Yamaha f350 compression

[aristos1978]

Hi,does anyone know the actual compression of F350 and the normal difference between cylinder.
We checked and found from 125 till 175psi .Is this normal?

[boscoe99]

There is no one absolute normal compression value. There are too many variables involved.

First, how was the compression check done? Should be battery fully charged, engine hot, all plugs removed, kill circuit activated, throttle wide open, calibrated gauge used, and the motor cranked until the gauge pressure stops rising.

Minimum compression value per Yamaha is 107 psi, with the test performed at an ambient air temperature of 68 degrees F. Usually one would expect to see 150 psi or more.

Also, balance between the cylinders should be within more or less 15% between lowest pressure and highest pressure.

How does the motor run? If it runs well, then more than likely there is not a problem. If a retest indicates the same pressures and cylinder differences then you can also perform a differential (aka leakdown) pressure check which hopefully will shed some more light on exactly what is causing one cylinder to have less compression than another cylinder. Maybe a decarboning treatment would be desirable. But again, if the motor is running well what is the concern?

[aristos1978]

First af all, thanks for your interest.
First we noticed some decrease of power and top end speed with same weights, so at local authorised dealer performed compression test and found 125psi till 175psi.
After that we used decarbon treatment and compression raise from 125psi to 135 psi at the same cylinder.Maximum stayed at 175psi (upper cylinder).
Next step was to make leak down check and found leak at exhaust valve.
So next step is to open head and clean valves but my concern is that even oficial dealer don't know the actual compression.
So if anyone know it would be helpfull.
Thanks.

[boscoe99]

First af all, thanks for your interest.
First we noticed some decrease of power and top end speed with same weights, so at local authorised dealer performed compression test and found 125psi till 175psi.
After that we used decarbon treatment and compression raise from 125psi to 135 psi at the same cylinder.Maximum stayed at 175psi (upper cylinder).
Next step was to make leak down check and found leak at exhaust valve.
So next step is to open head and clean valves but my concern is that even oficial dealer don't know the actual compression.
So if anyone know it would be helpfull.
Thanks.

What more do you want to know? See the excerpt from the Yamaha service manual below.

How much decreased performance have you seen? Where are you located? If you are in the southern hemisphere the temperatures are hotter than normal now. Engines will lose HP during hot and humid conditions. You might want to think twice before letting a dealer remove a head. Does your dealer have experience doing this type of work on an F350?

Did the dealer report how much pressure he was applying to the cylinder during the differential pressure check and how much was being retained in the cylinder?

[rodbolt17]

pulling a F350 cyl head isn't difficult, time consuming but not difficult.
its easiest to pull the power head and place it on a work bench.
that kind of work is difficult to do when its 8-10ft up in the air.

for my head machine work I use Flagship marine in Punta Gorda FL.

[aristos1978]

What more do you want to know? See the excerpt from the Yamaha service manual below.

How much decreased performance have you seen? Where are you located? If you are in the southern hemisphere the temperatures are hotter than normal now. Engines will lose HP during hot and humid conditions. You might want to think twice before letting a dealer remove a head. Does your dealer have experience doing this type of work on an F350?

Did the dealer report how much pressure he was applying to the cylinder during the differential pressure check and how much was being retained in the cylinder?

Hi,my concern isn't it hunt of the last knot we can and agree that is usuall to loose some power during summer here in Greece.Actually we noticed reduce about 5 knots with same conditions and after servicing,performed a compression test.Result was varying between 125 psi and 175 psi with 3 different tester and that is my concern, amount of diference!
Is that ok one cylinder to have 125psi and an other one have 175psi?
I think that 50 psi is too much!

[boscoe99]

Hi,my concern isn't it hunt of the last knot we can and agree that is usuall to loose some power during summer here in Greece.Actually we noticed reduce about 5 knots with same conditions and after servicing,performed a compression test.Result was varying between 125 psi and 175 psi with 3 different tester and that is my concern, amount of diference!
Is that ok one cylinder to have 125psi and an other one have 175psi?
I think that 50 psi is too much!

Greece is cold this time of year so temperature and humidity is not part of the problem. I thought that you might be from the land of Oz.

You now have to make a decision as to what to do. Run the motor as is or pay some bucks to have the head(s) removed for rework. Assuming that the compression differences can be corrected by reworking just the head(s) only. Maybe the piston rings are contributing to some of the difference. Then you are really getting into some major disassembly.

I know you said the motor was de-carboned but did the dealer use Yamaha Internal Engine Cleaner or did he use just a concentrated batch of Ring Free?

I bet that you have a Grady White. Correct? Good luck to you.

[rodbolt17]

you think 50 psi is to much?
I KNOW its to much.
there is an issue.
if a leakdown test has identified an ex valve leak the next step is find out why that valve wont seat.

could be a stuck valve,broken valve spring, insufficient valve clearance or a burned/bent valve.

ambient air temp wont affect that engine like an old carbed chevy.

that engine actually has an intake air temp sensor to adjust fuel based on air density(temperature) hence the term speed density EFI.
between the intake air temp and the key on barometric pressure reading the ECU gets a fair idea of air density.

at key on the baro pressure sensor feeds the ECU a value representing air pressure that stays locked into memory until the next key cycle.

most boats don't change Altitude much during the same key cycle.

the ECU does actively monitor intake air temp during the key cycle.

so back to the head removal.

its not difficult nor magical.

you MUST follow the steps in the manual as the valves wont clear the pistons nor each other.
the cam driven sprockets MUST be removed and installed correctly or you can destroy them.

like I said.
easiest way to remove the cylinder heads is with the powerhead removed and placed on a sturdy work bench.

takes about 1 1/2 hours to r&r the powerhead.

over the past 20 or so F350 block replacements I have done I do have some tricks.
its very similar to the F250 and I guess I have over 40 of them done.