Multiple problems

Hello all,

I have been lurking here for a while and figured I would ask the experts about some issues I am having.  I bought my son, who is five, a chinese ATV (not sure of make) off craigslist as a project.  After several days we finally got it running.  Its a 50cc fully automatic.  Here are my issues.

1. With the airfilter snorkel connected, the machine seems to lack power and will sputter at higher RPM's.  I can get the sputtering to stop by either disconnecting the airfilter snorkel or by slightly opening the choke.  The machine will run fine either way for a period of time (20-30 mins).  I have read the "carb adjustment" procedure listed here and there seems to be no difference.

2. Although the machine does ok for the first 20 mins, not withstanding the above, it does seem to lack low end power.  I have read that this is a problem with the fully automatics.  Is there any kind of clutch upgrade?  It seems that the clutch is engaging at too low of an RPM.  It does have an adjustment screw, however, this does not seem to have any impact.  Infact, I have had the side cover off, and there is nothing on the inside of the case that touches the clutch.  This screw will just spin when adjusted.  

3.  After 20 mins, the machine will bog down and barely move.  I notice a lot of smoke coming from the breather tube.  It is almost like the clutch is stuck ingaged.  Once it cools down, it will run as above.

Thanks in advance for any input.  This has been a fun project and my son has enjoyed it.  Next step, one for Dad too.  

it sounds like all of it is in the carb. have you cleaned it since you got it? or put new gas in it. also, im not to familiar with chinese quads, but just drill some holes in the air box lid. somehow it needs more air. and when dad gets one, dont go with chinese. get a honda!  
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2005 honda trx450 golden west graphics, hot cam, high compression piston, hmf exhaust, nerf bars,works shocks, golden west seat cover, one industries bars, and more

Thanks for the reply.  I have taken the carb off and cleaned it twice.  I actually ordered a new carb, turned out to be slightly different and I just swapped the jets and float.  I have probably gone through 2 tanks of gas since we got it running so I doubt it is bad gas.