compression percentage diffrernce.

[maryhannaj]

I recently posted about performace upgrades for a 1982 115 Johnson. My compression numbers at that time were somewhere between 105 and 115 or so. I recently ran another combustion chamber cleaner through it and ran it on the water today.
I gained about 2 mph and better bottom end. I came home today and did another compression test. The numbers have increased to 135, 130, 129 and 125.
Although veyr happy about the compression increase, I am a bit worried about the difference between the 135 and the 125. Opinions and advice are all eelcome as usual. If it makes any difference, I only achieved the 135 once out of six tries. Otherwise it was a pretty sready 130. Thanks for reading!

 

[metriccrescentwrench]

Re: compression percentage diffrernce.

That's a pretty insignificant difference.

 

[maryhannaj]

Re: compression percentage diffrernce.

I only ask because a local tech said something like 10 percent and I am trying to decide if now that the compression is up a bit if I am going to spend the money on some parts or not but thanks for the reply.

 

[Bosunsmate]

Re: compression percentage diffrernce.

I agree with Metriccrescentwrench,
id be happy buying a motor with those readings.
Buy a impeller, (any other waterpumps bits needed), some gear oil and sparkplugs and then try and keep restraint

 

[maryhannaj]

Re: compression percentage diffrernce.

Thanks all!

 

[maryhannaj]

Re: compression percentage diffrernce.

I put this video up to see if I could get more opinions on the way it's running. I hope it's the right one.

 

[maryhannaj]

Re: compression percentage diffrernce.

Here's another one just for the fun of it. Sorry it is filmed sideways. Im still trying to figure out this smartphone type of stuff. The water was a bit more rough out on the bay on this day. Lots of fun!

 

[Bosunsmate]

Re: compression percentage diffrernce.

Nice place to boat.
It idled and powered up well.
I couldnt hear the engine at high speed as the wind seemed to be stalling the speakers but if it was running consistent at high speed and no missing then id like it myself!

 

[maryhannaj]

Re: compression percentage diffrernce.

It is a great place. Great fishing too. It is a body of fresh water seperated from the Great Salt Lake by a dyke and some wet lands. Its very shallow. When the wind picks up just a little, it becomes rough very fast.
Anyhow, I just discovered a small drip of water coming from the starboard ban
k head gasket on the bottom so it is definatly time to look into it. Would you by chance know if the omc powerhead gasket kit comes with head gaskets too? I know it sounds like common sense but I would just like to know for sure before I order one. Thanks.

 

[keefallan]

Re: compression percentage diffrernce.

The gasket set does come with head gaskets. Water can also come out of the water covers too!! Before you take the heads off, just make sure that it's not the cover on top of the head. But it sounds like you want to open it up. If you do take the heads off just torque them down to 20lbs and start with the inner most bolts and work out in a circular pattern(a swirl).

 

[maryhannaj]

Re: compression percentage diffrernce.

It's definatly the head gasket. Thanks for the info on the torque specs and pattern.

 

[Bosunsmate]

Re: compression percentage diffrernce.

Thats amazng how its virtually adjoined to the great salt lake but is still freshwater.
In regards to headgasket, try and clean up the old mating surfaces as best you can as old gasket glue can be stuck on them.
Dont add gasket glue to the new head gasket as it should come with the correct amount.
I was just looking at one yesterday that was pushing througfh on the starboard side. It always seems to be that starboard bank that goes first in my history with them

 

[keefallan]

Re: compression percentage diffrernce.

Before you put the head back on, lap the head on some glass with sandpaper attached. I obtained a glass plate from Lowes. It was a 1/2" shelf that was 8" wide and 30 inches long.Use a figure 8 pattern and check to see if the head is flat. I start out with 150 just to see how flat the part is. When the scratch pattern is even all the way around the head, finish up with 320-400 grit. You now have a dead flat head......guaranteed not to leak. The head may be slightly warped. That's why i bring it up.

 

[maryhannaj]

Re: compression percentage diffrernce.

Thanks again all for the advice. As for lapping the head surface, that makes great sense, but won't that be nearly the same as milling the head surface and change piston clearance? If so, will it be insignificant enough to not have to make any other changes?
It is pretty cool about the bay being nearly connected. It provides some great recreational boating and sailing. Plus the fishing can be great.. I've managed to bring in some pretty nice wipers this year and some nice walleye too.

 

[keefallan]

Re: compression percentage diffrernce.

No, it won't make much of a difference at all. You don't have the high compression heads anyways. Yes you are doing a milling job on a small scale. But you are removing only enough to make sure it's flat. It's what the factory manual tells you to do to flatten the head. You won't have to change any jets.

 

[emdsapmgr]

Re: compression percentage diffrernce.

The decarb on your engine has had remarkable results. Normal compression numbers for that engine would be in the 125 range. I'd check to make sure both heads are a matched set. (140 heads run higher compression.) If they are not-that could explain a lot. Also acceptable to have compression within 10%. Not unusual to have the compression on one head vary 5 lbs from the other head, due to manufacturing techniques.

 

[maryhannaj]

Re: compression percentage diffrernce.

Thanks emsapmgr. How would I know if the heads are 140s? I did just buy this boat a few months ago and know nothing of it's history.

 

[maryhannaj]

Re: compression percentage diffrernce.

Ok. New problem. While I was doing the compression test, I cranked the motor for five seconds each time, not the suggested five turns as suggested on another old thread. Did this give me a false reading, or does it make a difference?
Not to answer my own question, but it seems to me that I may have just pumped up the guage. I do want to clarify that I tested it the same way I did when I got the lower compression readings.
Holy garbage bag leakage! What is the correct procedure for an out board two stroke motor? Thanks everyone.

 

[metriccrescentwrench]

Re: compression percentage diffrernce.

You'd want to let the gauge get as high as it can go. The first few revolutions you'll see the reading climb as the air in the gauge and fittings get compressed a bit more each time. Finally all the air in that "dead space" is compressed to the same level as in the cylinder at TDC and the gauge quits going up.

 

[maryhannaj]

Re: compression percentage diffrernce.

Thanks metriccrescentwrech. I will have to assume from your reply that the guage and procedure that I used gave a correct reading. The de-carb did really work pretty darn good. I must have had some really gummed up rings. I tried using the Power tune the first time and didnt get much immediate change. I then used a Yamaha de-carb product that did'nt require a long soak time and then used the Johnson /Evinrude de-carb engine tuner that required a soak time of 3 to 16 hours. Bad weather circumstances required me to let it soak for about 20 hours.
Apparently it worked. Can you tell me how I would identify if at some point if the heads were replaced with 140's? It was mentioned in an earlier reply as a possibility. But since you are on-line at the moment maybe you can tell me? Thanks.