Using a fuel system cleaner like Gumout to improve engine performance is a common practice. However, if your car starts to stutter after using it, there could be a few possible reasons for this issue. One possibility is that the cleaner is loosening and dislodging carbon buildup from the fuel system, including gaskets and injectors. As this buildup gets cleared, it may temporarily disrupt the smooth flow of fuel, leading to the stuttering.

In such cases, it's generally recommended to continue using the fuel system cleaner as directed, since it's designed to gradually remove carbon deposits and improve overall performance. The stuttering should diminish or disappear as the cleaner continues to work.

However, if the problem persists or worsens significantly, it's advisable to consult a professional mechanic. They can assess the situation thoroughly and provide more specific guidance tailored to your vehicle's needs.