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With out see it running on a scanner. Maybe the IAC or throttle body. Does it have any codes. Did you erase codes after rebuild? Any time I do a rebuild on engines and transmission, the codes have to to erased. Other wise you are trying to run a new part on old PCM codes. Codes have to be erased.
the question is did you remove the ridge at the top of the bore before you put the new piston and rings in. If you didn't then the knock could be from a broken top ring or broken ring land under top ring
all cylinders wear up to where the top ring stops and when new pistons and rings are fitted this wear point hits the ring as with a new piston the ring now travels further up the cylinder
usually a 2 stroke will chuff/backfire out of the exhaust if it low on compression, did you put the rings in the right grooves and up the right way and is the piston in the right way around as there is a front and back ,the ring pegs are supposed to face the inlet side of the barrel.
With any naturally aspirated motor, aside from timing, your fuel/air mixture is crucial at part-throttle as well as at wide-open throttle and off-idle. Sounds like your running too lean on the top end and too rich at part-throttle and off-idle. You should focus on either preliminary adjustments to your carbs; rebuilding or replacing them. You may also consider re-metering and re-jetting your carbs.
They can do this from time to time. What can happen is the expansion chamber gets pumped with fuel vapor and then can be ignited, causing a large backfire. I hate to attempt to answer your question with a question, but how old is the piston and ring? A ring won't last very long in a 125. For racing, it's changed every race. For joy riding 15-20 hours of run time and it's time for a ring at least. Weak compression can make it impossible to start. If you have perfect compression the cause for the non-start could be something as simple as a burned up or fouled spark plug.
Start by re-ringing the motor (you can do this for about $25 inside of 2 hours time). Clean the carb out, clean the air filter, fill the tank with fresh fuel and replace the spark plug (NGK B9ES - from Autozone for $1.89). If the bottom end of the engine is good, the above steps should get it fired up - providing your ignition system is good, and based on the apparent backfire, I would say you have spark.
My first thought is low compression... which leads to a few questions...
What fuel to oil ratio have you used?
What do you mean by 'fresh top end'? To me a fresh top end includes piston and rings and cylinder bore, gaskets and crank seals. Anything less on a 2-stroke is an incomplete job and usually leads to premature failure.
Have you followed break-in procedure on your fresh top end? Double check the head bolt torque.