Motorcycle Repair: Yamaha Grizzly 660 Top End Skip, yamaha grizzly 660, fuel additives

Question
I am not sure what caused my Grizzly to start doing this.  I have ran it through some mud, but I didn't notice it doing this any afterwards.  I did run some "sea foam" cleaner in the fuel tank, but that is supposed to clean out carbon buildup.  Anyway, when I reach about 57-58 mph (according to the speedo) I can let off the throttle about 1/4 inch and the skipping stops, and it will maintain 58 mph, but if I throttle it on up it starts skipping rapidly and feels like it is loosing its "oomph" and slows down a bit.. and toggles between 57-58, I can let off the throttle a tad and it feels like it got a boost, again.  Before I started having this problem, with no skipping, I maintained a 62-63 mph top end.  Any help would be greatly appreciated.

Answer
Hi Mike,

This is a tough problem to diagnose via the internet. I'd like to be present and perform a few checks.

Is the air filter clean and in good condition?

Using fuel additives can harm carburetor parts if label instructions are not closely followed. Sea Foam is the only additive I recommend using.

Can you describe the appearance of the spark plug?

What is cylinder compression?

Have you ever visited ATV Frontier? It's a great ATV resource.

www.atvfrontier.com

Respectfully,
Mark Shively