15 hp 85 Johnson compression issue

Khenders

Seaman Apprentice
Joined
Jun 3, 2007
Messages
38
My 15 hp Johnson has low compression; ~60-70 both cylinders.
It also has low flow out the tell-tale (I replaced the impeller with no real effect and will check the gromment on the water tube shortly). Hoping for the lesser of possible evils, I checked the head gasket hoping that a warm engine (due to a dislodged grommet?) caused it to leak. However, after pulling it last night, I could not find any indication that it leaked.

The cylinder walls are completely smooth and the pistions glide easily. I grabbed the pistons and tried to shake them to see how much play there was between them and the cylinder walls. I got very little movement as one would expect. But how much is too much?...I don't know.

What else could be causing low compression? :confused:

Any help or insight would be greatly appreciated.

Thanks in advance
 

wavrider

Chief Petty Officer
Joined
Nov 26, 2007
Messages
543
Re: 15 hp 85 Johnson compression issue

Did the water tube seat correctly? is the low flow at idle speed? thermostats?
Also need model number or year of the engine to help answer your questions.

Do a de-carb and see what your compressionis after wards, could be carbon making the rings stick a little.
 

iwombat

Captain
Joined
Jul 12, 2006
Messages
3,767
Re: 15 hp 85 Johnson compression issue

If you're getting 60-70psi on a pull-start. That's about all you can really hope for. I doubt you're reaching a consistent 300rpms on the rope.

Compression itself is simply a diagnosis tool for solving performance problems. What's going on with your motor that made you check compression?
 

Khenders

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Jun 3, 2007
Messages
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Re: 15 hp 85 Johnson compression issue

I'm not getting full power out of it. It runs an stays running and does not smoke or load up...it just won't open up.

I'm going to replace the head gasket, thermostat (and water gasket) and then try decarbing the engine and cleaning out the fuel system.
 

iwombat

Captain
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Jul 12, 2006
Messages
3,767
Re: 15 hp 85 Johnson compression issue

In my experience, poor compression will usually manifest itself first at idle. Are you getting good low-speed performance - no misses sneezes etc?

Is this a new condition w/ your boat & motor setup? If not, it could just be you need to do a little work setting up your motor with your boat.
 

wavrider

Chief Petty Officer
Joined
Nov 26, 2007
Messages
543
Re: 15 hp 85 Johnson compression issue

If it is not a new set up I would lok at your spark to make sure you are getting a good 7/16 blue spark out of each coil.

2 strokes will idle and run on one cylinder but not get up to speed.

Start the engine, preferably in a barrel so there is back pressure, if not then on muffs. Use insulated pliers to pull one spark plug wire off at a time, pull it by the boot of the wire and listen to see if any noticeable loss in engine rpm. the engine should decrease with the wire pulled off and increase when you re-install the spark plug wire.

Use insulated pliers and for safety sake perhaps stand on a piece of wood, 30,000 volts going through that sparkplug wire will light you up. I know.
Let us know what you find out.
 

mikesea

Lieutenant Commander
Joined
Oct 1, 2006
Messages
1,830
Re: 15 hp 85 Johnson compression issue

you might have a broken cam that is on the timer base under the flywheel,it has a plastic piece that has an adjust by allen screw ,they break or the roller itself might have issues,as for the water ,there is a grommet that is on the top of the water tube that in time can collapse therby restricting flow,.i have made a cutting knife by taking a piece of clothes hanger wire,cut a long piece and mash an end,the sharpen ,put other end in a drill and wallow out the rubber ,it works most time,i have worked on many small 9.9 and 15 your problems are the common ones
 

iwombat

Captain
Joined
Jul 12, 2006
Messages
3,767
Re: 15 hp 85 Johnson compression issue

I've replaced a few badly grooved roller cams myself. They can sometimes be flipped over, or you can epoxy the groove for a temporary fix. I think they're all of $5 at the dealer.
 

patrickray

Petty Officer 1st Class
Joined
Feb 15, 2008
Messages
256
Re: 15 hp 85 Johnson compression issue

iwombat, you bring up a good question. Im going to a compression check on my 1972 evinrude 9.5, 9222M. If 60-70 is all I can expect by pulling on the rope. Is that good or bad? Thanks

Patrick
 

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
Re: 15 hp 85 Johnson compression issue

Just a thought---I have checked all my old motors that are all pull start. I get 80 to 85lbs. on 10hp's, 108-115 on 18hp's, over a 100 on my 15hp. The only motor I had that only showed 60 lbs. has a bad piston and rings/cylinders which was a 10hp. So if all you get is 60 or 70 lbs. and your guage is reasonably accurate, it's too low. The one I had with bad compression on one cylinder would idle, but wouldn't get up on plane. My thinking is your 15hp '85 model should have over 100 lbs. of compression.
Just my opinion,
JBJ
 

iwombat

Captain
Joined
Jul 12, 2006
Messages
3,767
Re: 15 hp 85 Johnson compression issue

After a freshly rebuilt '87 15hp I was able to crank out 95psi on the rope. Unless you can really spin it, the important test is to pull evenly for each cylinder and see if they match up.

Really guys, your motor will tell you if you've got a compression problem. Don't use compression tests by themselves to tell you that you've got a motor problem.

If it's running well, baseline the compression so that if it doesn't you can compare. Hanging your hat on a specific number just isn't going to be worth the effort.
 

Khenders

Seaman Apprentice
Joined
Jun 3, 2007
Messages
38
Re: 15 hp 85 Johnson compression issue

iwombat -It runs fine at idle...even at low to mid range. It's just a bit sluggish when i apply throotle and never reaches full rpm.
Cylinder compressions match up pretty good...low but a match.

Wavrider - spark is good and plugs are clean

Mikesea - Interesting thought...i'll run out and look. (tick tock tick tock) Your right...it's the cam...the roller is good but the cam follower is broken and the link is gone. :eek:

Now I need to reassemble the head, get another cam follower...and pieces, and figure out the low water flow.

Thanks all for your help...I'll keep you posted. ~Ken
 
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