Bike starts and runs, but has weak throttle. Left cylinder seems to not be firing. Has spark but not sure about compression. Could the valve seals be bad, causing it not to fire?
I just replaced the valve seals on my 250 I had the head off for this and used an automotive valve spring compressor to remove the keepers and springs, the seals were relatively easy to replace. They have nothing to do with the compression issue they only seal against oil leaking down the valve guide.Never replaced the valve seals on a Rebel engine, but you should be able to do so without pulling the head. You really need a service manual to do this. It will give step-by-step instructions along with pictures, provide torque values, etc.
The longer the disassembled carb soaks in Chem Dip, the more likely it will be completely clean.