i know this is an old thread but will post in here since its related
i got yamaha 9.9FMH and i just replaced the reed valve to 15hp.
the day after i went to test the motor in the sea the head cracked and water start comming out. some ppl told me that it doesnt have to do with the reed valve and maybe the head was corroded.
also attached there are some pics of the motor. and got some questions.
1 : which metal do i have to bend. in order to get more speed? (already replaced the reed valve)
2 : when i removed the head i found some salt etc. with what can i clean it?
3 : the pic when i removed the cover.
4 : close up of the cracked head.
i already ordered the head +gasket, and wanted to check what could have caused the crack so that when i replace it, it doesnt happen again.