Moped only goes max 15miles per hour??

Its a deristrected 50cc Chinese scooter db50qt -11 and it was running fine 45moh max i used to get until one sunny day couple of days ago where it would firstly not go faster thsn 37mph which i thought okay its boiling hot so the cooling might not be ideal as it could get really hot ewsliy so it wont let me ride faster. I decided to clean my ped as it wqs a nice weather and after that it wqs the same 37mph max but the next day it wont go above 15 or 20 maybe (hardly) it sounds alright and i have no clue whats uo. Oil was changed 600km ago plus im about to fit a new oil filter and air filter but if that doesnt work. Im clueless

 

Boiling hot days won't make any difference to the speed in this country, you should still be getting 45 mph.
Strip and clean the carb, blow all the holes through, check the belt and rollers for wear.
Check the clutch for wear

 

What air filter you hot on you say you washed bike what did you use to wash as might have either soaked air filter or got water in carb

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im putting money on carb being full of water, or maybe no oil? have you checked your oil recently? i check mine once a week maybe more...
welcome by the way!

 

My bike used to do this after a jet wash as had a open cone on it lol had to drain carb all the time lol

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