low compression 70hp omc

[Cut n' Jump]

I just reringed my 77 model Johnson 70hp. It runs great on the muffs although the idle seems lean[no smoke and white plugs after an hour of running]. It smokes real good when you romp on it. Just for grins I checked the compression with a known good gauge. 80psi on every cylinder. It had 120/120/100 before. The bottom piston got scored from a leaky exhaust gasket. I retorqued the head and still 80psi. I noticed the head gasket was twice as thick as the old one. This thing fires up on the first bump and runs very smooth and quiet. Will my compression go up after breakin? Also should I run extra oil and for how long[50-1 is normal]?

 

[WillyBWright]

Re: low compression 70hp omc

Compression readings are fairly meaningless until you break it in. I assume you honed it? If not it may never break in and give good compression. Run double oil for the first tank, 1 1/2 times the oil for the second tank, then 50:1 after that.

 

[Cut n' Jump]

Re: low compression 70hp omc

Yes, it was honed just enough to completely deglaze. A straight hone was used and the walls were round. The bottom cylinder got an extra thousanth or two to smooth the scoring. Only 1 used piston was installed. Once I found the price of rings I almost said put em back- they are new. Also I use synthetic oil. Synthetic sucks for 4 stroke rings. Would regular oil be better?

 

[Dhadley]

Re: low compression 70hp omc

Your going to lose some because the cylinders are past the wear limit on size. The pistons are going to rattle some too. The thick head gasket will drop it some more. <br /><br />Although its not up to spec it will probably run pretty good.<br /><br />You dont need the extra oil while youre in the idle only part of the break-in. Once you put it under a load you should run the extra oil for a while -- 10 hours. It probably isnt as critical since 2 of the pistons are worn and the cylinders are certianly not tight.

 

[Cut n' Jump]

Re: low compression 70hp omc

Is there anything other than rings or combustion space that affects compression? Ran 8 gallons thru it the other night. Still 80psi. There was only .001 taken off the cylinders during honing. I think it has .004 clearance. The pistons do rattle slightly. For some reason the darn thing loads up at low rpm's now.[needle&seat?] Runs great at higher rpms. If I run at speed to clean the plugs and stop it jumps on plane real quick. Just going slow fouls the plugs. I'm gonna find a thinner head gasket and maybe mill the head a little. I don't see how it runs so well on 80psi compression.

 

[marinemech]

Re: low compression 70hp omc

run tcw3 non synthetic oil to break in new rings<br />also moter was probably designed to run on leaded fuel to componsate the head gaskets where probably desgined thicker to lower compression