as much as it sounds like carburetor problems, I am going to try to send you in another direction.
Get a good spark tester and see what kind of spark you have. You actually could have a bad electronic ignition causing this proble.
You also may need to check the valve settings. The valves should be reset about every 100 hrs of use. The clearance should be .004" between the pushrods and the rocker arms. when the valves are completely closed.
Also you should take a look at your gasoline. If you didn't clean the gas tank and replace the fuel lines when you pu on the new carburetor, you could have the same problem because of old or bad gas...water in the gas, or more. If this is the case you will hae to clean the tankd, and clean the carb again.
I hope this gives you something else to try.
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