Sounds like a mixture issue to me.
First things first, get the bike hot, switch off, then carefully remove the spark plug.
Watch out! It won't be cold !
Have a good look at it and identify the insulator next to the sparking point. Note it's colour.
If it's white, the mixture is lean, if it's sooty or oily black, the mixture is rich.
Clean, gap and refit the old plug.
Using the stand on a chock of wood , lift the rear wheels off the ground then use my mixture setting guide in the zx50 thread area.
Once the mixture is set...
If your plug insulator was originally white, your good to go.. But,
If it was sooty/oily black you'll need to swap it now with a new plug.
Don't think you can refit a stinking plug because we need a "white sheet" to start the next step...
A plug colour check !
With a new/or white tipped spark plug fitted, Take the bike on a good run for say ten miles, then when you get home remove the plug and have a look at the colour of the insulator.
It should be a biscuity, ginger colour with a greyish tinge.
This is the correct colour.
If it's sooty black or oily black, your mixture is too rich and you need to down jet in the carb
If it's as white as it went in, your mixture is too lean and you need to up jet the carb.
There's idiots guides on YouTube how to do this.
When the mixture is correct and the spark plug is ginger, the bike will rev higher..
It's not rocket science kids...more revs means more speed !
