Before you get the wrong idea, let me make myself perfectly clear: I’m talking about the balls in your fuel line–not the ones in your pants. Those fuel line balls are an important part of your fuel system, and if you keep an eye on them, you can learn a lot about what’s going on with your outboard.
Let’s say your engine ran fine for a few minutes, then quit. It’s still cranking, but the fuel line ball is COLLAPSED. That tells you that the fuel line’s venting system isn’t working properly. The issue could be as simple as forgetting to open the thumb-screw vent on top of a portable fuel tank (who hasn’t done that at some point in their boating career?) or it could be a plugged vent line—often the result when mud wasps build a nest in the vent cap and hose. In any case, your engine is struggling to pull gas from the tank but air can’t replacing it.
Now let’s pretend the motor cranks but won’t start, or it starts, then stalls. You pump the ball to get some fuel flowing, but it remains SOFT. That indicates an air leak in the lines. It shouldn’t take more than six or eight pumps to get that ball firm, so you’ll have to locate the leak and plug it until you can properly service or replace the line. Wet the fuel line down with soapy water and look for bubbles, which are a tell-tale sign of air leaks.
If your ball is HARD and stays that way, it means you probably have a faulty fitting, a fitting that isn’t seated properly (which is very common where the line meets the tank and where the other end meets the engine), or the fuel line is installed backwards. The fix is as simple as jiggling or re-seating the fitting, unless it needs replacement. And try reversing the line even if the arrow on the fuel ball is pointing the right way. A few years back thousands of fuel balls were manufactured with the arrow pointing in the wrong direction, and a few of them could still be out there.
When you see a CRACKED ball, it’s time to replace the entire fuel line. Common rubber fuel lines have a limited lifespan (be happy if you get five or six years out of one) and often the first part to go is the ball. You can replace it and re-use the rest of the lines, but this is usually a bad move—when that ball gets cracking, the rubber hoses usually aren’t too far behind.