From a similar experience, I would check the engine chain rubbing against the side of the upper motor mount. Even if you have a small gap between these two, the shaking and tightening of the chain while moving will cause it to rub against the upper mount bracket or bolt.
This is probably because the motor is mounted slightly too far right causing that bolt to angle left and chain to pull in towards the engine. I used bolt cutters to cut the all-thread back and left mine to squeel, file down or break the chain. See, If i mount too far left the right bottom mount bolt will cut out flush against the head when rubbing against the inside of the bottom mount bracket. Additionally, the outside edge of the left bolt hole and bolt will saw out and break evenly from vibration. the bolt and bracket are all aftermarket parts, And the dax bottom mount bracket advertised as "will Never Break"
So, to avoid the engine popping out using the upper motor-mount as a pivot; which will break your cdi, lock the back wheel, bend your chain and make you rework the bottom bracket and drill out the broken bolts
LEAVE SPACE BETWEEN THE BOTTOM LEFT MOUNTING BOLT AND BOLT HOLE... SPACE ON THE LEFT SIDE OF THE HOLE THEN TIGHTEN DOWN THE UPPER MOUNT SO IT DOESNT PIVOT> thats the side that the back gear and motor chain are on.
The motor also pivots from the center of upper mount that causes alot of change left and right on the upper engine but less change on the bottom. The bottom brackets bolts would be loose to allow them to angle left and right. Bottom bracket might pivot here too.
Hope that makes sense.