Speedfight3 50cc LC sticking at 6000rpm

Hello all first time user so apologies if this has already been covered.
My son blew up his SF3, we have replaced head, rings, crank, bearings, piston and had carb cleaned and of course new plug.
Now it's just sticks at 6000-7000 rpm and won't get past that, in fact it has major problems on hills just bogs down. Any advice or points in the right direction would be amazing.
Have also replaced belt, new fuel filter and had the variator apart all seems fine.
Please please help save bike from a long walk off a short pier

 

My first check would be the rollers in the variator. If you fit lighter rollers then you will see an increase in rpm allowing the bike to reach its power band, possibly 8000-9000 rpm on yours. Once it can use its power then hill climbing will improve. You will find that once it hits its power band the overall 'thrust' will be better.
If, having fitted lighter rollers and it revs up to 9000rpm nicely but still bogs on hills then it will be time to look at the clutch Contraspring. One thing at a time though.
If you don't know what weight rollers are in the variator because the writing has worn away then check what the stock roller weight is and try about 1.5g lower. My stock ones are 5.6 and I fitted 4.0 rollers for the same reason. Different bike so your stock will be different but you get the picture.
Your aim is to use rollers that allow your bike to accelerate quickly to its optimum rpm for its power band and maintain that rpm until full open throttle at max speed. If you find it now reaches high revs but feels like its lacking top speed then the rollers may be too light, its a trial and error game to find the right weight rollers for your bike and rider.

 

Hi Buzz.
Many thanks for response, we have lightened the rollers as some came with the new exhaust we fitted, I use the term rollers cautiously as they were more sliders by the time we changed them.
This has improve ability to reach power band.
May have to have a play with some different weights.
Thanks again for response