9.9 hp vibrates at higher speeds

[Nightler]

I have 2003 Mercury 9.9 hp motor 4 stroke. It starts and runs smoothly. Put into gear and it runs smoothly pushing the boat at a low speed until it is pushed to a higher speed. It acts as if it is miss-firing at the higher speed. drop it down to a slow speed it runs smooth. Could the stator be causing this? I does not build up in vibration as a miss aligned prop would give.

 

[Texasmark]

I agree it seems to be a misfiring issue from your input. Before you get too involved, put in a new set of plugs and go from there. Plug problems will show up at the higher speeds has been my experience.

 

[Nightler]

Thanks, I will change the plugs just to be on the safe side.

 

[Sea Rider]

If it's not plugs related, could have a shot head gasket issue ? At speed small water particles are let into combustion chambers and engine starts to miss, at slow speeds none. It's a cocktail of head dilate & temp along a faulty, dried, cracked head gasket failure combination.

Happy Boating