I'm curious if you have a cable binding not allowing for 100% full throttle

 

Or taped over.
Is the air filter clean and in good condition.
Is there a mouse nest in there?

From watching the video, I take it you were in 3rd gear WOT and it wasn't climbing much. Is that correct? If so, that doesn't seem to be right at all.

For those with more knowledge - long shot here and not the first place to look but wanted to through it out - could this be a mis-seated or small tear in the carb diaphragm?

Poorly flowing fuel (fuel filter dirty or other issue)?

Poorly jetted carb. Even though the PO says its stock jetting, it may not be. It may be time for you to get to know your carb better.

When you have it going WOT, reach down and pull out the carb (enricher) **** and see if that makes any difference.

 

Nope, there are no mods and the stock snorkel and lid are intact

 

How would I check that?

 

It has all brand new filters and the airbox is clean. I will check the carb to see if there are any leaks, but there is no gas leaking out at least.

 

This bike has 6 gears, us all of them if you want to go faster.

 

Watch the video, its topping out at 38mph in 3rd gear at 6500rpms

 

Good point. How much of a turn do you get out of the throttle grip? I think mine is a little over 1/4 turn, like maybe around 5/8 turn. IIRC, there is a throttle stop on the carb. Is the carb-end throttle reaching its max open position at WOT?

 

I was thinking this, but you'd expect surging and sputtering, I didn't notice any of this in the vid. Seemed like the bike ran smooth up to 6500 rpm then hits a wall. What happens when you shift to 3rd or 4th?? I'm thinking something is amiss with the ignition system...check and replace the plug.

 

1/4th turn, but I dont gain any more power when I get passed half throttle. Half throttle and full throttle give the same amount of power, but it still turns all the way.