79 100Hp Johnson compression

ceddon46 · Aug 19, 2013

While pulling a tub yesterday , I noticed a loss of power , I pulled boat out of lake and put in on ears in drive way and found cylinder # 2 not pulling its load it had fire so did a compression test and it was low around 70 with all others around 135 , a week ago I had 135 on this one as well but my question is I ran about 16 gallons of premium gas through engine this weekend and have always ran regular , being that it runs a little hotter could it have gotten my rings ? the main question is could I put some marvel mystery oil in that cylinder in hopes that it can correct the problem if just the rings are stuck not cracked would it hurt anything ?

JB · Aug 19, 2013

Re: 79 100Hp Johnson compression

Run a decarb treatment through it. The hi-test fuel probably didn't cause it. It is probably just a stuck ring.

Marvel mystery oil wouldn't fix anything.

ceddon46 · Aug 19, 2013

Re: 79 100Hp Johnson compression

Did one last night did not help still low compression , since I am probably looking at a rebuild what is the best manual to guide me through the process of a tear down and a rebuild with torque spec's etc ?

Bosunsmate · Aug 19, 2013

Re: 79 100Hp Johnson compression

Pull the head and have a look.
You might be lucky and its just a blown gasket on that cylinder. You can also look for evidence of big end bearing damage, that normally means a much more expensive rebuild.
Cant advise on manual, i just use online searches and on here when needed for torque values etc

emdsapmgr · Aug 19, 2013

Re: 79 100Hp Johnson compression

Agree. Pull the head and check the seal ring on the head gasket for that cyl. If the head gasket seal is ok, the tear the carb down, esp the side that feeds the low compression cyl. You may find a fuel restricton in one of the jets. If you need a manual, get a factory manual only. Check out: outboardbooks.com

ceddon46 · Aug 20, 2013

Re: 79 100Hp Johnson compression

pulled head and this is what I found , notice damage to piston at 8 oclock and a few scratches on cyclinder wall only one can grab a finger nail , can this be honed out ya think ? what might have caused ring failure like this just age ?

Bosunsmate · Aug 20, 2013

Re: 79 100Hp Johnson compression

Yes age, heat, running lean, running too cold.
Remove the bypass port for that cylinder and see if the rings have spring in them or a snapped, see if they were frozen in the grooves.
Hows the head look?
Yes a light hone and brand new rings could work

ceddon46 · Aug 20, 2013

Re: 79 100Hp Johnson compression

Head look's good , bypass port? guess I will have to wait on my manual to get here on that one ,this will be my first outboard not a newbie to engines by all means so I think I am just going to do a total rebuild on it , kinda worried about finding someone around my area to bore an outboard so far no luck on that.. someone said use scotchbrite on it to knock off any burs that might be on it, then hone it ... what you guys think ?

ceddon46 · Aug 20, 2013

Re: 79 100Hp Johnson compression

Model # on this engine is 100TLR79S by the way

V153 · Aug 20, 2013

Re: 79 100Hp Johnson compression

Sounds to me like ya blew a head gasket. I'd start there. If it's never been replaced it's not at all uncommon for those gasket(s) to fail on these old V4s. The powerhead to mid gasket does too. So be prepared ...

If it is in fact the head gasket just make sure to clean up the mating surface on the block good & have the head decked.

Bosunsmate · Aug 20, 2013

Re: 79 100Hp Johnson compression

Bypass port is on the side of the engine where the intake manifold comes from the crankcase towards the entry into the cylinder.
You can see four covers over each of them, two on each side of the motor, one for each cyinder. There is normally four bolts holding these covers on and they will have a hose to it from the carbs and it will normally have a few grounds bolted on it too and often other parts and pieces.
Once you remove that cover for that cylinder you will be able to see into the cylinder and you can turn the flywheel until you see the side of the piston so you can then check the rings are compressing and havent snapped.

Bosunsmate · Aug 21, 2013

Re: 79 100Hp Johnson compression

Heres a picture of a disassembled one. If you look on the right, on the cylinder block you will see ive removed the lower bypass port cover. Thats because this one had a low compression reading on that bottom cylinder. By removing that bypass cover i could put a screw driver through those four holes and test that the springs werent snapped. But these were snapped on this engine. Hence the disassemble and later honing.

Note the bypass port cover for cylinder number one is still attched

ceddon46 · Aug 21, 2013

Re: 79 100Hp Johnson compression

Ahh ok I see , good pic Thanks... , on the parrt of the engine to left there is two ears for bolts is that the motor mounts , if I remove those I can remove power head from rest of the motor ? trying to get an early start on rebuild manual and parts be here in few days

Bosunsmate · Aug 21, 2013

Re: 79 100Hp Johnson compression

to remove the powerhead you need to remove the lower cover under the cowling and then all the long powerhead bolts coming up from underneath. And the short ones at the front

79 100Hp Johnson compression

Seaman Apprentice

Honorary Moderator Emeritus

Seaman Apprentice

Admiral

Supreme Mariner

Seaman Apprentice

Admiral

Seaman Apprentice

Seaman Apprentice

Lieutenant Commander

Attachments

Admiral

Admiral

Seaman Apprentice

Admiral