Rear tire and Sproket

My friends KQ 300 starts fine and runs, as soon as you put it in gear and try to accelerate its bucks and farts like crazy, not moving... I cleaned carb and it worked good for 1 day... now back to same problem.... could it be such as a bent valve????

I’m looking for a top end kit for a Yamaha. I would like to see how well the engine runs before spending $6-$7 hundred on oem. Niche has complete kits from $1-$2 hundred.  Anyone have experience with Niche or similar Chinese companies? 

I am working on a basket case for a friend.  It is a 98 Big Bear (YFM350FWB).  It came with a parts machine which is a 96, which I am coming to find out is VERY different from his machine.
 
Anyway, first thing is first.  I checked compression, 150psi.  Valves are set correctly, and the engine is in time.  I cleaned the carb, and there was some junk in the passages.  It is a chinese carb, which I figured from the lack of markings or lettering altogether.
 
The bike runs.  It idles ok, but breaks up badly under acceleration.  Pulling a new spark plug shows it is running lean.  White strap, with no black whatsoever.  So I have the carb adjusted with the mixture screw out way too much (about 5 turns), and the diaphragm needle lifted to the top slot.  It is still not getting enough fuel (or too much air).  It backfires through the carb, and cannot get above idle without carrying on.  The carb boot is in good shape, so it isn't pulling any air there.  I did test this as well by spraying ether around the boot when running, looking for intake leaks.
 
Anything big I am missing here?  I am starting to believe that this chinese carb is the issue.  This quad came with the original carb, which I would like to rebuild and use, but the mixture screw is totally stuck, and stripped way down in its recess.  
 
Thanks for any assistance.

I'm assuming, it's alright to test the engine after adjusting valves without the rear axle installed? Can someone please verify this? I'm waiting on axle parts and wanting to get that done while waiting. Thanks! 🙂

Where do yall ride at in South Florida other than the MX track on Krome Ave?