The answer to 'why is my shut-off valve foul smell' is that a foul odor coming from the shut-off valve is typically caused by a build-up of bacteria, mold, or other organic matter in the valve or the surrounding pipes. This can happen if the valve has not been used in a long time or if there is a leak allowing stagnant water to collect.
Common symptoms include a rotten egg or sewage-like smell near the shut-off valve. To solve the issue, you'll need to clean and disinfect the valve and surrounding pipes. This may involve turning off the water supply, disassembling the valve, and thoroughly cleaning all components. If the problem persists or you are uncomfortable working on the plumbing, it's best to call a professional plumber to diagnose and fix the issue.