what could this be caused by?

[xNOLOVEx]

You might have answered your own question. You added pipe and didn'y change jets, it had a dragging feeling. The air temp changed at least 50* and you didn't change jets and it did the "Drag" thing again, hmmmmmmmmm. I have a dollar says you partially seized the piston. And wether anyone has injection or premix, ya gotta make sure it's rated for cold weather. Bean oil isn't good below about 40*

Pull your pipe off at the motor and take a peak at the piston. The ex side is where damage usually occurs first. If it looks bad try to post a pic. If it has scratches deep enough you can feel them with a finger nail, don't ride it, you will only cause more damage which is more $$$$

yeah i kinda knew the whole time it would be the jet n the mix is too lean. but just wanted to make sure it wouldnt be anything else cause the light vibrating sounds came from the top where the piston is.
the guy i bought it from there were 2 owners before him thats why i replaced half the things on there. and every so 2 months i replace gear oil.
soon im going to rip the whole thing apart n replace with new all racing parts and NOT RIDE IN WINTER lol