Yamaha C85 compression pressures-

Status
Not open for further replies.

Tommy in FLL

Seaman Apprentice
Joined
Apr 10, 2009
Messages
44
The engine is a 1996 C85TLRU, a pretty typical 85 hp motor. It is essentially the same engine as the oil injected 90 hp motor, just with lower compression. 5.2:1 to be exact.

This engine has a problem with overheating after about 30 seconds under full power. It squirts water just fine, though with the thermostat and the water pressure regulator removed.

Initially I thought I had a head-gasket issue, since when I ran the engine with the thermostat cover removed, exhaust came out. I replaced the head gasket, and in doing so noticed that the outside of one of the barrels was chipped and repaired with marine tex. That and there was considerable corrosion and "spooge" in the water jackets. After the gasket went in, the symptoms were the same, still overheating.

Compression check: 85 psi/ 85 psi/ 88 psi, checked using a borrowed tool from the parts store and done according to the Yamaha manual.

Does this sound low?

Thanx in advance!
 
Joined
Oct 28, 2010
Messages
923
Re: Yamaha C85 compression pressures-

The general rule of thumb is if it is within 10lbs or 10% of each other it is decent anything over that presents a problem.
 

Tommy in FLL

Seaman Apprentice
Joined
Apr 10, 2009
Messages
44
Re: Yamaha C85 compression pressures-

I guess the head gasket probably isn't an issue then.

How does one get all the crud out of an engine? This has been used in brackish water all its life-

Thanx!
 

sflwesty

Recruit
Joined
Aug 23, 2010
Messages
3
Re: Yamaha C85 compression pressures-

You probably have to put the thermostats back in.My buddy has an e-tec and 1 time he forgot to screw in the pee hole back into the hose flush and the motor would run fine at low speeds, but at high speeds it would overheat due to the motor not having enough backpressure.The water was not staying in the block long enough to cool it.
 

Tommy in FLL

Seaman Apprentice
Joined
Apr 10, 2009
Messages
44
Re: Yamaha C85 compression pressures-

Funny you should mention the t-stat/pressure regulator.

I put those back in today, and started the engine. It no longer peed, but never overheated [did not take it about 1200 rpm or so], and a pyrometer aimed at the head showed about 135 degrees.

Hmm....
 

Friday

Recruit
Joined
Nov 19, 2017
Messages
1
What should the compression for a 1995 Yamaha C85 outboard motor be at when it was brand new
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
Re: Yamaha C85 compression pressures-

Funny you should mention the t-stat/pressure regulator.

I put those back in today, and started the engine. It no longer peed, but never overheated [did not take it about 1200 rpm or so], and a pyrometer aimed at the head showed about 135 degrees.

Hmm....
Maybe stick a section of monofilament/weedeater line into the pee hole to unplug any restrictions....
 

GA_Boater

Honorary Moderator Emeritus
Joined
May 24, 2011
Messages
49,038
7 years old. Tommy has been gone for 5 years.

Friday - Start a new thread of your own.

Closed.
 
Status
Not open for further replies.
Top