81 15hp low even compression

[Pussiwillow]

Hello I have a 1981 Johnson 15hp that is reading 50 psi in both cylinders, dead even. I've done a decarb on it with no success, I pulled the cylinder head and the walls were shiny and glazed so I rehoned it and took the glaze out. There was no scars in the walls just real light scuffs near exhaust ports that cleaned up. I looked over the head, that's about all I did I didn't square it up or anything, but did "look" ok, and the head gasket looked good too no burn spots or cracks. I did not replace head gasket as I did not have one, I do now and will change it I wanted to run the motor after the hone to see if I got improvement, I did not, still reading 50 even, starts first pull the fuel pump is pushing fuel great so I got enough vacuum. I took to cylinder to tdc locked the motor from turning, with both plugs out, took my compression gauge fitting and hooked 50 pounds to it, had pressure seeping into the other cylinder and out the exhaust, what should I check next should I check head for warpage and change gasket? Thank you!! I just want to go fishing again!!

 

[JB]

Re: 81 15hp low even compression

Howdy.

Welcome to iboats.

"Starts first pull". Does it run and work??

Why did you do the compression test??

Sounds like you might be trying to fix something that ain't broke, Pussiwillow, unless there is more that you haven't told us.

 

[AlTn]

Re: 81 15hp low even compression

if the compression is dead even, you may have a blown gasket between the cylinders or the head may be warped between then..possibly < not likely > the deck is dipped between the cylinders...a straight edge will be beneficial in determining a head/deck issue

 

[Pussiwillow]

Hey thanks for the replies, well I wanted to do a compression check because it wasn't idling low enough and stall and really had no top end power pretty much pegged out at 10 mph in my 12 foot flat bottom Jon that weights 95 pounds haha I rebuilt carb and fuel pump new plugs and fuel lines that's when I checked the compression

 

[Chinewalker]

Re: 81 15hp low even compression

I'd try another gauge. I doubt that motor would run at all on 50psi...

 

[Pussiwillow]

I fired it up on one cylinder with gauge in the other cylinder, I did this for both of them, and just above idle speed it pushed to 80, I didn't rev it or anything just above idle

 

[Pussiwillow]

I used 2 separate gauges and were 5 pounds within eachother

 

[Monte1961]

Re: 81 15hp low even compression

80psi is plenty on a pull start! Thinking yer chasing a rabbit!

 

[1946Zephyr]

Re: 81 15hp low even compression

you actually ran it on each cylinder with the compression guage in the other? Hmmmm, never tried that one before. At least you know it's firing on both cylinders. On a 15, you should get upwards to 100 psi on each cylinder, I would think. If the cylinders look good, then it's looking more like the head isn't quite flat and you might be getting blow by to each cylinder. Like AlTn said, the flatness of both the cylinder head and block should be checked. If the motor got hot in the past, it could have warped the head.

 

[Pussiwillow]

Monte, I'm only getting 50 on pull 80 with it running just above idle speeds

 

[nwcove]

Re: 81 15hp low even compression

ill go with a warped head, (and re-using a head gasket is never a good idea , especially when trying to diagnose a compression issue).
and i hope you didnt borrow the guage ! its now well soaked internally with fuel/oil mix.:facepalm:

 

[Will Bark]

Re: 81 15hp low even compression

nwcove, maybe that's why he's getting 80psi now.

 

[nwcove]

Re: 81 15hp low even compression

nwcove, maybe that's why he's getting 80psi now.

yep, probably so! but if the motor even runs on one cyl, barely above idle, even 80psi is a suspect number. (i'd do the same process, run it on one cyl with comp guage installed, but maybe mix it 25/1......the numbers might keep increasing! )

 

[Pussiwillow]

I stripped the gauge down as far as lines, they were clean I ran 25:1 when I ran it after the hone, I only reused the head gasket to run the motor for 30 minutes to re seat the rings in a hope that they were the suspect, the numbers didn't budge one bit from the hone, it lead me to believe the head is the suspect. I pulled it with trottle wide open and warm got a reading of 50psi on each cylinder, then I did it again but ran it on one cylinder for both of them, they both jumped to 80 just above idle, I killed the motor and re checked it by pulling like I did the first time I got 50psi again, so I don't think oil got in my gauge and gave me a bad reading I think I got a warped head in between cylinders and running it at 800 or so rpm built up more compression then It could leak hens the jump in numbers, me pulling it doesn't turn it nearly enough as it did at 800 rpm or so or as fast as say an electric starter. Just my thoughts

 

[Pussiwillow]

I ment to put 100:1 after the hone I got caught up reading the 25:1 and wrote that haha but no it was 100:1

 

[Rick.]

Re: 81 15hp low even compression

100 /1 is not enough oil. 50/1 minimum for that engine. 25/1 is even better right after honing. I would put a new gasket on the head and torque it properly and then run sea foam thru it just to see if the it loosens the rings a little. Perhaps they are gummed in their grooves. Beyond that, if it starts and runs ok I would just use it. Best of luck. Rick.

 

[Pussiwillow]

Thanks Rick, 100:1 was suggested by a boat shop mechanic after the hone he said to run it about 2000 rpm for 20 minutes, I was weary as I always run 50:1, I also ran a decarb and after let's it soak in the cylinders, this weekend I'm pulling the head checking for straightness and putting a new head gasket and calling it! Thanks for all the replies

 

[Rick.]

Re: 81 15hp low even compression

After you run it a while the comp. numbers may well improve and even more with the new gasket and flattened head. Rick.

 

[boobie]

Re: 81 15hp low even compression

I guess I'm really stupid. Why would anybody run a hone in a cyl with out tearing it down and putting new rings in it?? If you want to seat the rings, start it up and run some Bonami through the carb throat. Seats them real quick.

 

[Pussiwillow]

Hey boobie thanks for the suggestion if my head and gasket don't work I'll give that stuff a try for sure