Have a yz 125 2001 help coughs and sputters

I just bought the same bike. I had to choke it hard to even get it to start. Seemed as if it was running lean. It sputtered under acceleration and would die when I pushed the choke in. My first step I filled the tank the tank with 93 octane gas. This is a high compression motor. After gas swap my bike jumped out of it's skin. No more choking and very smooth through the power band and does not sputter no more. A totally different bike from then

most likly needs new rings-or bad plug if you are lucky

Possible water in the carb float bowl. Go over a whoop and a bit of water gets picked up and into the engine. Water doesn't burn well. The water is condensation from the gas tank. Try draining the carb and see if that helps. Also be sure the spark plug cap and battery terminals are on securely. Check the side stand kill switch to be sure it is securely mounted. Please rate my answer. Thanks!

Not 100% on the KPS but my guess would be a 20 year old bike I doubt if a after market cylinder would be available so more than likely still the original? The stamp KPS may even be Kawasaki Production System logo but again not 100 % Low compression would be a likely contributor to spits and splutters at high revs - some other likely possibilities would be spark plug breaking down at high load, fuel mix bit high in oil, partial blockage in carbi jets may be limiting fuel when wide open throttle.Any of these could give coughs and splutters at high RPM's These days you generally only resleeve the cylinders rather than rebore but if your cylinder is in not to bad a shape then a simple honing tool can clean them up quite nicely If you about to rebuild with new piston kit and gasket kit I would recommend full compressed air clean of the carbi as is all off anyway would only add hour or so to do then with your new piston kit you would be back to full power again
I not certain with Kawasaki ratings but all the Yamaha YZ's I build and race the pistons are only rated for 20 hours use your's would not be to much different I would guess
Good luck with your rebuild from the sound of things you should be back to normal as soon as you button her back up
regards Jamie

Have you checked the condition of your reeds? This sounds like a good place to start troubleshooting.

125 bottom end power is almost non existent, but u could look on yds possibly for jetting and carb tuning specs, it all depends on altitude and temperature.

mess in with the engine will need a carb rejet the only way to fine out what the right jetting is needed is to run the bike on a rolling road and test how much power its engine is putting out under full load,,and to fine out what its power band is now,,,thats if its has one now?? ,so you see its not just a big game of bunging on a bit here and a bit there and saying iv have sooped up me bike as it will now be a lot slower than it was before you toyed with it,,and buy the way you also need to change the air filter as well,,,now you may see were all that money go's when your racing bikes,,,1000, of bucks for only half a second off the track laps! if you realy wont to go faster,,,
get a biger bike!

your carburetor might need cleaning. The low air needle might be clogged. Try this take the air filter off, get a carburetor cleaner, start the motorcycle and right there at the air manifold(where the filter u take out)spray it with the carb cleaner and rev it but not too much. Carefully use protection for your eyes. Might be lucky and u don't have to get out the carb to cleaned it