Without going back and re hashing everything that has been done or discussed, I think I would start over from square one and re verify what you have done. From all indications and tests, this motor should start and run.
How I attack a non running GY6
Ensure that the battery is fully charged, check kill switch is in the proper position. ( I have missed this step and felt stupid when I found it)
Remove the air filter tube from the carb and squirt a small amount of carb cleaner directly into the throat of the carb, if it starts or attempts too, troubleshoot the fuel system.
% of my fuel issues have been plugged idle jets and dirty carbs.
Check for spark, I have an inductive spark checker for this, that is reliable and quick just to see if the coil is firing, If so, then I have eliminated the stator as an issue, and normally the CDI. I have had one or two CDIs trick me that they were working but the machine still did not run until I replaced them.
Next I check the compression. I kill a few birds with one stone at this step. Remove and inspect the plug. I also ground the plug to the case with a modified short car jumper cable, and check the intensity of the spark, if weak I change the coil after I have done a compression check. A gy6 requires a minimum of
PSI to run, anything above 120 PSI is acceptable. Low compression can be caused by many things, from a simple valve adjustment to a hole in the piston.
Next, I remove the valve cover, and check the valve adjustment. I do this a bit different than some. I rotate the motor until the exhaust valve is fully open and then adjust the intake valve to .004 inch. Then rotate the motor so the intake valve is fully open and adjust the exhaust to .004 inch. This has worked well for me for many years. Then I pull the inspection plug on the fan shroud and check that the cam timing is correct at Top Dead Center.
If no joy by this point, which is really rare, I put everything back together and get out my battery powered inductive timing light. Connect it and check that the motor is actually firing at the "F" mark on the flywheel.
Odd stuff I have run across, when all the above tested "good"
A plug that will not fire under compression
A CDI that did fire the plug but was bad anyways.
A weak coil that also fired the plug and appeared to be strong
An intermittent Trigger on the stator.
Good Luck
Jim
% of my fuel issues have been plugged idle jets and dirty carbs.
