1991 15 hp johnson i just traded for has almost 90 psi on top 0 on bottom

[werewolf]

I just picked up this 91 Johnson 15 hp on a trade deal, bottom cylinder is about 90 lbs on guage top cylinder is zero compression, had to put the rope starter back together because it had a broken rope, when I was checking compression I found if I had the top cylinder all the way up I could put about 60 psi of air in the plug hole and you almost couldn't hold the flywheel , I used a large flat blade against starter drive to hold the flywheel and it holding air pretty strong, my question is would the reeds cause no compression issue or what ? the top cylinder will hold air pressure with some leakage but crank it with rope and has none . im soaking the cylinders with some carbon cleaner over night to see if it changes.

 

[racerone]

Reed valves have nothing to do with compression in the cylinder.--------Take the cylinder head off.------My guess from outer space is a blown head gasket

 

[werewolf]

I asked about the reed valves because of a article I read where dirt dobbers got in a 9.9 Evinrude and built their nests and the upper side was covered in dirt nests blocking the reeds , I could not figure out how they could have gotten inside it, if it couldn't get air it would not have compression ? that motor had 120 on bottom cylinder and zero on top cylinder .

 

[racerone]

Air gets into the cylinder via the exhaust ports.-----That gets you a good compression test.---Read all about how simple these motors are.

 

[Tim Frank]

I asked about the reed valves because of a article I read where dirt dobbers got in a 9.9 Evinrude and built their nests and the upper side was covered in dirt nests blocking the reeds , I could not figure out how they could have gotten inside it, if it couldn't get air it would not have compression ? that motor had 120 on bottom cylinder and zero on top cylinder .

I thought you said 90 psi on bottom?

 

[Tim Frank]

anyway, does not matter.
What Racer said....take haed off and see what is there,,,,and condition of gasket.

 

[werewolf]

ok the bottom cylinder is still 90 to 95 the top cylinder was zero, after soaking overnight the top cylinder came up from zero to right at 35 psi , this motor has sat for awhile [had dirt dobber nests on both sides of the motor , from what I could see the reed plates look brand new . what im trying to determine is - why does it hold air pressure about 60 lbs is what I have my regulator set at , it will try really hard to turn motor over I have to hold the flywheel with a large screwdriver and I put my stethoscope on the water port and don't hear any air coming out or at the inlet for the water pump. im beginning to think the second ring is stuck and also the top ring on the top cylinder. , going to keep soaking the top cylinder and see if it changes, if the head gasket was leaking the only place it could do so is either in the water jacket or between the cylinders, also checked the bottom cylinder and don't hear any air coming from it .

 

[tomhath]

Taking the head off a 2-stroke is trivially simple, just remove the bolts and you're there. Check to see if the bore is damaged or cracked.

 

[racerone]

I have 20 new head gaskets on hand.------The head would be off that motor in a few minutes !!

 

[werewolf]

ok I may be getting somewhere the blue and yellow paint is exhaust , blue and green is water inlet, the blue only paint mark is blowing air out of the holes when I apply air to the top cylinder at top dead center , what is the port where the blue only paint mark is ? didn't remember seeing this before ? is it another water port to exhaust excess water from pump ?

 

[Crosbyman]

you are making this very complex … just soak the top cyl a few days with some kerosene of PB blaster … make certain the cyl is high enough to close the in and out ports so the juice stays in the cyl.

another option is to just run the engine with engine cleaner like Mercury powertune and redo all your testing

some folks use SEAFOAM (put a can in one gallon of fuel and smoke the place up)

I am all for pulling the cyl head but you could snap a a bolt head or two…. so go slow , be patient and do the easy stuff first

 

[racerone]

Just pull the head off !

 

[Sea Rider]

I have 20 new head gaskets on hand.------The head would be off that motor in a few minutes !!

Right, don't want to be a bad omen, let's hope that no head bolt breaks inside the power head when beeing removed specially if said motor was used in salt water...

The only way to have 0 reading inside a combustion chamber would be to have the upper piston's rings fully stuck closed inside the piston canal.

Happy Boating

 

[racerone]

Blown head gasket for zero compression.

 

[werewolf]

ok I was able to confirm the real issues , the port I asked about is for excess water from engine, I appreciate racerones suggestion about pulling the head , I for one want to know what caused the problems first, it was the head gasket blown right at the water jacket and didn't affect the bottom cylinder, so I applied air to bottom cylinder and no air came out the lower water port, positive proof of a head gasket , I have to say thanks to racerone and the others , ive retired but I did transmissions for 46 years and good diagnoses is very important before you dive in, ive fixed a lot of transmissions that others could not, have one here now that a shop did 6 times and lasted 5 days , I did his original trans two years ago and he said the other day hes keeping the truck now . ok water pump impeller failed and they ran it trying to get in got hot and gasket said see ya later. , water pump complete with plate and housing ,thermostat,head gasket from Johnson, two new spark plugs , starter drive. oh and fuel pump too its missing from motor , spark from wires will jump two inches so should be fine. thanks and I will surface the head and check liners and block too

 

[tblshur]

glad you got it fixed but could you tell me how it held 60 psi in a cylinder with the gasket blown . anyone? thanks

 

[Tim Frank]

It doesn't matter.

As a guess the gasket allowed pressure to equalise across both cylinders and the position of the pistons was such that the ports were closed off so both pistons were holding pressure.. i.e. luck.

The only rational explanation for a zero compression reading in one cylinder is )hope for it) a blown head gasket ~ or ~ some catastrophic and expensive internal failure....IMO.

 

[werewolf]

TBISHUR i have looked at the gasket when the head was off it was blown into the water jacket but the bottom cylinder was still sealed , so the gasket was acting like a bleed orfice into the water jacket, thats way i asked about the water ports on the
outside of motor so i could determine where the air i put in top cylinder was going, if i found it was going inside crankcase and the water jacket it would had been severe damage and not worth going any further, ive had some old 12 hp evinrudes and the 25 s break off head bolts so i wanted to be sure before taking any head bolts out. thanks

 

[tblshur]

thank you tim frank i was just curious and thank you very much werewolf for posting final results :joyous: