stock compression numbers

[swyne]

Hello guys new to the forum. Question? I have a 1987 88hp Special. My model number is J88MSLCUS. The compression numbers are not good. 3 of the cylinders are around 100psi each and one around 48-50psi. It does feel like its running on 3 cylinders. What should they be? Thanks for you time.

 

[wilde1j]

Re: stock compression numbers

There are no published compression numbers. For most motors, > 90 and w/in 10% of each other is fine. Some motors are typically lower than others and some, like an '85 J 140 I had, are significantly higher (140 to 150 psig).

Your one low cylinder clearly has a problem. Start by pulling the cylinder head and carefully examining the head, gasket and bore. Could be a bad head gasket, stuck piston ring, scored cylinder walls, broken piston ring, damaged piston, etc.

 

[swyne]

Re: stock compression numbers

On this engine, (Belongs to a friend of mine) It was running rough. Like on three cylinders. He honed out replaced the piston, rod and rings in the one of the cylinders. He had not ran it since he did this because he wanted the carbs cleaned first. Which is why he brought it over for me to do. I decided to just for the heck of it to do a compression test, and that what I found. The low compression is on the cylinder that had the work done.

 

[R.Johnson]

Re: stock compression numbers

If you were to hone out all damage mark's, and did'nt have a mean's of mearsuring the bore! That could be the result.

 

[wilde1j]

Re: stock compression numbers

Needed to bore that hole, not hone it and put in oversized piston and rings. He wasted his time (and money). ... and ya' gotta measure!

 

[swyne]

Re: stock compression numbers

Ok looking in to this matter I found that the head gasket was installed improperly. upside down. I reinstalled the head gasket and now the compression went from 48-50 to 90-95. and the bottom one gained 5psi to 105psi. from 100psi. I would like to remove and soak the carbs overnight. Question is there any breakdown to these carbs to remove certain parts that can't be cleaned in this manner? I know on small engines you have to remove the seat from the carb and bowl etc. I have never done an outboard carb before. I really appreciate the help guys, thanks

 

[CATransplant]

Re: stock compression numbers

Remove anything not made of metal before soaking the rest. Have a carb diagram on hand, or take some digital photos you can consult.

I used to be able to keep the parts placement in my head for reassembly, but I quit doing that a couple of years ago. I get distracted too easily these days.

 

[swyne]

Re: stock compression numbers

Ok got everything all cleaned. Just wondering what are the exact torque figures for the head bolts. I found 30ft/lbs but it dosen't seem like enough. And it also seems like they were on tighter than that before I started. In which if thats the case the head gasket was already crushed to a higher spec. It will more than likely fail. right??

 

[wilde1j]

Re: stock compression numbers

OEM shop manual will have correct torque value and torque procedure sequence.