I have a 250 chinese atv.It starts cold.Very hard to start. When you give it gas it bogs then accelerates.When its warm it runs ok.Whats up  

the cylinder head is supposed to be retorqued after 1 month of riding.  but be sure to retorque to factory specs.  
overtorque will result is a warped valve.

as well sometimes the carb needs to be adjusted the fuel/air mixture.

turning the screw left will allow more air into the mix, turning it right will allow more fuel into the mix.

mark where it is setting before you start, and make your adjustments in small increments.

it should be set between one and a half - 2 turns.

hopefully you are burning 92 octane or higher in it.  it's what is reccommended.
change the spark plug as well the factory plugs tend to leak compression between the ceramic and the nut.
and check the motor mount often, they are really bad about breaking or pulling the weld loose.  

Is that like the same as the chinese dirt bikes(pit bikes - 50cc bikes)?  

not super familiar with pit bikes,  
it's always good to check torque on the bolts after a little while riding. remember you are dealing with aluminum engines.

they swell with heat.

if it were me i'd get a torque wrench and a list of factory torque specs and go over it periodically.

you can't hurt by torqing the bolts to what they are supposed to be.

on the 92 octane, it is what is reccomended for all atv's and dirt bikes, sadly alot of people burn the cheap stuff.
it causes unnecessary carbon build up on the piston and cylinder head.

not to mention you get a little more umph with 92 octane or higher.

the fuel air miture should be the same for all  2cycle and fourcycle alike, unless you get into fuel injection, but then you don't have a carb with injection.


most carbs set anywhere from 1 turn out - 3 and half.
just depends on the engine size and the carb cfm.

but the 50cc should be somewhere in the 1 or 2 turns range.