Johnson 3HP JW-17 Compression woes

[outbdnut2]

For example: I once read 60 pounds on a 3 HP JW with the checkvalve at the gauge, and if I remember right, it was 80+ with the valve a the cylinder, but it will vary with the length of the hose and the inside diameter, like Frank said – it’s the volume of the hose, and different brands willuse different lengths and ID.
Dave

[Buccaneer]

That would be quite a difference in readings. Thanks for sharing
Frank and Dave.

quote outbdnut2:

For example: I once read 60 pounds on a 3 HP JW with the checkvalve at the gauge, and if I remember right, it was 80+ with the valve a the cylinder, but it will vary with the length of the hose and the inside diameter, like Frank said – it’s the volume of the hose, and different brands willuse different lengths and ID.
Dave

Prepare to be boarded!

[jerry-w-bert]

To Jerry Ahrens :Found 3 rings which would not move at all . Two on upper Cyl . where comp is worse . The one that did move only pushed in slightly. I put in some Seafoam with the engine on its side like you suggested ,came back later and the one that did move ,depressed more. I have a can of deep creep ,best penetrating oil I’ve ever used. The can said it works on carbon deposits .I squirted some in and 10 minutes later 3 out of 4 are depressing enough to squirt out the deep creep in the ring grooves. So I am going to let the one soak over??!!!!! night and check it again in the morning .I will wash it out with carb clean when they are all good and free.

To Outbdnut2 , Buccaneer and FrankR, Thankyou ,my check valve is at the gague. I hope that is a factor in my favor, however i used the same gague for both sets of readings . Since i have found 3 totally stuck and one partially stuck ring I am hoping that they are the main problem because 3 out of 4 are free now. I will proceed to redo the head properly and then take readings again and try to crank it up. Thanks to all and i have a question to all .

Question : I did not use gasket sealant on the head , should I ? Maybe part of the problem? I am waiting for a new exhaust cover plate gasket . None found locally. Also what are your recommended initial needle valve settings to start the engine.

[fleetwin]

OK, well don’t let that cleaner soak too long, overnight is fine though. Be sure to fill the cylinders with oil afterwards or get the engine started to get rid of the cleaner…
Yes, probably a good idea to use the OMC gasket sealer on the new head gasket, but this is only helpful for external water leaks. Gasket sealer will not help with leaks into/out of the combustion chambers. If you decide to pull the head again, you should replace the head gasket again, don’t reuse it, I would probably rough up the cylinder walls with some fine emery as well. The head bolts are torqued to just 80 inch pounds, in stages, starting with the center bolts.
I would probably use the OMC typeM-847 sealer on that exhaust cover gasket, those bolts love to loosen up causing leaks.
The high speed needle (bottom one) is backed out 3/4 turn after being gently seated.
The low speed needle (top one) is backed out 1.5 turns after being gently seated.
These settings will get the engine running, don’t forget to make sure the packing nuts are tight enough so the needles don’t move on their own…
You will be able to make some adjustments in the barrel/tank, but it can only be "dialed in" while running on the boat. Dial in the high speed needle first, then the low speed needle…

[jerry-w-bert]

ok Thanks Fleetwin , ill follow your recommendations and repost with my results …I am learning from you experts and I appreciate you taking the time to help me.Any other advice from anyone is welcome. JWBert

[jerry-w-bert]

To all who have responded , I have not been able to work on this engine this week, We are having a huge function at our home 50 + people and so I have have been working day and night ..I have poured in Marvel M. Oil in the mean while . I’ll get to it late next week …Thanks

[bob-d]

Have always liked this small Mercury compression guage.
Fits in tight spots, no hose to throw off the readings on small outboards.
Mercury 91-29287.
Bob D

[garry-in-michigan]

My favorite also – In the mid ’50s I paid one tenth what they cost today . . . 😆

[jerry-w-bert]

Got to this engine today .Ok , the top ring on the top cylinder only moves a little bit , not like the other 3 ,which depress about twice as deep . I was patient squirting more juice in every other day ,cleaning with carb clean and comp’d air. It never moved any more than from the first .But since it did move I put on the new head gasket following fleetwins instructions . I put head on my table saw and checked for warping by doing figure eights on fine grit emory paper. No high spots. Unfortunately I don’t have a torque wrench that goes down to inches so I just snugged the head down little by little going by the diagram . Compression is 55 bottom and 40 top … There is no way I don’t have that head on in a satisfactory way . I have not bothered to gas it up and try to start it with current Compression . The 55 number (bottom cyl ) was the same as before and now the top is down to 40 and not 45 . This supports 2 things , 1: the gauge is consistent whether perfectly accurate or not . So compression is indeed low . 2: My installation of the head appears not to be the cause of the low compression because the readings are fairly consistent both before and after this work.

I am looking for a torque wrench that will go down to inches since I will need it for the rods and etc to replace rings.

So it still appears that maybe rings are needed???, But I need expert advice . What would you do next? What do you think could be another cause of the extremely low number in the top cyl. ..This engine was running last year ,before I touched it ,so I’m really down about it… Help please!

[fleetwin]

Did it run OK last year…If so, I would get it running, and I’ll bet the compression will come up a bit and it will run fine….

[Author]

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