Cope Marine Advice: Why Your Outboard Loses Prime & How to Fix It
At Cope Marine, we often hear boaters ask: “Why does my outboard lose prime?” The answer usually comes down to one thing — an air leak somewhere in the fuel system, typically between the engine and the fuel tank.
Here’s our expert advice on how to diagnose, troubleshoot, and fix the issue so you can get back to smooth running on the water.
Common Symptoms of an Air Leak
You’ll generally notice one of two things:
The engine runs out of fuel and dies unexpectedly. After sitting for a few minutes, you have to re-prime the bulb to get it going again.
The engine struggles at wide-open throttle — sputtering, surging, or running rough because of aerated fuel.
Both of these point toward a fuel system air leak, not necessarily an engine problem. Let’s walk through how to find and fix it.
Different Engines, Different Behavior
4-Stroke & Fuel-Injected Engines:
These engines store fuel in a Vapor Separator Tank (VST) or Fuel Supply Module (FSM). Since they have electric fuel pumps, they rarely lose prime unless there’s a serious leak. Even with a small leak, they’ll usually fire right up when you turn the key — until that leak gets bad enough to cause problems.
2-Stroke Carbureted Engines:
These are much more sensitive. Because of their carburetor design, fuel can siphon back into the tank when the engine sits overnight. You’ll often have to re-prime before each morning start. If you have to prime every time you start up — especially after sitting overnight — there’s probably an air leak somewhere in the fuel line.
Step 1: Visually Inspect Your Fuel Lines
Start at the engine and follow the fuel line all the way back to the tank. Move it around and look for:
Cracked, brittle, or soft hoses
Loose or missing hose clamps
Damaged or leaking primer bulbs
Extra splices or connectors (each one is a potential weak spot)
Rusty, loose, or corroded fuel filter fittings
A rusted or loose filter housing can let air in — allowing fuel to drain back toward the tank and leaving your system unprimed.
Step 2: Troubleshoot with a Clear Hose
Once the visual check is done, it’s time to get hands-on.
You’ll need:
A 1-foot length of clear fuel hose
A hose barb fitting the same diameter as your fuel line
How to test:
Remove your engine cowling and locate the fuel line where it enters the engine (usually at the fuel filter).
Disconnect the line and insert your clear hose.
Prime the bulb and start the engine.
Watch the fuel in the clear hose closely.
If you see a stream of air bubbles coming from the tank side — bingo! You’ve found evidence of an air leak upstream.
Step 3: Follow the Bubbles
Reinstall the fuel line where you started, then move down the line toward the tank, testing at each connection point with your clear hose until you find where the bubbles stop.
You can also start directly at the fuel tank pickup tube if you suspect the tank. If you see bubbles there, the problem may be with the pickup tube or its fittings.
We’ve seen it all — from cracked hose barbs to corroded fuel filter brackets that looked fine on the surface. The clear hose test doesn’t lie!
Step 4: Fix It Right
Once you’ve found the leak, repair or replace the problem part:
Replace damaged hose ends and re-clamp securely
Swap out cracked fittings, filters, or brackets
Replace weak or old hoses entirely
Then re-prime the system, test again, and get back to boating!
Important Tip: Know Your Fuel Hoses
Not all fuel lines are created equal. Blue and gray hoses — especially the blue ones — are notorious for internal liner separation, especially when exposed to ethanol.
When the liner collapses, the fuel pump can’t draw properly, leading to fuel starvation and erratic running. If you spot these hoses, plan to replace them sooner rather than later with ethanol-resistant marine-grade hose.
Step 5: Properly Prime Your Primer Bulb
Here’s something most boaters don’t realize — there’s a right and wrong way to prime a bulb.
If the bulb is held horizontally, it may not move any fuel at all. Always hold it vertically, with the arrow pointing upward. This ensures the internal check valves work correctly and fuel flows toward the engine.
If the Primer Bulb Is Air-Locked
Sometimes fuel drains too far back into the tank, and the bulb can’t pull it up again. Here’s how to fix that:
Disconnect the fuel line from the engine side of the bulb.
Hold the bulb vertically, arrow pointing up.
Squeeze it until it’s completely collapsed.
Cover the open hose barb with your finger, then release.
Repeat several times until you feel fuel reach the bulb.
Reconnect the line and prime as normal.
If you still can’t get fuel, double-check that your tank has fuel and that the pickup tube is submerged.
Final Advice from Cope Marine
Air leaks in your fuel system are frustrating, but they’re usually simple to fix once you know where to look. By following these steps — visual inspection, clear-hose testing, and proper priming — you’ll track down the problem, fix it right, and keep your outboard running strong.
If you’re unsure or need help diagnosing a persistent loss of prime, bring your boat to Cope Marine. Our certified technicians can quickly locate and repair leaks, replace worn components, and make sure your fuel system is running safely and efficiently.
Because at Cope Marine, we don’t just sell boats — we keep you on the water.