What is compression spec for 1998 Yamaha 90hp?

[Mark42]

Found a used 98 Yamaha C-90 for sale. Want to check the compression, but I don't know what it should be.

Can anyone supply the compression spec for the 98 C-90?

Anything unique to Yamaha I should be aware of? (I am familiar with Johnson and Force designs, but not Yamaha).

Thanks for the info and any advice!

Mark.

 

[robert graham]

Re: What is compression spec for 1998 Yamaha 90hp?

Same motor as mine. I believe compression readings are supposed to be generally in the 90 to 120lb. range, but more important is that each cylinder should be nearly the same reading, give or take a few pounds. For instance, if you had 110LBS on #1cylinder and 120lbs. on #2cylinder, but say 70lbs. on #3 cylinder, it could indicate an issue on #3cylinder. If I was doing a compession check on mine, I'd take my time and get several readings on each cylinder to be sure they were correct. I've had great performance from my 90C since buying it new in 1999, with no problems at all. Install a fuel/water separator and use the Stabil Marine all the time.

 

[Mark42]

Re: What is compression spec for 1998 Yamaha 90hp?

Same motor as mine. I believe compression readings are supposed to be generally in the 90 to 120lb. range, but more important is that each cylinder should be nearly the same reading, give or take a few pounds. For instance, if you had 110LBS on #1cylinder and 120lbs. on #2cylinder, but say 70lbs. on #3 cylinder, it could indicate an issue on #3cylinder. If I was doing a compession check on mine, I'd take my time and get several readings on each cylinder to be sure they were correct. I've had great performance from my 90C since buying it new in 1999, with no problems at all. Install a fuel/water separator and use the Stabil Marine all the time.

Robert, Thanks for the reply. The owner of the motor says the compression is 125 each cylinder, but I have read posts where compression was as high as 129. Just trying to get an idea of how much time/wear is on the motor based on compression drop from stock spec, so the factory spec would be a help. Do you have a manual for the C-90?

Glad to hear the motor is serving you well. I chose this as a replacement for my '85 85hp Force because of its light weight, simple 3 cylinder design, and power tilt/trim. Hope it works as well for me as it does for you!

 

[robert graham]

Re: What is compression spec for 1998 Yamaha 90hp?

I'll check in my manual tonight and get back with you. Do you have any reason to believe that there's any problem with the motor? Best way to tell for me would take the boat/motor out to the lake and run it, fast, slow, start, stop, idle, troll, etc. Main issue for me would be if it's a saltwater or freshwater motor. A motor that's been in saltwater for 12 years may have issues, whereas a freshwater, well-maintained motor could be pretty much like new, mechanically. I know mine runs just like when I bought it. Check that motor out on the water and good luck!

 

[Mark42]

Re: What is compression spec for 1998 Yamaha 90hp?

I'll check in my manual tonight and get back with you. Do you have any reason to believe that there's any problem with the motor? Best way to tell for me would take the boat/motor out to the lake and run it, fast, slow, start, stop, idle, troll, etc. Main issue for me would be if it's a saltwater or freshwater motor. A motor that's been in saltwater for 12 years may have issues, whereas a freshwater, well-maintained motor could be pretty much like new, mechanically. I know mine runs just like when I bought it. Check that motor out on the water and good luck!

I just see different compressions listed in different ads. One says "120 psi on all cylinders, just like a new motor" another ad says " 125, 127, 127 like new". So I'm wondering what is the spec?

 

[99yam40]

Re: What is compression spec for 1998 Yamaha 90hp?

there is not a written spec.
If they are within 10% of each other call it good.
Find out when the last servicing was done and what was done at that time. Hopefully with receipts to make sure they are not just saying it was done. Other wise you will have to do it all to make sure it has been done