Does your Carburetor have an Carburetor Diaphragm if so here a tip. I had the same issue with mine.
Mechanics Note: If the engine is sluggish or won't allow you to run her up much past half on the throttle, and you are having difficulty getting her to full speed, look in the carburetor with the intake boot off, and see if the diaphragm is going all the way up. If it fails to do so, it is torn or leaking and you have found your problem.
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