Ive been fighting this problem for awhile now.  Can not get my quad to run right. One minute will have top end then no topend or no bottom end  then tons of low power. I checked stator meet all ohms spec. Checked coil good Switched Cdi boxes. Biggest problem im having now is cant get it to pull clean first threw 3rd with 410 main. But 4 threw 6 will. 4 threw 6 will pull clean with 450. All grounds are clean and greased..  All electric connectors are clean. Kill switch works proper. Spark on plug looks strong. Key for flywheel is straight. Ive checked everything you can check. Its about to drive me nuts.

Reeds are good.  Brand new carb. With jeting spec builder said to run.

U may have an air leak, when u rebuilt it this winter did u split cases for any lower end work.? I was chasing jetting all over and it ended up being air leak from cases around crank, atleast i think anyway, everything led to that, when i revved the engine i would get a small amount of pressure on transmission side from vent, after i split cases i found it was assembled with no silocone or gasket, i resealed bottom and now runs better everywhere

Yep leak down test it before I fired it up.  I might have couple possibilities it coule be right now.  Motor getting a nasty vibartion in it. About to redo crank ane crank bearings and  balancer bearings.