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I have a Troybilt self propelled 6.75. I rebuilt the carburetor and put fresh gas in.It started right up but it was running bad kinda like surging. by that I mean it would speed up then slow down. I put a new Carburetor on and it does the same thing. Does anyone have any ideas what could cause this?

Could be your intake tube is cracked. That will allow air to enter the fuel system past the carb. Does this engine have a mechanical or Air vane governor?

post complete model & serial numbers

Did you replace in intake gasket(s)? Surging is sometimes caused by a lean condition.