81 15hp low even compression

[AlTn]

Re: 81 15hp low even compression

just guessing, but I believe boobie is pulling your leg....I've heard of them doing this back in the '40's to old car engines where, hopefully, the bonami exited with the exhaust gases with little or no mixing with the intake mixture, however, you really don't want bonami to be pulled into the crankcase and across the bearings and crank surfaces, not to mention the gummy mix that's going to try to find its way into the ring grooves

 

[Pussiwillow]

Got to pull the head today and square it up. It is warped in the middle, approximately .004 definitely enough to see a decent gap under it, I think I found my problem, going to mill the head within the week and see what I got

 

[Pussiwillow]

Well I installed my head after milling it down and squaring the block to, new head gasket, torqued down 50 pounds both cylinders still, time to re ring it

 

[AlTn]

Re: 81 15hp low even compression

have you tried another compression gauge?....as I understand it, the motor starts and runs, does it exhibit problems at top end?..idle?...if it runs "well enough" you might at least delay the ring job until winter

 

[dazk14]

Re: 81 15hp low even compression

How many pulls on the starter rope are you using to test compression? Those motors usually hit 50psi on a good gauge 1st pull, then build from there.

Does your compression gauge(s) have a shraeder valve on the plug end and does it hold pressure until until manually released?

How low will the motor idle?

Here's a Po-Folk way of testing the compression... Take plugs out. Put finger over plug opening and pull cord. Don't get zapped by the plug wire. Yes a little awkward and you can't get a perfect pull - no problem. Does it blow your finger off?

Go get em'

 

[Pussiwillow]

The motor starts first pull, idles like garbage under load has no top end power mid is good, pegs out bout 10 mph on my 12 foot Jon that weighs less a hundred pounds, I've used 2 comp gauges both being within a few pounds of eachother and no it doesn't put enough psi to really pop my finger off the hole, I can turn the flywheel with one hand with both plugs in with ease I'm looking at a 1984 15 hp johnson tomorrow was used as a kicker motor and says it runs excellent, from the pics looks like it is in great shape, 300 bucks, I'm snagging it if compression is good

 

[AlTn]

Re: 81 15hp low even compression

price sure sounds right...

 

[Pussiwillow]

The motor I just looked at had the same issue, 50 pounds both cylinders, this motor is spotless and clean it's in amazing shape, hardly ran I guess it was a kicker motor, it has been sitting for a few years am gonna go back thursday and look at it again after he runs it, my motor was the same thing it sat for a few years, I really think this other motor has potential, when I go back Thursday what should I do to see why it has low compression? Maybe it just need a good hard run to re seat the rings? What do you guys think I may offer him 200

 

[AlTn]

Re: 81 15hp low even compression

with the '81..where is the av plate in relation to the keel with the motor mounted on the boat?...an inch or so above or below?

 

[1946Zephyr]

Re: 81 15hp low even compression

Well, if your cylinder walls are in decent shape on your motor, then you could put new pistons and rings in it, for about the same price as buying that other motor. And since winter is practically here now.........

http://www.marineengine.com/parts/j...turer=Johnson&section=Crankshaft+&+Piston

 

[Pussiwillow]

The cav plate sits dead level will bottom of boat

 

[1946Zephyr]

Re: 81 15hp low even compression

That's perfect. At least your motor is matched with your boat.

 

[gdavies07]

Re: 81 15hp low even compression

I'm far from an outboard mechanic, and I'm still learning day to day. But I have a similar problem with my 30 hp Johnson (55psi both cyl, but cyl walls looked great, could still see hone marks, replaced hd gasket, planed head, no change in compression) and from what I've gathered, It could also be a crank seal or the reed plate. Like I said, I'm far from a true outboard mechanic but from info I've gathered about mine, it might be worth while having a mechanic diagnose the problem. I did the same as you, I went and bought a true, high compression & good running motor, but I will investigate my motor to learn & fix what I have to try and recover some of the costs. Good Luck!!

 

[Pussiwillow]

I had to put a jack plate on to make it work as my transom is a 12 inch transom, I know what you mean the compression issue just rattles your brain I'm gonna have to rip this thing apart and see what's going on

 

[tugboat80]

Re: 81 15hp low even compression

Just for a baseline comparison: My 92 Johnson 15 rope start shows 114/116 psi. I had it trolling down to 1.7 mph pushing a 14' open alum purring even. It runs same boat to 22 mph. Gotta love Evelynn Johnson.

 

[jmendoza]

Re: 81 15hp low even compression

The rough running is most likely a plugged up carb, or carb problem, overhaul it. For low compression, to unstick the rings I would try getting it hot, put it on tilt, pop off the cowl, remove the plugs, and put one piston at the top and spray in some Valv Tect marine motor de-carb solution, let sit for 15 minutes and put the other piston at the top and repeat. Put the plugs back in and fire it up, run for at least 1/2 hour at moderate speed.

 

[Pussiwillow]

Tugboat, thanks for rubbing it in hahaha but serious I know these motors are awesome, when they run right, I really wanna get this thing cooking again.

Jmendoza, I overhauled the carb, it doesnt run "rough" it is just gutless and won't idle in the water with back pressure or load at all, the carb, fuel pump, lower unit, impeller, plugs, head gasket and a few other new parts, as far as the rings, I did a decarb were it sucked the can til empty at fast idle then I let it stall at the end and soak for 20 minutes, then ran it fast idle for 15 minutes, would that accomplish the same? I'm just trying to think, what else can interfere with compression other then the head and rings, but I imagine it has to be the rings, could they get glazed from sitting and prevent them from seating against the cylinder walls, or you think they could still be stuck? I can wiggle the piston in the cylinder And the ring stays pressed against the walls... I'm just at a loss and pissed I bought this thing just weird it had compression when I bought and hardly ran it then this

 

[AlTn]

Re: 81 15hp low even compression

you can pull the bypass cover and at least see if the rings have any spring left in them or if they are stuck in the grooves by pushing on the them with a softer surface non scratching object...I've used a popscile stick....you can tell if the rings are stuck or not, at least in that location

 

[jmendoza]

Re: 81 15hp low even compression

Try this:

Squirt some 2-stroke oil in the clylinders and then do a compression test, throttle open, and make sure the o ring on your compression tester seals to the surface of the head. If the compression goes up, you may have worn out rings, and/ or blowby.

Badly carboned /stuck rings require a direct treatment as I outlined, the method you used is per the directions, but for engines that are not badly carboned up, that proceedure is done as a preventative maintenance step. The top ring being a Dykes type rarely gets stuck, it's the second Keystone ring that gets stuck. My 1975 9.9 took 3 treatments to get the compression from 70 PSI to over 100.

If you have not lapped the head and replaced the gasket, try that too.

If the engine idles and tanks good, but fall on it's arse when in gear, it's a sign that the low speed needle is set a tad too lean, try opening it 1/2 turn and try again. You did replace the carb to manifold gasket with a new one? Make sure the carb's throttle arm is synched to the cam on the magneto plate too.

 

[Pussiwillow]

Jmendoza, you just planed my weekend, I'm going to do what you said to free rings, the head has been resurfaced with new gasket straight and true, the carb gaskets are all new, the motor pulled good from idle to about half throttle, then from there on no power, I removed the head after decarb and the pistons still had a bit of carbon build up so I'm lead to believe the rings are most likely stuck, which I hope thanks so much!!