high compression, wrong f90 head?

[oldbay616]

chasing ongoing idle issues...took my boat to a mechanic who said my 2007 f90txr compression was 270s said it was way to high and will never run right and according to yamaha would never run correctly... after it ran perfectly fine for about 100 hours..

after he contacted yamaha he told me the head on my motor should have been a 6d8 head and instead had a 67f head

so my questions are,
is the compression too high ?
and 2 is there a difference in the combustion chamber sizes between these two heads

[boscoe99]

1. 270 psi sounds way too high. It also sounds unlikely. Is it the situation that all four cylinders have this same value?

2. Assuming a calibrated gauge was used, and the pressure is indeed 270 psi, then maybe the head needs to be pulled for an inspection. Excessive carbon build up can cause higher than normal compression pressures.

3. How did the shop confirm that the motor has a 67F prefix head installed?

4. Only the engineering department at Yamaha Japan will know the difference in size between the two cylinder heads.

5. Our resident master mechanic says that a direct compression test is a teat on a boar hog. Maybe the reading you got should be ignored.

[oldbay616]

i have a cheapo harbor freight gauge that read 250-260 when i checked it at home, this is a reputable local yamaha certified master tech mechanic.
they said they did a carbon treatment on the motor and the compression didn't come down at all, they noticed the head was painted a slightly different color and they said the 67f was stamped on the head.

he called up yamaha and they said based on the serial number the place i bought the boat from called them about the same motor (shortly before i purchased the boat)apparently the previous owner severely overheated it and they rebuilt it... yamaha also told the current mechanic the correct head was a 6d8... coincidently the first time i took the boat after being on plane for 5 minutes i got the hot horn and took it right back to them and they told me it was the water pump impeller... they said oh by the way there was a small oil leak so were gonna a change a gasket while were working on it... makes me wonder if they weren't talking about the head gasket.

[oldbay616]

i think i might have answered my own question, if you look at the earlier f90s that have the 67f head they have the same stroke and bore and displacement and compression ratio. they do use different pistons, however i am hard pressed to believe there is that much difference between the volumes between the combustion chambers in the head and the dishes in the pistons to create that much of an increase in psi... and yes i get the leak down vs direct compression...however i don't have low compression symptoms nor do i have widely varying psi between cylinders... so i basically have a holding pattern with a mechanic saying well this is definitely the problem

[rodbolt17]

if it ran ok before, don't sweat it.
about the worst thing that could happen is piston detonation.

my 455 olds ran about 275 PSI.

high compression wont cause an overheat.

[throrope]

i have a cheapo harbor freight gauge that read 250-260 when i checked it at home, this is a reputable local yamaha certified master tech mechanic.
they said they did a carbon treatment on the motor and the compression didn't come down at all, they noticed the head was painted a slightly different color and they said the 67f was stamped on the head.

he called up yamaha and they said based on the serial number the place i bought the boat from called them about the same motor (shortly before i purchased the boat)apparently the previous owner severely overheated it and they rebuilt it... yamaha also told the current mechanic the correct head was a 6d8... coincidently the first time i took the boat after being on plane for 5 minutes i got the hot horn and took it right back to them and they told me it was the water pump impeller... they said oh by the way there was a small oil leak so were gonna a change a gasket while were working on it... makes me wonder if they weren't talking about the head gasket.

Sounds to me a registered YAMAHA dealer didn't disclose a known issue with the sale and you have cause to request a factory remanufactured power head. That error suggests others were made. If I were you, I'd return and ask for the regional manager.

[99yam40]

severely overheated and rebuilt motor and it still overheats, not a good sign

[boscoe99]

There is no such thing as a factory remanufactured power head. Not from Yamaha anyway.

I see nothing to indicate that the motor was bought from a Yamaha dealer, and if so, on what basis? Might have been as is where is.

Am I missing something?

[throrope]

There is no such thing as a factory remanufactured power head. Not from Yamaha anyway.

I see nothing to indicate that the motor was bought from a Yamaha dealer, and if so, on what basis? Might have been as is where is.

Am I missing something?

Making assumptions to the benefit of the OP.