I've always been concerned about my first gear speed. I can only reach about 30 mph (maybe 35) and then i have to shift into 2nd. The bike only seems to pull up to about 4500-5000 rpm, and then after that, it revs, but there isn't really a corresponding increase in speed.
Also, this happens in 2nd, though to a much lesser extent.

I've youtube vids of an F3 reaching about 90kmh in first, which i believe is about 55 or 60 mph.

Any ideas? All the other gears seem to be working fine.

 

i cant believe you can get an f3 to start to pull under 7000 rpms, before that mine is pretty lame

 

That's the thing, it still doesn't really pull. Like I said, it can only get up to 25 or 30.

 

i think that when mine is bouncing off the red line its doing roughly 50-55ish mph...i dunno man, you dont have a stunt sprocket on do you...haha

 

Sounds like you have a higher gear ratio than stock.

Other than that, it might be that you're not accelerating after 4-5k RPM because the clutch is slipping. In that case, it will rev, but you won't go anywhere.

 

Can the clutch slip in one gear only?

 

Yes it can slip in first gear only. Sounds like that clutch plate has burnt up. Easy fix also.