?????? Question Johnson QD-16

[Buccaneer]

Who knows, maybe it will be a good runner when back together.
Let us know what happens!
I’ve never researched rings for a Scott yet.
Possibly you could obtain some from this guy …….
http://pistonrings.net/

tinkerman wrote:

No sir that is the picture of a Scott Atwater model 497 …. May go ahead and clean the carb and fix the pull rope and see if it’ll start

Prepare to be boarded!

[wbeaton]

I think you are looking for a problem that may not exist. The fact that every motor reads 50 psi is a red flag. I have never seen a 1950’s OMC with compression below 60 psi that would run. In fact, 60 psi seems to be the basement of whether it will run or not. I’d prefer to see 70 psi or better. Every gauge reads differently, but the difference shouldn’t be that big. I suspect your gauge is reading low or not sealing properly. Either way, don’t start tearing things apart, yet. Start with the basics first.

Wayne
Upper Canada Chapter

uccaomci.com

[Mumbles]

What Wayne says, X 2.

[phil]

He wrote:

tinkerman wrote:

I have a 30 hp mariner on my boat that runs great and it shows 85 psi per cylinder so I’m thinking my gauge is ok.

http://www.omc-boats.org
http://www.aerocraft-boats.org

[wbeaton]

I missed the 85 psi part on the Mariner. However, 85 psi is still low for a motor of that horsepower and age. I still suspect the gauge.

Wayne
Upper Canada Chapter

uccaomci.com

[David Bartlett]

wbeaton wrote:

I missed the 85 psi part on the Mariner. However, 85 psi is still low for a motor of that horsepower and age. I still suspect the gauge.

Especially if it runs well.

David Bartlett
Pine Tree Boating Club Chapter

"I don't fully understand everything I know!"

[jeff-register]

Looks like you have the pistons out. Take a ring off & put it in the cylinder & measure the ring gap at the top of the bore, next do the bottom of the bore to check for cylinder taper.

[tinkerman]

No I didn’t take the pistons out yet, I was contemplating it. I think I’ll get a better compression gauge and see what happens. It makes sense if my mariner runs like a brand new engine then 85 is really low for that motor. It’s an 89 model. So maybe if the mariner is actually better compression than the gauge is showing (even 110-115) then maybe my vintage motors are closer to 80-90 which would be ok for an old motor. I wasn’t crazy about buying ANOTHER gauge but I think at this point I NEED to rule it out. Thanks for the advice

It's a jungle out there.

[Biddaddydan]

The reed valves are indeed thin layers of metal that bend in (thus their name) to allow air and fuel mixture coming from the carb to journey to the combustion chamber. Their are located on the INTAKE side, in front of the motor , just behind the carb. Your pictures show the EXHAUST side (rear section) of the motor head.

Pistons seem fine, rings probably are too, I’d dissassemble the intake, get the reed plate off and make sure there is no light visible at the edges between the plate and the reed when held against a source of light. If so, replace or realign the reeds leaves until they completely block the opening.

Also replace the intake gaskets as their perfect sealing is crirtical. While you are at it, change the head gasket and see your compression readings soar. In all cases, thorougly clean the mating surfaces when changing gaskets.

Have fun !

There are no old outboards, just machines so well built, they have outlasted their owners !

[fleetwin]

I’m with Frank, that powerhead looks pretty good to me also…You’ve got the intake and exhaust covers off, so am assuming the rings aren’t stuck and there is no major scoring or scuffing…I would question your compression gage…Perhaps the head gasket is NG…I wouldn’t take that powerhead apart unless you see something wrong that might explain the low compression…

[Author]

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