Low compression on one cylinder 15 hp Johnson 1981

[sweetmarie]

I've got only 40 psi on top cylinder and 100 on bottom. Removed head cover and noticed two small "holes" on the sides on the top piston. Do you think this could the reason? The other possibility I could think of is the top reed valve assembly. Posted two pics.
TIA

 

[racerone]

The reed valves have nothing to do with compression in the cylinder.----Unless of course a reed valve broke and damaged the top cylinder.-----It does look like some " loose item " bounced around in there.-----The motor has to come apart for major work.

 

[boatwhisperer]

Since you're new here - Take the time to read our rules.

 

[sweetmarie]

The reed valves have nothing to do with compression in the cylinder.----Unless of course a reed valve broke and damaged the top cylinder.-----It does look like some " loose item " bounced around in there.-----The motor has to come apart for major work.

Ok, I always thought that if one or both reed valves for the same cylinder were broken, the back pressure would come out through the carb and hence lower the compression reading. But I guess that if I'm getting 40 psi, there's got to be something else going on like you said. The cylinder wall looks good from what I can see with the head cover removed. Oh well, I've got 3-4 months to rip it apart to see what's up, this will be my winter project, living in the northwestern part of Quebec, Canada.
tks.

 

[racerone]

Take a look.---When piston comes up in the cylinder the exhaust ports are the last to close.-----Then compression starts.-----No reed valves are in this picture.------Reed valves could be laying on your work bench and you would still see good compression on a motor in good condition !

 

[Crosbyman]

" the back pressure would come out through the carb and hence lower the compression reading.."

well not really

the reeds only hold the piston back pressure when the piston comes back "down" after the explosion above the top cyl section

this forces the fresh load of fuel fumes into the cyl via the IN ports and the reeds make certain that this happens instead of the fuel load being pushed back out he carb face.

cyl compression only exists when both in and out (exhaust ports) are closed on the piston up stroke.

nothing to do with reeds ​

 

[sweetmarie]

I've guess you've figured out by now that I'm not a mechanic by trade?? Well at least now I know where to start looking, rings, gasket, etc. thanks again for you guys input.

 

[thatone123]

Your in a remote area like me. Many times you can find these 15's so reasonable it is not worth the time and trouble to rebuild. Check Craigslist for the nearest town or city. Unless you really know what you are doing the new used motor will last longer than many rebuilds

 

[racerone]

Just picked up a 1992 model 15 Evinrude myself.----Said to have low compression and poor spark.----Easy fix as I have all the parts on hand.----Piston standard / piston rings / pistons O/S / gasket kits / ignition parts etc./ lower unit seal kits / water pump kits.----All picked up for next to no investment----

 

[oldboat1]

The water passages look remarkably clean, but the pistons look to have been hammered pretty well -- looks like the plug reach was too long. What plugs are you using?

Holes in the piston crown edges seem kind of mysterious to me. Maybe overheating, plug too hot, carbon pieces burning in an overheating condition (no experience with that -- might be a stretch).

 

[racerone]

On the 81 models the top piston ring may be very close to the top of the piston.-----Plus the ring has a wild taper on it.----That leads to broken rings due to the design of the rings.----Motors use a 1/2" reach plug and I doubt that it is due to too long of a sparkplug.