Johnson 9.9 randomly knocks off

[DoingTheBestICan]

I have a Johnson 9.9 model J10RCRA . Was out on the lake the past 2 weekends and it runs great then all of a sudden it knocks off. I prime it again and it starts right up then later it does the same thing. What could be causing this? Maybe air getting into the fuel line or fuel pump issue ?

 

[racerone]

Does it slow down and die ?-----Does it lose power instantly like you turned a key off ?------Is the fuel tank properly vented ?

 

[DoingTheBestICan]

Yes when it knocks off its kinda a quick cut off, it might die just a little bit then off. The fuel tank is a 6gal and sits at the back of the boat uncovered.

 

[racerone]

Likely running the carburetor empty due to a fuel supply issue.-------Test run with another tank and hose.

 

[boobie]

Put a timing light on it and look into it. If you lose your spark when it quits you know it's ignition.

 

[oldboat1]

^^(loss of flash=loss of spark. It's a useful trick for doing a cylinder drop test while underway.)