I have a 2017 125D quad. It is nearly new and still shiny. A month ago it started cutting out and backfiring when accelerating. Seems to be worse after warming up.
I am a decent mechanic and i have done the following.
Replaced the carb. The new ones have air adjustment anyway so I was going to replace this in due time anyway. This made no difference.
I replaced the plug, checked the valve adjustment. Checked fuel flow to the carb.
I have 2 of these so I swapped out the CDI but still no difference. I just pulled the side cover and visually inspected the magneto. Nothing looks wrong but hard to tell without testing it.
Quedtion. Can the woodruff key have sheared a little?
before I start tearing apart my other quad and using the parts on the other can anyone here identify the issue or give me some other things to check???

 

Yes the key can get a "step" in it. Rare though, they usually do a full 180 degree spin so TDC mark is at BDC. I would try swapping coils next.

 

Ok, when I swapped out the coils on the 2 quads I noticed that one of the wires on the coil just came off without any pulling at all. I went ahead and swapped them out but I also squeezed the connector some with pliers so that it was hard to install and the fit was tight. I rode it for a while and all is good. I then fired up the other quad and it runs good as well. So I think the loose connector was the issue.
In my attempt to fix it I had replaced the carb and now this one runs better than the other. It has much more throttle response and I think it is due to the adjustable air bleed. Is there a way to make the original card adjustable or will I just need to buy another one?

 

My grandson has a tao tao 125 and same issue. the two spade connectors going to the coil pack for the spark plug were loose!
The wiring on these things is very poor and needs constant fixing
Thanks to Aw68 for his post... It helped me out alot
Jon