Johnson 3HP JW-17 Compression woes

[jerry-w-bert]

I have a Seahorse that started fine before I worked on it! It had gas leaking out the lower end from this hole .

I replaced some seals , and that fixed the leak . I also replaced the head gasket hoping to increase compression because the old one was pretty raggedy . Compression was 60 and 65 before replacing gasket. Now it is 45 and 55 and will not start !!. ,(GOOD SPARK!) OK , because of my ignorance about marine engines , I did not resurface the head nor did I have any idea for head torque sequence or correct foot pounds . I just used common sense and was careful . I just found the sequence and values here in this forum . I also repaired a rub thru spot on the plug wires where one had worn down to almost bare wire and installed some chafing gear. Also put a new impeller in the Water Pump. I have a new set of piston rings I want to install . So I’m guessing the loss of compression is probably due to not using sequence chart and possibly a warped head as well? I have stripped off gas tank , Needle valve/choke plate, intake pipe,and exhaust plate. I do not see scoring of rings or pistons thru exhaust ports, rings are so tight a fit that I can not really tell if they are frozen or not , (when I had head off I poured in Marvel Mystery Oil just in case ). OK so that’s where I am , What is your recommendation for my next move? Should I proceed with the ring job of first go back to the head and see if compression improves first ? As you can tell I’m a novice and any advice will be appreciated greatly . Thanks Jerry

[frankr]

Compression should be about 80psi. But those motors are fussy about what kind of gauge you are using. I can’t imagine losing all that compression just replacing the gasket, assuming you at least cleaned the surfaces and bolted it down somewhat tight.

Have you pulled the flywheel? I suspect ignition trouble (cracked coils??). Never mind it has spark–the great deceiver.

[fleetwin]

What seals did you replace?
I’m with Frank, I don’t think changing the head gasket would lower the compression unless you really botched something badly…Like Frank says, these small combustion chambers are real fussy about the compression gage, so I would not put too much confidence into the readings you are getting…
Sounds like you just patched up the chafed plug wire, you should go through the entire ignition system and replace both plug wires. Are you sure you have the plug wires on the right plugs? Is this engine new to you? Did you ever have it running? Perhaps it is drooling due to a carb or ignition problem…
No, I would not replace the rings. I’m guessing there is something else keeping this thing from firing up….

[Steve D]

quote FrankR:

Never mind it has spark–the great deceiver.

Hahaha, I love that! I’m gonna mount that on a plaque over my work bench. 😆

[jerry-ahrens]

You may very well have stuck rings causing your low compression #s. If in fact your gauge is accurate… if you have the exaust cover off, a quick way to check for stuck rings is to use a small (pocket size) screwdriver, and gently push the ring straight in to the piston ring groove. If it springs back out slightly when you let off the screwdriver, the ring is probably free. Stuck rings won’t move in and out at all when pushed on with a screwdriver. This is due to carbon build up that glues the ring in the groove tight. It takes some patience, but I’ve freed some stuck rings up with Quicksilver Powetune, or BRP (OMC) Engine Tuner. Lay the motor on it’s side so the product has a chance to soak in. Sometimes though, they can be stuck so bad that the powerhead will need to come apart.

[jerry-w-bert]

To Frank R , Thank you ! Yes I cleaned both surfaces very well with acetone. and bolted it down snug… little here ,little there like I would any head . . I too am flabbergasted that the compression dropped so much !! I will be getting to it next week and I’ll keep you posted , I’m going to redo the head using the torque specs and correct sequencing and then try again …

To Fleetwin , Thank you ! It was a year ago but I remember one was an o ring at the top of the drive shaft at the powerhead. There was nothing there . The other was down by the water pump..I think …It was a bugger getting the old one out as I remember. Probably an original seal. Yes it has run, I had used it maybe 8 or 9 times in the past 8 years always started right up and no problems . But it has set unused for 3 years with gas properly drained and run out of carb . It started up fairly easy last year when I was going to take it out and then I noticed the gas leak after pulling it out of the water barrel , and started working on it. Then I got cancer, surgery , ect. so it has set for another year and I’m just getting to it now. Can’t go wrong on the plug wires top one has a metal band on it says top. Yes I pulled the flywheel last year everything looked shipshape in there, no visible cracks ,no evidence of overheating or water ingress. I’m going to check to see if rings are stuck as per Jerry Ahrens . Yes I am new to this engine, this is the first marine engine and the first 2 stroke I’ve worked on. I’m 66 years old and spent 32 years at sea in the Merchant Marine ,but I was in the Deck Dept ,so I didn’t do engine work. But I figure your never to old to learn …,Re: carb I did repair the valve at the gas tank ,had to solder the wing to the valve stem ,this valve is a very pricey item found one valve on line for 98$ …too much for me. Carb seems ok , but may I ask you for the correct initial needle valve settings on the hi and low . Thank you very much for your input I will repost as soon as I get back to it ..hopefully by Wednesday.

To Steve D.My 3rd cousin twice removed is head bag boy at that piggly wiggly…:) I hope you didn’t off him !!

To. Jerry Ahrens Thank you for your input. I will check the rings as you said. I had watched a YouTube video where a guy did a comp test on a similar engine and it had 60 & 60 but wouldn’t start ..he claims he changed the rings and that fixed it .. Hence I bought the rings . I’m going to redo the head again , and if rings are stuck I’ll try the easy fix with a product such as you say , How about seafoam? I have some in the shop and I also have Lucas injector cleaner I use in my Ford 7.3 liter pickup. I really like this little Johnson it works perfectly for my McKenzie wood drift boat . I am going to know this engine inside and out when this is over.

THANK YOU TO ALL WHO HAVE REPLIED , Also thanks for having patience with me … a novice ..Look for a new post/update next Wednesday or Thursday .. Sincerely J.W.Bert

[jerry-ahrens]

Jerry, I would highly recommend Quicksilver Powertune over Sea Foam… it is strong stuff. It also is excellent for carburetor cleaning. On carb cleaning, follow that up with a rinse of Gumout carb cleaner.

[outbdnut2]

To expand on what Frank said, If your compression gauge has the checkvalve up at the gauge after a length of hose, the volume of the hose can be significant to give you a lot lower reading on these small cylinders. I have one compression gauge like this that I don’tr use anymore for that reason. You want the checkvalve right at the cylinder.
Dave

[Buccaneer]

Interesting about the compression testers with check valves
at the gauge, like mine. Anyone ever note any actual pressure
reading differences between the two type of gauges?
If so, did they differ by 5 psi, 10 psi, etc?
I have an old Craftsman gauge with a foot long flex hose
that works great, of course the check valve is by the gauge.
I’ll keep using it, but now be aware of the situation.

Prepare to be boarded!

[frankr]

The point is that there is a certain number of cubic inches of volume within the hose. That number of cubic inches is additional to the number of cubic inches within the cylinder. So if you have a honking big cylinder, the cubic inches within the hose is much smaller than within the cylinder, percentage-wise, and doesn’t affect the reading as much as if it were a small cylinder. The percentage can be quite large on those little 3hp cyinders.

Putting the check valve at the cylinder, eliminates the problem.

[Author]

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