johnson 115 low compression

[mieyk]

heres the story... i bought the motor from this guy that said it ran great 2 years ago when he upsized to a new motor. been sitting ever since in his garage.

installed it on my boat, checked compression and all but one had 120 psi.

the low one had 90. i havnt tried starting it yet but when i checked compression, the three with 120 had a fast crank to it but as soon as i moved the compression tester to the one with 90 it was a slower crank.(spark plugs out)

why is it that the low cylinder cranks slower than the rest when on the compression stroke? should i start it and check again or is there something else i should check for?







---A BAD DAY OF FISHING IS BETTER THAN A GOOD DAY AT WORK.---

 

[flabum]

Re: johnson 115 low compression

Yes it is strange that the engine cranks slower on the low compression cylinder. One thing that would come to mind would be possibly stuck reeds.With the engine stiiting that long it is possible there was fuel left on the reeds that "glued" them shut. If this is the case, get some OMC Engine Tuner or Mercury Power Tune and spray it down the carbs and let soak for an hour. Then go ahead and get her started. With the engine sitting so long, you will need to rebuild the carbs and replace the water pump as well.
After you get it running and warmed up, check the compression again.

 

[mieyk]

Re: johnson 115 low compression

good response... i will pull the airbox and check for suction before i do that to see if that is what is causing it.

i was in a rush to beat the cold so i didnt get a chance to really check anything. i didnt even get a chance to hook up the cables but it sounds about right.

thanks and any other ideas welcome.








---A BAD DAY OF FISHING IS BETTER THAN A GOOD DAY AT WORK.---

 

[flabum]

Re: johnson 115 low compression

You will not be able to tell if the reeds are opening on that cylinder by checking for suction since one carb runs two cylinders.Take the airbox off, set the throttle to wide open and look iside at the reeds on the affected cylinder. If you see brown gummy residue on the reeds, they are prob stuck. Before you start that engine, it would be a good idea to rebuild those carbs first. They are likely gummed up from sitting so long. While the carbs are off, you can get a better look (and better spray) at the reeds.

 

[tashasdaddy]

Re: johnson 115 low compression

here's some really good reading and advice, especially, 'awakening sleeping outboard' , 'engine won't start', ' decarbing'. decarbing after you get it running, i highly recommend. if has been known to free up stuck rings, and bring the compression back up to where they belong.
http://www.iboats.com/bbboard/bbBoard.cgi?a=viewforum;fid=36

 

[ezeke]

Re: johnson 115 low compression

I think you should cut to the chase, get a new head gasket and pull the cylinder head so that you can see what's happened.

 

[newtong_ware]

Re: johnson 115 low compression

I agree with pulling the head and looking. No telling what might be going on in their every time you crank it. A stuck ring could do some nasty stuff if you just keep going.

 

[BF]

Re: johnson 115 low compression

is it just that for whatever reason it cranked slow for that hole? If it had cranked faster, like the others, maybe the compression would have read higher (?). I can't imagine how the slower cranking could be related to which hole the tester was screwed into (unless it was the last one and your battery was getting low).

I'd charge the battery and redo the test, starting with the suspect hole before I tore into it.

 

[ezeke]

Re: johnson 115 low compression

Respectfully, taking a cylinder head off a 115 is not exactly "tearing into it". It is prudent, however, considering the symptoms.

 

[tashasdaddy]

Re: johnson 115 low compression

i agree with ezeke, 8 - 10 bolts, and a new gasket. just don't break the bolts.

 

[BF]

Re: johnson 115 low compression

well... agreed pulling a head off isn't major... but I still think the "symptom" might just be a errant reading caused by slow cranking from a weak battery. If that's true, pulling the head won't show anything.

IMHO, if a re-test with proper cranking speed still comes out low, then for sure there's something to pursue.

that's my $.02 FWIW