kx 125 stumbles in first

[dirt track killa]

I just sold my buddy a 91 kx 125,I had it for a couple of months.
When I got the bike it was spoogin like crazy,quick carb tune up and it was great the few times I rode it.

So my buddy buys it from me and he has had it for a few weeks now.

For some reason it has developed a big stumble in first gear only,when transitioning into the power band.This happens no matter what throttle postion.
I have no idea where to begin to look.I checked to see if the exhaust valve is opening at rpm,and it snapped open and closed just fine.I checked the reeds they are fine.I checked for air leaks none.

I opened the carb and cleaned the heck out of it still the same.

I am lost could it be electrical?
Any other gear and it pulls hard and revs out. :bang:

 

[dirt track killa]

ttt

 

[trial_07]

For some reason it has developed a big stumble in first gear only,when transitioning into the power band.This happens no matter what throttle postion.

Could you clarify this?