It doesn't slip when going up hill unless you try to put one gear down it kinda jumps.. What to look for in the oil? Check temp of oil after starting? I noticed it gets very hot also and can smell an odor. The noise is a very noticeable and doesn't sound rite like something slipping out of 3rd gear grinding

 

O sorry didn't see comment about metal shavings lol we'll check that out, Ty

 

please make sure that the exhaust pipe at the jug/cylinder is not glowing red. if it is, your carb is running too lean and needs to be richened up a bit. hmmm...grinding in 3rd. hopefully it is a vibration as mentioned. maybe i missed it, but how fast is the quad going when this sound shows up? about how many RPMs is it? 1/4 or 1/2 throttle or more?

 

I don't think it gets red hot but will check.. What does richen carb mean? I doesn't burn much gas I noticed if that makes a difference. We will drain oil tomorrow and put new oil, people who we bought it from let I it sit all winter. I'll let you know what we find then.

 

O, the noise happens when going lil over half throttle very noticible to where u have to stop just for it to quit

 

by richen it, i mean you are running too lean. not enough fuel to air ratio. if you richen it up, the exhaust temps will be cooler cause you have more flue gas exiting your exhaust valve pulling heat away from the engine.
this quad was a semiauto tranny, right? now i'm curious if the centrifugal clutch may have an imbalance or a sticking clutch part. hmmm.....like buttons? i'm no clutch expert though.