Anyone have any insight as to why my jest ski will only start when I remove one plug?

Question

It cranks but wont turn over. I have tested both plugs and they are fine. The only way the jet ski will start is if I have only one plug in, not both??? Why?

Answer 1

I can only think of one reason:

The starter motor isn't able to spin the flywheel fast enough to generate the power that provides spark when both plugs are in.  With one plug out, it's only pushing against half as much compression.  

Possible causes:  weak battery, corrosion/loose terminals, worn out starter motor needs rebuild.

Answer 2

try switching the spark plug wires whith one plug in...you may-be

getting spark from 1 of the wires.