ar125 difficult to start and no power to run when cold?

hi guys, im not sure im in the right place for this but here goes anyway lol, ive searched my problem on google and i ended up joining here hoping someone can help.
i have a kawasaki ar125 that has a couple of problems im hoping someone can help me with, the bikes around 20yrs old (1990 model)

when the outside temp is warm the bike starts and runs sweet as a nut, but when the outside air temp starts getting cold the bike is hard to get started and when it does start there is no power to pull away or even move above 10mph. the engine runs really really slowly (changing the idel speed does nothing to help this either) and when u open the throttle it just makes a kinda boom boom noise like maybe a drum being hit and i have to leave it ticking over for anything between 5-15 mins to get warm and then it has full power and runs perfectly. ive recently replaced the piston and rings and tried a second hand carb i got off ebay but the same thing still keeps happening.

any ideas would be greatfully recieved right now as i dont know what else to try to get this problem fixed.