Not sure what a vacum piston in carb are you sure thsta what and are you 2installed vacuum piston. I can assure you you have a va um leack for sure.if you can't find it spray some aresol carb cleaner. Spray all around vacuum hoses. You know when you found leak engine will change the way it was running.
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I changed the main jet from a 140 to a 170 and it runs great now thank you for the help.
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