Sounds like you need a new power valve in the cylinder. They can go bad and thats how they will act. You have to take the cylinder off to change it. If your that far you might as well put a new piston and ring in it.
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There is reputed and there is actual horsepower!
A reputed 52.5 Hp for the 1990 is not right.
A reputed 57.5 Hp for the 2007 maybe right...at the crank.
You lose 8%-10% at the wheel, 5% more with an O-ring chain.
Standard KX250 would be around 42-45 Hp in peak condition. Intake mods, jets, expansion chamber and some porting changes may give you more but it's give and take...you will lose bottom or mid-range and tractibility will go out the window with a very peaky motor. Computer search KX250 forums and canvas the responses and also look for generic KX250 link's.
The power band is the point wheneverything is nominal and and peak performance is realized. The air/ fuel ratio , timing, rpms It's mor predominant and noticeable in a motorcycle because of handling and weight All engines have them and these "power bands are really the peak of what s also called the torque curve Band meaning sp[ecific point and duration everything is working together. a gas lawn mower engine is set up to run inside this band. Thats why it doesn't fall on its fsce on the first patch of grass.
A diesel engine has a specific rpm range it has to maintain under load to get nominal horses.. If everything is functioning at nominal, max power is put out. Surprisingly enough it can be felt in all engines if paid attention . That's all it is, no special "bands" or parts. Knowing where the power band(range is makes a big difference on any machine. On an electric motor peak torque is @ let's say 1850 rpms. go faster useless lower toyque. go slower power drops To keep the range of the power band constant. everythinhg needs to be constant, like fuel mix, spark plug condition ,air filter andcarb adjustments.. I had a bike I hate and feared the power band. I would fall into it not paying attenyion and it would litterally crawl out from under me. All it is is that narrow band of peak performance..
You can experiment with different expansion chambers. The cone shape of the chamber bounces a sonic shock wave back to the exhaust port. The wave forms a pressure "wall" at the port to "seal" the exhaust port so intake gasses don't escape. Change the wall timing and you change the powerband. This is the expensive way and with no guarantees. The bike may already have the best possible chamber.
I suggest you go a different route. Just go up about four teeth on the rear sprocket or down one tooth on the front sprocket. This will mean the bike will have a lower top speed but more torque and earlier powerband relative to mph speed. Bottom line is you will come out of the hole quicker with higher revs and reach the powerband sooner. The loss of top speed is overcome by the speed out of the corners. Generally, more time is spent in first, second and third than the brief moments spent in fifth or sixth.
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Try the air filter first (make sure it's clean). The needle valve on your carb slider could be bent, or not seated correctly or just set wrong. If you have a power exhaust valve, it could be set wrong or malfunctioning as well. Good luck.