one cylinder low comp out of 6, now what?

[chrishayes]

I did a compression test last night on my 74 merc 1500 OB and here is what I got...

#1 91psi
#2 119psi
#3 120psi
#4 120psi
#5 122psi
#6 120 psi

Now what? the #1 cyl is not within 10% of the rest...shame, what a shame. I have this sinking feeling. I have done all this research on how to get this thing up and running again and was having a lot of fun working on it and the boat. So, I assume that rebuild is the only option besides a new motor...what is the going rate for a power head rebuild from a shop? I have rebuilt fully the 4.0L straight 6 in my 94 Jeep and that turned out great, so is it much worse to do an outboard? What are your thoughts?

 

[chrishayes]

Re: one cylinder low comp out of 6, now what?

edit: this was with fogging oil sprayed in the cylinders with the motor cold. The impeller needs replaced so the lower unit is currently off. I didnt want to fix anything until I did this test.

 

[Laddies]

Re: one cylinder low comp out of 6, now what?

Because the engine has not been run and tthe top cyl. could be as simple as sticky rings pull the intake porty cover off and inspect the cyl. for damage if it loks OK spray iit with Powertune and see if it will loosen the rings. Also it won't hurt to fire the engine up for 20 or 30 seconds to clean it out without the lower unit on and if you want to run it for more than that just hook a water hose to the supply tube to cool it.

 

[chrishayes]

Re: one cylinder low comp out of 6, now what?

that was my first inclination, pulling the insp port but did not know if there was a gasket that I needed to source before doing this or if it is a RTV type gasket? thats a good idea about the water tube...duh. I have got to stop over thinking this 2 stroke. I know engines, this is just a different kind and the mistakes are more costly so i dont want to make them.

If it matters, I did try to put some sae30 in the top cylinder as my manual suggests but it did nothing for the compression.

Also, I first did the test with no fogging oil and my numbers were around 60psi and the motor labored to turn over.

Thanks laddies, you have been a huge help to me.

 

[chrishayes]

Re: one cylinder low comp out of 6, now what?

oh, and the plug in this cylinder looked differnt than all the rest. it was dry and had no oil on it where the others all had a slight oily residue. it was whitish in color. I did some research and found that I needed a 76 plug not the 77 that are in it(champion). would this make a difference?

 

[chrishayes]

Re: one cylinder low comp out of 6, now what?

one more thing to report. I dont know if it is normal or not, but when I sprayed the fogging oil in the cyl's some of it leaked out through the square tube in the foot, which I assume is the exhaust. Normal?

 

[Laddies]

Re: one cylinder low comp out of 6, now what?

Yes, it just coming out the exhaust. I think I would hook it on the hose and start it, if it will, you don't care if it's running alright or not just try to get it to clear it's self out. Double mix some fuel for extra lubrication as it's been setting and the bearings need a little break to loosen up. if you can get it to run use some powertune as described on the can.

 

[chrishayes]

Re: one cylinder low comp out of 6, now what?

Yes, it just coming out the exhaust. I think I would hook it on the hose and start it, if it will, you don't care if it's running alright or not just try to get it to clear it's self out. Double mix some fuel for extra lubrication as it's been setting and the bearings need a little break to loosen up. if you can get it to run use some powertune as described on the can.

How would you suggest that I hook the garden hose up to the water tube? it looks to be a 3/4 copper line. DO I need this to be water tight or can I makeshift duct tape it on?