1977 Johnson 4HP Rings

[Hank007]

Hello. I am a newbie to the site and to outboards in general, but have a question and this looks to be the best site on the web for information. I have a 1977 Johnson 4HP. It has 45 psi on each cylinder and I was told it should be 80 minimum, and preferably in the 100-115 range. It was suggested to change the rings. My problem is that I have found that there is a standard set of rings and an oversized set. I have not opened the motor yet but wanted to get the parts on the way. I have a set of calipers to check the size of the cylinder. Does anyone know what the standard size measurement is so I can determine which set I need? Thanks for any help.

 

[HighTrim]

Re: 1977 Johnson 4HP Rings

Welcome to iboats.

I would probably open her up first before purchasing any parts to determine exactly what your problem is, and whether you want to persue it.

May also want to try a decarb before that, then try testing again with a different gauge if possible, although if it is 45 psi, that is a tad low to be needing just a de carb, but it cannot hurt.

 

[Hank007]

Re: 1977 Johnson 4HP Rings

Thanks HighTrim. I opened her up and pulled the piston. I was told it would be stamped with either an "s" or "o" depending on whether it is oversize or standard. I don't see either on it. There is an "L" but that is it. Does that mean anything? If not, is there another way to tell? Thanks.

 

[tashasdaddy]

Re: 1977 Johnson 4HP Rings

what do the cylinder walls look like, scarred, does it need to be rebored, that determines what you need.

 

[JB]

Re: 1977 Johnson 4HP Rings

Your real problem, Hank, is bad information and bad advice.

If both cylinders match that close I would call that a healthy engine.

Too many people get hung up on psi. That is not the important thing; the important thing is the balance or span of psi among the cylinders. Both cylinders getting sick the same amount at the same time is nearly off the charts as far as possibilities go. The only thing I can see pulling compression down in both cylinders is a blown head gasket between the cylinders, and even then the psi are almost never close.

With a small, rope start engine compression readings can vary all over the map because of differences in guages, technique and strength of the guy pulling the rope.

As long as both cylinders show close psi, I think your engine is okay.

You can do a decarb and get a weight lifter to pull the rope with a different guage if you like. You will get different readings, but they will still show a healthy engine.

 

[F_R]

Re: 1977 Johnson 4HP Rings

Thanks HighTrim. I opened her up and pulled the piston. I was told it would be stamped with either an "s" or "o" depending on whether it is oversize or standard. I don't see either on it. There is an "L" but that is it. Does that mean anything? If not, is there another way to tell? Thanks.

Standard bore is 1.5625". Oversize pistons are usually marked with the oversize i.e. "020". The L designates the 'loose" end of the wrist pin in the piston. Important that you understand this if you plan to remove/reinstall the pin. You push it out through the tight end and put it back in through the Loose end. Otherwise you distort the piston.

 

[Hank007]

Re: 1977 Johnson 4HP Rings

Thanks F_R , JB and tashasdaddy for the info. I have the pistons out now and will have the parts in a few days. I am replacing the rings and all the seals. The pistons look clean. The cylinder walls are smooth. No scarring or scuffs. I did find that the rings seem worn and do not fit snug into the cylinder, so maybe that will resolve the issue. I will keep you posted on how it turns out. This should keep me going for a while. Thanks so much!