I have worked on so many of these carbs it is not even funny. There is a list of things that I go by to find the and fix the issue. 1. Clean the carb 2. Check for air leaks at intake manifold (if it is rubber metal ended, found a lot of them that you cannot see unless you slightly twist the manifold and then you will see crack if it has one) 3. Float valve 4. Ignition system 5. Air filter system (has to have the air filter in place for carb to function correctly). 6. Check engine valves clearances both intake and exhaust 7. Check compression Your problem sound to me like you have a faulty float valve or the Oring under the valve seat is leaking by. The easiest way to find out is with the carb off. Hook the fuel line up. Hold the carb level and turn the petcock on. Hold it in the level position for at least two minutes. If any fuel comes out of carb, then the float is leaking by. As to the fuel blow back when the carb is on without the air filter on it you will have some at full throttle but should not have any at mid throttle. This indicate the fuel mixture is a little too rich, but I have found on some carbs the machine will not run at full RPM without having a little. I do not rebuild any carb that has a float valve problem. The main reason is the rebuild kits are not specifically for that carb. They are always for several different carbs and says they will work on that machine. But I have found they are not exact. Best to buy new carb from OEM for the machine. I know the OEM are more expensive but you get what you pay for. I think I went through buying about 3 carbs for my Yamaha before biting the bullet and getting one from Yamaha. The ones they sell online for cheap price most likely will not be setup for your machine. If you notice in what they will fit is a range of different size machines.