What is good compression for a 9.9?

[merconeman]

I have a 1996 Johnson 9.9 that was overheated at one time. The compression reading now is 60lbs on both cylinders. It has a hard time starting and wont stay running for long. I took the cylinder head off to see if the cylinders were scored and it looks fine with no marks. <br /><br />I imagine that 60lbs is kinda low for this and it should be up near 100 am I right?<br /><br />Any advise would be appreciated!

 

[JB]

Re: What is good compression for a 9.9?

Howdy, Merconeman.<br /><br />Welcome to iboats. <br /><br />Compression in the 100s for the 9.9 would be exceptional, but in the 60s seems low. I say, "seems" because of all the various factors that affect readings.<br /><br />Because the cylinders read about the same psi I suspect that your engine is mechanically okay and that your problems are related to fuel, ignition or cooling.<br /><br />Give us more specifics about what "hard starting" and "wont stay running long" mean.<br /><br />When was the impeller last replaced? Is it electric or pull start? What were the compression readings before the overheat? What was the cause of the overheat?<br /><br />Let us know, and good luck.

 

[merconeman]

Re: What is good compression for a 9.9?

I am working on the motor for someone so it is not mine. The motor overheated because the waterpump went on it I believe. It melted the thermostat housing so that was replaced. It is a manual start motor.<br /><br />Does sierra make a head gasket for this model or will it be a dealer part only?

 

[Paul Moir]

Re: What is good compression for a 9.9?

An overheat would certainly account for the low compression. Does the old head gasket show any signs of burn-through?<br /><br />The Sierra 9.9 hp head gasket does not fit the '96 model. There was a change made in '92. You must get your head gasket from a dealer.

 

[merconeman]

Re: What is good compression for a 9.9?

The head gasket looked fine. The cylinderhead shows paint bubbling from the heat. The cylinders like I said look good. What is the next step? <br /><br />Is 60lbs real bad? I am usto motors with 120lbs

 

[Snowdrggn]

Re: What is good compression for a 9.9?

I have a 1989 15hp Evinrude, same powerhead as 9.9, with about 600 hours on it. Shortly after a thorough decarb, it has 104 psi on the top cylinder, and 106-107 psi on the bottom cylinder, when taken on a warm engine. The calibration of the gauge was verified within a couple of weeks before the reading was taken. This engine has been run at mostly higher rpm ( not a kicker or used for trolling).<br /><br /> For what it's worth?<br /><br /> Cheers,<br /> Snow.

 

[steviecops]

Re: What is good compression for a 9.9?

I've got a 1986 9.9 and the compression is 105 and 107. I recently got rid of a 1977 9.9 and the compression was 110 in both.

 

[wayne h]

Re: What is good compression for a 9.9?

i have a 9.5 with 70 and 72 on the cylinders the motor runs great so i am not touching it. like was said before in an earlyer post there are alot of factors.are u testing it at wide open throttle also its a rope start so u might not be pulling her fast enough as an electric start would. how long has the motor been sitting sounds like u might have a carb problem.

 

[Paul Moir]

Re: What is good compression for a 9.9?

I'm no expert on this, but I have visions of piston rings stuck from scraped aluminium. You know, the sort of damage you get when an engine overheats and the piston siezes in the bore. <br />Surface the head to make sure it's flat and goop up the old head gasket (just for testing) and bolt the head back on. If your compression improves to good, take it off, clean it up and order a new head gasket (and count buddy lucky!).<br />If not, my next step would be to take off the intake cover (on the port side?) and poke at the piston rings to try to see if they're stuck.<br /><br />Conceivably, the rings could just be stuck with carbon, and a good decarb might break them loose. But I belive that's wishful thinking.<br /><br />Tell us what you find!