"1/2 the speed seems more like
"1/2 the speed seems more like a one cylinder issue than a spun prop. A spun prop will only allow very slow movement (trolling) but as soon as you give it more gas the rubber hub on the prop will disengage from the drive and it is effectively like having no propeller on the motor. The motor just revs up (since the prop. is spinning on the hub).
The quickest way to determine if both cylinders are working is to run the motor at a slow idle and pull one spark plug boot at a time. The motor should stay running on one cylinder. If you pull a boot and the motor stalls, it tells you that the other cylinder is not running. Run that test first. Use insulated plyers to pull the spark plug boot as you may get a little shock through the rubber when using your fingers. It is a very high voltage that goes to the spark plugs."