Mercruiser 470 Compression test question?

[jstaguy24]

Posted here recently about a boat I just bought. Had sat and had some rust in front cylinder. Boat had sat apart for awhile. I honed out the cylinder. cleaned up the block and head. No signs of warping or cracks. got new head gasket today and reassembled head. When I do compression test I goes to about 30 psi first stroke then 60 seconds stroke then releases the pressure. Feels like it coming out the exhaust port. I have not mounted the exhaust or intake manifold yet. Wanted to make sure the head and block is ok first. Am I missing something or are my valves just opening to quickly before the pressure builds? have only used this compression tester on my quads will try a test on something else tomorrow to make sure my tester is ok. any help would be great so I can get back to re assembly
thanks David

 

[Don S]

Re: Mercruiser 470 Compression test question?

When I do compression test I goes to about 30 psi first stroke then 60 seconds stroke then releases the pressure.

Not exactly sure what that means

Follow this service bulletin
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

How sure are you that the valves are adjusted correctly? Are you using the OEM service manual?

 

[jstaguy24]

Re: Mercruiser 470 Compression test question?

when it cranks it hits 30 psi first then I think when the piston hits top dead center the second time it goes 60 then you hear a hiss or release of air pressure. Feels like its coming out the exhaust valve. dont know how else to describe it

 

[Big Boat Bay Buster]

Re: Mercruiser 470 Compression test question?

Did you follow the manual? There is a certain way the head has to go on. The 470 is not a normal engine when it comes to putting it back together, including the valve train.

 

[Bondo]

Re: Mercruiser 470 Compression test question?

when it cranks it hits 30 psi first then I think when the piston hits top dead center the second time it goes 60 then you hear a hiss or release of air pressure.

Ayuh,... Sounds like your compression tester is Beat....

 

[stonyloam]

Re: Mercruiser 470 Compression test question?

Just the front (#1) Cyl? How are the others? Does the compression go up when you add a little oil? If oil does not help you may need a valve job.

 

[jstaguy24]

Re: Mercruiser 470 Compression test question?

I got about 60 psi on all cylinders today. Adding oil made no difference. I dont think im getting a good reading on my compression tester. Also dont have a battery in it yet. Just cranking off of jumper cables so it cranks slow whenever I have anything restricting the spark plug hole like my compression tester it really struggles to turn over strong. I did put it back together according to the manual. I think its ok as im getting the same reading on all cylinders. need to get a new tester i believe.

 

[jerryjerry05]

Re: Mercruiser 470 Compression test question?

Hook up the battery and do it right.Remove all the plugs,open the throttle all the way then crank till the guage stops(use a screw in tester ONLY).Your comp should be a lt more than that.J

 

[Don S]

Re: Mercruiser 470 Compression test question?

Did you read the service bulletin I gave you a link to? Doesn't sound like you did.

 

[jstaguy24]

Re: Mercruiser 470 Compression test question?

I just read the link you put up Don. SOrry missed it the first time. I will get a battery next and get it hooked up so I can get a better test as well as trying my compression tester on something else to verify if its good or not. IF I dont have my intake and exhaust manifold on yet will it make a difference? it says open throttle plate completely but I have not put any of that back on.