johnson 112 low compression, next step please

[66 joe]

Ok guys, pulled head off rough running 94 johnson 112. One cylinder had 100psi, all others had 125.
Expecting to see blown head gasket, I dont think Im seeing one. Motor is recent rebuild. Other than a white looking hot spot with some wear, theres no nicks or damage. Still cross hatch in bores.
Should i , new headgasket and try again? decarb?
Dont really wanna go any farther this year, Thanks , Joe

 

[Daviet]

Re: johnson 112 low compression, next step please

If every thing looks good inside, I would replace the head gasket. Run it up to temp and retorque the gasket and recheck the compression.

 

[SparkieBoat]

Re: johnson 112 low compression, next step please

yes do a decarb and what Daviet said...the 100 psi is probably not your motors poor running issue..you need to trouble shoot your motor, instructions in my sig below.

 

[66 joe]

Re: johnson 112 low compression, next step please

yes do a decarb and what Daviet said...the 100 psi is probably not your motors poor running issue..you need to trouble shoot your motor, instructions in my sig below.

Kinda the what I wanted to hear,,, ordering a head gasket and when time permits retest, thanks

 

[66 joe]

Re: johnson 112 low compression, next step please

ok, 2 last questions, I'm looking at cylinder 1 (bad one, 100psi) Im guessing a hot spot toward top, looks smooth and remaining 90% has crosshatch around. But no damage, nothing can catch my finger nail. So a small spot in bore kinda off color no bigger that 1 sq inch. Do I do anything to that spot with piston at bottom? Scotch brite, fine sand paper? I dunno, got both bores coated with marvel mystery right now. Until I get head gasket.
And anyone got quick part # for 94-95 112 spl head gasket? thanks again, Joe

 

[Daviet]

Re: johnson 112 low compression, next step please

Here is a 95, you can cross check it to see if the 94 is the same.
http://www.ishopmarine.com/ishop/js...CatFilter=BRP&eCatSupFldr=supplier-100-005-EJ

 

[emdsapmgr]

Re: johnson 112 low compression, next step please

While you have that head off, how do the rubber water diverters look? Are they correctly located?

 

[66 joe]

Re: johnson 112 low compression, next step please

While you have that head off, how do the rubber water diverters look? Are they correctly located?

Thank you, yes I see the ruber diverter in place. Just ordered head gasket, and I only took head off of low compression side. If I have a bad ring, would I still get 100 psi with no damage to bore?

 

[emdsapmgr]

Re: johnson 112 low compression, next step please

One piston can still make decent compression with only one ring. The top (outer) ring is usually the one that goes first. A couple of things you should look for to confirm that the outer ring is in place: 1. Often the bore will be ok. Look for dents on the piston and head. Usually when it goes, the ring will break. The broken end will catch on one of the exhaust or intake ports and pcs of the ring will get forced into the combustion part of the cyl. As the piston continues to go up and down, that ring pc will get caught between the crown of the piston and the underside of the head-repeatedly. The piston will pulverize the ring and cause numerous dents on the piston crown and underside of the head. It's unusual for a ring to break and not have this type of visible damage. 2. The piston usually will have slop between the side of the piston and the cyl wall. You should be able to move the piston around slightly (with your hand) in the hole when the piston is at top dead center. You can shine a flashlight in the small crack and see the top ring. You should be able to move the piston around enough to see if the top ring is in one pc or not.

 

[66 joe]

Re: johnson 112 low compression, next step please

One piston can still make decent compression with only one ring. The top (outer) ring is usually the one that goes first. A couple of things you should look for to confirm that the outer ring is in place: 1. Often the bore will be ok. Look for dents on the piston and head. Usually when it goes, the ring will break. The broken end will catch on one of the exhaust or intake ports and pcs of the ring will get forced into the combustion part of the cyl. As the piston continues to go up and down, that ring pc will get caught between the crown of the piston and the underside of the head-repeatedly. The piston will pulverize the ring and cause numerous dents on the piston crown and underside of the head. It's unusual for a ring to break and not have this type of visible damage. 2. The piston usually will have slop between the side of the piston and the cyl wall. You should be able to move the piston around slightly (with your hand) in the hole when the piston is at top dead center. You can shine a flashlight in the small crack and see the top ring. You should be able to move the piston around enough to see if the top ring is in one pc or not.

I know what youre talking about, having seen dents before from things inside the combustion chamber, in this case, there are no dings on head or piston at all. The piston moves very minimally side to side, just like the good one below. Im going to check again with better light, and see if I can see anything different. Thank you, Joe

 

[SparkieBoat]

Re: johnson 112 low compression, next step please

your piston is probably just fine, replace the head seals and properly torque the head back on, I use black permatex for the water holes. do a decarb and your compression will probably even back up, but it would run ok like it was, although it is better to get it within 10 PSI.

 

[66 joe]

Re: johnson 112 low compression, next step please

Ill post results, hopefully by this weekend; thanks everyone for help, JOE

 

[66 joe]

Re: johnson 112 low compression, next step please

Ill post results, hopefully by this weekend; thanks everyone for help, JOE

ok,,, new head gasket and i now have 125, 125, 120, 110 psi; so the 100 shot up to 110 with new head gasket;
Im going to get new spark plugs and lazer temp reader and start her this weekend. I will prob de carb also.
thanks again, Joe