Even without a power valve, way back we had Harry Klemm build a 250r motor for baja. all low and mid power to pull tall gears, by 7500 rpm it was done, but hitting 6th gear was like making the jump to light speed. bonus points as it was an easy motor to ride, very smooth, didnt wear you out and ran forever.

Check jetting 1st. Make sure your gas mixture is about 40:1 upto 20:1. Your jetting will change with this. A simple way to check the 1st step of jetting is to get in 3rd gear on a long smooth section of road. Hold the throttle at 1/3 opening and see what it does after a few seconds. But keep it there longer. If it starts to surge your probably lean. If it sound very high pitch and feels wrung out. It's very lean. If it's deep sounding and doesn't instantly respond to more throttle it's rich. At this point I would check the needle valve and see where the clip is. Raising the needle/lower the clip with richer the midrange. Lower the needle will lean it out. If it's better but not good. You can go up or down on the main jet. If it idle and responds off idle you pilot jet is close. All the jets are marked. Bigger the #. Richer/larger jet. Hope this helps.

plug the exhaust port, get a pvc fitting that fits the intake manifold with a tube, then a low pressure gauge then a shrader valve.. pressurize it to about 5-6 psi. it should hold that for at least 5 minutes. If it doesnt. use a squirt bottle and soapy water to find the leak. Air leaks are the enemy of 2 strokes and they are easy to get. If you have an air leak you will never get the jetting right and you will melt it down.

When jetting, get the main jet pretty much spot on, but run the needle a clip rich.. A ,lot of people seize a motor and blame the main jet, but the truth is the main jet was good, the needle was ahair lean. You run it long and hard, then back off, if the needle is the least bit lean, the minute you back off the temp spikes and it sticks.

When I rebuilt the carb and was reassembling I noticed they were running the needle either up or down one notch (5 setting with number 3 being the middle it was off one notch). I think I will start with that and see where that is set.....

I raced for a long time and worked at a bike shop for about 7 years was always tuning bikes, you can call the pipe manufacture and they will give the jetting specs to you, id start there and then set the needle in the middle clip position and set the low speed air screw usually set to 1 1/2 turns out when first starting to tune it. you can read the plug by running wide open and shutting it down the lower the clip position will give it more fuel and vice versa for less fuel. two strokes like to be wide open and blown out often i always ran motoil and mixed 32 to 1

What year is it? Are you coming from a 4 stroke? "Partial" throttle on a 4 stroke vs a 2 stroke is a big difference. 1/4 throttle on a 4 stroke just purrs, 1/4 throttle on a 2 stroke just bucks and farts...

Run it about 1/2 throttle from mid to high RPM, the powerband should consistantly pull you into higher RPM.

Was it ported? If somebody got there hands on it and did more of top end porting that could make it even worse.

Take it out, run the pizz out of it like you stole it and report back... Then we can properly diagnose.

The newer style carbs with the needle clip position is probably your best bet. The owner's manual should tell you how to do this. Back when I was riding this was almost a weekly adjustment. Have fun!

I used to ride an old CZ. you had to wind it up just to get it rolling..........but when it hit the power band you better be holding on.

It's probably carb jetting. Make sure you have the stock jets, etc, and go from there.

If everything was done correctly mechanically, i would say you're looking at jetting issues. Does it start and run at low rpm ok? Something as simple as you're float height could be too high. Pull all of your jets out and see what they are.