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Author Topic:   Question regarding testing engine compression
Greenhead posted 11-07-2002 09:32 AM ET (US)   Profile for Greenhead   Send Email to Greenhead  
Would anyone know what the desired range of compression would be for a 1987 OMC 90 HP engine w/VRO (i don't have any other specs on the engine unfortunately)? Also any suggestions at to where to buy a decent quality compression gauge would be much appreciated.

Thanks,

Hank

JBCornwell posted 11-07-2002 10:21 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Ahoy, Greenhead.

Almost any Auto Parts store can sell you a quality compression tester. Be sure to get the type that screws into the sparkplug hole.

OMC manuals ask for at least 100psi, but specify that each cylinder should compress within 15% of the others.

On the big OMC V4s it is typical to get 120 to 145lb.

What compression over 100psi you get is not important because of differences in technique, engine temp and lubrication (and differences in guages).

The "within 15%" is important.

For technique, search for "compression".

Good luck.

Red sky at night. . .

JB

Cpt Quint posted 11-07-2002 10:56 AM ET (US)     Profile for Cpt Quint  Send Email to Cpt Quint     
or
http://continuouswave.com/ubb/Forum3/HTML/003024.html
Greenhead posted 11-08-2002 09:26 AM ET (US)     Profile for Greenhead  Send Email to Greenhead     
Thanks very much for the all information as it was very helpful.

Best regards,

hank

The Chesapeake Explorer posted 11-10-2002 08:12 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
I have never seen over 120 lbs on my 1988 V4 90 Evinrude even when new. With 1500 hrs on the motor (and more hrs going on yet) I have a compression of 110 on 3 and 100 on 1 cyl. The important thing is that the motor be within a compression range of about 15-20 lbs.I have not done a compression check in a while now and need to do so and post.
The Chesapeake Explorer posted 11-12-2002 11:15 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
Just did a compression check yesterday. Port side cylinders, top and bottom 118, Starboard side top cyl. 104 Bottom 98 . This motor has over 1500hrs.. 1200 plus on hr meter put on in 1992 and estimated 300 more since I bought the boat new, The motor had been slightly siezed in its early days due to VRO problems and TCWII oils.Motor is 1988 Evinrude V-4 90HP. To test compression I warm motor up running it then.I remove all plugs then set throttle wide open..insert compression gauge and crank about 2 seconds.check reading then move gauge to next cylinder. This motor still idels fairly well! and runs good but has lost a bit of top end but I am not pushing her at top end..letting her cruise in her sunset years.
Bigshot posted 11-13-2002 09:23 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Try decarbonizing it. Rings just may be a tad stuck.

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