprocket then it will just take time for the motor to break in. Until they do break in they tend to struggle a little.

I put 17t in front have carb rejetted. I think maybe it's a carb issue when going slow around twenty it has a little shutter to it. It's also 20 degrees cooler out since last ride.speed was checked by GPS as my speedo is off by atleat 5mph.

So around 20mph, 2nd or 3rd gear, light throttle? Is it like a hesistation or stumble when you hold the throttle steady? If so, that is a lean condition and generally means you want to step up a size on the pilot jet. A common attempt to band-aid this is to raise the needle a notch or two and/or richen the idle mixture. The standard pilot jet is a 40, and for the most part is adequate for the Hawk with no mods, but it can still be a bit lean. A 42 or 45 pilot is more ideal for cooler temps and/or low altitudes. The only current source for said pilot jet is through nzbrakelathes on this site, so you can get in contact with him.