What this part does Manual primer bulb that draws fuel from the tank into the supply line and holds it there until the engine is started. The built-in check valve prevents fuel from flowing back to the tank. Why you need it Reliable starting: primes the fuel line quickly so engines start more easily. Prevents fuel loss and air entry: keeps fuel in the line to avoid repeated priming and reduce hard starts. Durable under real-world conditions: resists collapse, ballooning, UV exposure and alcohol-enhanced fuels for longer service life. Key features and materials Multi-layer rubber bulb construction resists collapse and ballooning for consistent pumping. Quality rubber bulb contains a built-in check valve to maintain one-way fuel flow. Clear PVC tube provides a visible fuel path so you can confirm flow and spot air or contamination. Compatible with Mariner and Johnson outboard fuel systems (as noted). Performance highlights One-way check valve holds fuel in the line to reduce the need for repeated priming. Resistant to UV and alcohol-blended fuels, making it suitable for modern marine fuel blends. Clear tubing gives a quick visual check during priming and troubleshooting. Typical applications Priming the fuel line on Mariner or Johnson outboard motors before starting. Replacing a worn or leaking primer bulb in a small boat fuel line to restore reliable starting. Visual inspection of fuel flow during on‑boat maintenance or troubleshooting. Installation and use tips Ensure correct hose orientation for the check valve (flow direction toward the engine). Inspect tubing and clamps for secure connections and replace if cracked or hard. Prime until fuel is visible in the clear tube and the engine has fuel pressure. This primer bulb combines a durable multi-layer rubber bulb, an internal check valve, and clear PVC tubing to provide visible, reliable fuel delivery for Mariner and Johnson outboard fuel lines.