Sounds like youre 4-stroking, firing every fourth stroke instead of every 2nd. Buy some jets, theyre pretty cheap. I think whats in the stock carb is a 70, or so i remember someone telling me, a 66 runs great in most cases on stock engines. Get multiple sizes, not just a 66, depending on your elevation that may not be what you need. I have a 63 in mine, debating if i need to change it or not, still testing, juat put it in yesterday, and it solved my 4-stroking issue, (yep, same issue) but im worried if its goin to get to hot. Ill revaluate that after a few rides.
Edit: i also want to mention i dont have a stock engine, upgraded carb and expansion chamber muffler, an a 32t sprocket. Debating switching back to my 36t, or buying a 34, slows too much on hills for my liking. Got a pullstarter otw in the mail, and plans to buy a better head in a few weeks.Thus leading to my decision to try the 63.