thermostat?
I have read the responses about a thermostat keeping the water from coming out the check hole. This has not been my experience on our newer model Honda 9.9 4-stroke, and water came streaming out at start up every time. It sounds like you are still getting good flow if the water eventually comes out strong. After running for a while, it the temp of the water acceptable? (You're not overheating are you?)On my old Johnson, I had to clean out the salt/crud in that check line periodically. Whenever the flow became weak, before going after the impeller, I would first get the WD-40 and using the little straw spray a few good shots backwards down the check hole.90% of the time this freed up / cleared whatever was blocking the flow, and it was fine again.So, before taking it apart, you might try the WD-40.In any case, if the impellor is years old, its worth changing.Let us know what you find out-